Hot tub TDS: what it means and when to change water

Hot tub TDS levels tell you how much has dissolved in your water since you filled it. The number only means something when you compare it with your fill water baseline, so test the tap water on fill day. When TDS has risen by about 1,500 ppm above that starting figure, most guidance says it is time to change the water.

This guide explains how to take a reliable meter reading, what a rising number does to your sanitiser and your skin, and why a saltwater tub needs its own baseline. You will also find a simple plan for draining, refilling and rebalancing when the meter shows the water is spent.

What does TDS mean in a hot tub?

TDS stands for total dissolved solids. It covers everything dissolved in the water that you cannot see and cannot filter out: minerals, salts, chemical residues and the broken-down remains of whatever bathers bring in. It is measured in parts per million (ppm), which is the same as milligrams per litre.

A TDS reading is not a measure of dirt. Clear, sparkling water can have a high TDS, and cloudy water can have a low one. The particles that make water cloudy are a job for the filter, while dissolved solids pass straight through it.

What counts as dissolved solids?

• Minerals already in your tap water, mainly calcium and magnesium, which are higher in hard water areas.
• Residues from sanitiser, such as the salts left behind after chlorine or bromine has reacted.
• Balancing chemicals, including pH increaser, pH decreaser, alkalinity increaser and calcium increaser.
• Shock, clarifiers, foam remover and scale or stain treatments.
• Sweat, body oils, lotions, cosmetics and detergent from swimwear, once they have broken down.
• Sweat in particular, which carries dissolved sodium, potassium and urea straight into the water during every soak.
• Sunscreen, which is a notable source of dissolved residue, especially in summer when bathers get in straight from the garden.
• Salt, if your tub uses a saltwater sanitising system.

If you read the labels on your chemicals, the usual sources are easy to spot. Sodium bicarbonate is sold as alkalinity increaser and sometimes pH increaser. Sodium bisulphate is the usual pH decreaser. Calcium chloride is calcium increaser, potassium monopersulphate is non-chlorine shock, and dichlor granules are stabilised chlorine. Tap water also brings its own sodium before you add anything.

Is a hardness test the same as a TDS test?

No. Calcium hardness is only one part of TDS, so a hardness test cannot stand in for a TDS test. It misses sanitiser by-products, broken-down organic matter and stabiliser residue, which all add to the total.

There is another difference. You can raise calcium hardness with calcium increaser, or keep it in check with scale control products. TDS cannot be adjusted with chemicals at all, because every product you add only increases it.

Why does TDS keep rising?

Almost nothing removes dissolved solids from a hot tub except draining. Every chemical dose adds a little, and every bathing session adds more. When water evaporates, the solids stay behind, so topping up with fresh water restores the water level but does not reverse the rise.

In fact, topping up after evaporation pushes TDS higher over time. Only pure water leaves as vapour, while the concentrated solids stay in the tub. Each top-up then brings in the fill water's own dissolved minerals on top of them, so every cycle of evaporation and topping up adds a little more to the total instead of diluting it.

Hot tubs build TDS faster than swimming pools because the water volume is small, the temperature is high and the number of bathers per litre is large. A typical tub holds around 1,000 to 2,000 litres (about 260 to 530 US gallons), so body oils and chemical doses are concentrated in far less water.

Why does your fill water baseline matter?

Tap water is never pure. It arrives with its own dissolved minerals, and the amount varies widely across the UK and Europe. Some soft water areas start below 100 ppm, while hard water areas can start at several hundred ppm before you add a single chemical.

This is why an absolute TDS figure tells you very little on its own. A reading of 1,600 ppm could mean heavily used water in a soft water area, or fairly fresh water in a very hard water area. The useful number is the rise: today's reading minus your starting reading.

Here is a worked example. If your fill water reads 250 ppm, add the usual 1,500 ppm rise to it. Your change point is about 1,750 ppm on the meter.

How to set your baseline

1. Run the tap or hose you use for filling for about a minute, so you are not testing water that has been sitting in the pipe.
2. Collect a sample in a clean cup, test it with your TDS meter and write down the figure with the date.
3. Fill the tub, add your start-up chemicals and let the water circulate for a day.
4. Test the tub water again. This second reading, taken once the balancing chemicals are mixed in, is your true starting point.
5. Keep both figures in a log, in a notebook or on your phone, along with the dates you add chemicals and clean filters.
6. Write the baseline figure and the fill date on a label and stick it inside the equipment cabinet, so the number is always to hand when you test.

UK water companies publish water quality information for their supply areas, usually searchable by postcode. These reports often list hardness and individual minerals rather than a TDS figure, so treat them as background rather than a replacement for your own test. The Drinking Water Inspectorate regulates drinking water quality in England and Wales.

Your baseline can change over time. Water suppliers sometimes blend or switch sources, so test the tap water on every fill rather than reusing an old number.

Tap water TDS can also vary from season to season, even when your supplier has not changed anything. A baseline you recorded in winter may not match the water you fill with in summer, which is another reason to test the tap water every time you refill.

What hot tub TDS levels are normal?

The most widely used guideline is to change the water when TDS has risen about 1,500 ppm above the starting level. Some manufacturers use a different figure, and a few quote an absolute maximum instead of a rise. Check your owner's manual and follow it where it differs.

The 1,500 ppm rise threshold comes from the Pool & Hot Tub Alliance (PHTA), the main industry standards body for pools and spas in North America. It is the benchmark that professional spa technicians generally work to, including in the UK and Europe.

Some manufacturers set a stricter limit, around 1,000 to 1,200 ppm above the fill water. If your manual gives a figure like this, use it as your change point instead of 1,500 ppm.

It helps to read the rise in stages rather than waiting for one alarm figure. The bands below are a practical way to interpret your log. They are not an official standard.

None of these bands depend on the age of the water. A lightly used tub can stay in the first band for months, while a tub used daily by several people can pass 1,500 ppm in a few weeks.

A dated log shows how fast your own tub's TDS rises. If it climbs by roughly the same amount each week, you can predict when the next water change will fall due and book the time in advance, rather than finding out when the water is already spent.

How do you read a TDS meter accurately?

A TDS meter does not count solids directly. It measures how well the water conducts electricity, then converts that into an estimated ppm figure using a built-in factor. Some dissolved substances, such as certain oils and organic matter, conduct poorly and barely register. The estimate is still reliable for tracking change over time, as long as you test the same way every time.

Because the meter only measures conductivity, it cannot tell harmless minerals apart from sanitiser by-products or other residues. A reading tells you how much has built up, not what it is. That is why the rise above your baseline is used as a trigger to change the water, rather than as a measure of what the water contains.

Taking a reading step by step

1. Check that the probe is clean and free of scale or residue, then rinse it in clean water.
2. Take a sample from about elbow depth, away from the jets and away from any spot where you have just added chemicals.
3. If your meter does not have automatic temperature compensation (often marked ATC), let the sample cool towards room temperature. Hot water conducts better and gives a falsely high reading.
4. Dip the probe to the depth marked on the meter, which for most pen meters is roughly 2 to 5 cm (about 1 to 2 inches). Stir gently to clear any air bubbles and wait for the number to settle.
5. Record the figure and subtract your baseline to find the rise.
6. Rinse the probe in distilled water rather than tap water, so leftover minerals do not affect the next reading, then replace the cap before storing it.

Meters, strips and shop tests

A digital pen meter is the most practical option for home owners. Basic models are widely available at modest cost, and a meter with automatic temperature compensation and a calibration option is worth the small extra. A good digital TDS meter is typically accurate to within about ±2% of the reading, which is plenty for tracking a rise of several hundred ppm.

TDS test strips exist, but they give a rough band rather than a precise figure, which makes a slow rise hard to see. Ordinary multi-test strips for chlorine, pH and alkalinity do not measure TDS at all. Some pool and spa shops will test a sample on a bench meter, which is a useful cross-check for your own readings.

TDS strips are good enough to confirm that the water is still well below the change point. Once the rise approaches about 1,000 to 1,500 ppm above your baseline, switch to a digital meter, because the decision to drain needs a more precise figure than a strip can give.

Many pool and spa shops will test a water sample free or for a small fee. This is handy for checking your tap water baseline before you fill, especially if you do not yet own a meter.

Keeping your readings consistent

If your meter can be calibrated, do it with a calibration solution following the maker's leaflet. Use the same meter for your baseline and for every later reading, because different meters can use different conversion factors and give different figures for the same water.

If you live in a hard water area, test TDS more often than an owner in a soft water area would. Your fill water already starts with a high mineral load, so you have less headroom before the total becomes high and the readings become harder to interpret. Frequent tests make the rise above your baseline easier to follow.

TDS is only one test. It tells you how much is dissolved in the water, not whether the water is safe or balanced, so keep up regular checks of sanitiser, pH and alkalinity. Our guide on how to test hot tub water and when sets out a simple routine.

What does high TDS do to your water?

As dissolved solids build up, the water becomes a harder place for your chemicals to work. Sanitiser has more to react with and its readings can fall quickly after a dose. Many owners respond by adding more, which raises TDS further and makes the problem worse.

• Sanitiser readings drop soon after you dose.
• pH and alkalinity drift and will not stay in range.
• Foam builds on the surface when the jets run.
• The water looks dull or slightly hazy even with a clean filter.
• Scale forms at the waterline or on heater and jet parts, especially in hard water areas.
• A stale or chemical smell remains after shocking.
• Skin feels dry or itchy after a soak.

Not all of these signs come from TDS alone. Low sanitiser, a dirty filter or high calcium can cause the same symptoms. Your TDS log helps you tell the difference: if the rise is past 1,500 ppm and the symptoms keep returning after you correct the balance, the water itself is the problem.

Skin, eyes and when to stop

High TDS on its own is not generally considered a direct health risk, but water that is hard to keep sanitised can be. If the water smells wrong, looks hazy or your sanitiser will not stay in range, stop using the tub until you have tested and corrected it or changed the water. The HSE publishes guidance on Legionella risks, including in hot tubs.

The risk of skin and eye irritation from poorly sanitised water rises with longer soaks and hotter water. Keep sessions short when the water is tired, and keep the temperature at or below the usual limit.

People with sensitive skin or eczema are more likely to react to tired, poorly balanced water. If that applies to you or someone in your household, change the water a little earlier and shower after every soak.

Children, older people and anyone with a weakened immune system should be more cautious about soaking in water that is hard to keep sanitised. Poorly sanitised water can also raise the risk of ear infections, so keep heads above the water when the balance is in doubt.

If a rash, itchy bumps or sore eyes do not settle within a few days of stopping, speak to a pharmacist or your GP. If you develop a fever, cough or breathing problems after using a hot tub, contact your GP or use NHS 111, and mention that you have used a hot tub.

Effects on equipment

Old water with a high mineral load is harder to keep balanced, and poorly balanced water can leave scale on the heater element or attack metal parts. Changing the water on time costs far less than repairing a heater or replacing pump seals.

High TDS also makes the water more electrically conductive. This can speed up corrosion where different metals meet, such as in the heater, jets and fittings. It usually shows as staining or small pits on metal surfaces.

Why is rising TDS the clearest signal for a water change?

Many owners change their water on a calendar, often every three to four months. A calendar is a sensible backstop, but it cannot see how hard the tub has been used. Two quiet months and two months of busy family weekends leave very different water.

TDS tracks the thing that actually wears water out: the total of everything that has gone into it. You can correct sanitiser, pH and alkalinity with a dose of chemical, but you cannot dose TDS down. Once the rise passes about 1,500 ppm, no product will return the water to a fresh state.

The most reliable approach combines both. Change the water when the TDS rise reaches your threshold or when your manual's maximum interval arrives, whichever comes first. Our guide on how often to change hot tub water explains time-based guidance and how the number of bathers affects it.

Test more often during busy summer months, or in any season of heavy use. More bathers, more chemical doses and more evaporation make TDS climb faster, so the water may need changing earlier than your calendar suggests.

As a guide, treat four or more bathers using the tub several times a week as heavy use. At that level TDS builds much faster, and each fill of water will last noticeably less time than it would in a tub used by one or two people.

Other signs still count. Change the water sooner if it stays cloudy or foamy after shocking and cleaning the filter, if it smells unpleasant despite good sanitiser readings, or after any contamination incident in the water.

How to slow the rise

• Ask everyone to shower without lotion or make-up before getting in.
• Rinse swimwear in plain water, because detergent residue adds to the load and causes foam.
• Add chemicals in small, measured doses and retest before adding more.
• Keep the cover on between uses to reduce evaporation and debris.
• Run the tub a few degrees cooler when you can, for example around 37°C (99°F) instead of 40°C (104°F). Cooler water evaporates noticeably less, so TDS concentrates more slowly between water changes.
• Clean the filter on the schedule in your manual, so the sanitiser is not fighting extra debris.

How do saltwater hot tubs change the TDS reading?

In a saltwater tub, a chlorine generator turns dissolved salt into chlorine, which turns back into salt once it has done its work. To make this possible, the water carries a deliberately high salt level, and salt is a dissolved solid. Your TDS meter cannot tell salt apart from everything else, so the reading will be far higher than in a standard tub.

The target salt level varies between systems. Many run somewhere in the region of 1,500 to 3,000 ppm, but some are lower or higher, so always use the figure in your manual. Never add salt to a tub that was not designed for a salt system.

Setting a salted baseline

1. Test your fill water before adding anything and note the figure.
2. Add salt to the level your manual specifies, let it dissolve and circulate, and confirm the level with salt test strips or a salt meter.
3. Take a TDS reading once the salt has fully mixed. This salted figure, not the tap water figure, is your baseline.
4. Track the rise above the salted baseline from then on, exactly as you would in a standard tub.

What to watch in a salt tub

Salt is not used up in normal running, but it is lost when water splashes out and diluted when you top up. When you add salt to correct the level, your TDS reading will rise as a result. Note every salt addition in your log, so you do not mistake it for a build-up of other solids.

Because the chlorine comes from the salt rather than from added granules, other dissolved solids can build more slowly in a salt tub. This does not remove the need to drain. Body oils, cosmetics and balancing chemicals still accumulate, so plan a change when the rise above your salted baseline reaches about 1,500 ppm, or the limit your manual gives.

If you have no salted baseline

If you did not record a salted baseline, you can make a rough estimate of the other solids. Subtract both your salt test reading and your tap water baseline from the TDS reading. For example, a TDS reading of 3,200 ppm, minus 2,500 ppm of salt, minus 250 ppm from the tap water, leaves about 450 ppm of other solids.

Treat this figure only as a rough guide. A TDS meter and a salt test do not convert salt and other dissolved solids to ppm in the same way, so the result can be some way off. Set a proper salted baseline at your next water change.

How do you change the water when TDS is too high?

A full drain and refill resets TDS to your fill water level. A partial drain lowers the rise in proportion to the water you replace, so replacing a quarter of the water removes about a quarter of the build-up. A partial drain can buy time, but a full change is the only complete reset.

Partial drain or full change?

To work out a partial drain, use this formula: the share of water to replace is roughly (current rise minus target rise) divided by current rise. It assumes your refill water matches your baseline. For example, cutting a rise of 2,000 ppm to 1,000 ppm means (2,000 minus 1,000) divided by 2,000, which is 0.5, so replace about half the water.

A partial drain only makes sense when the rise is just over your threshold. Once it is well past 1,500 ppm, a single partial drain rarely brings it back to a comfortable level, so do a full change instead.

How to do a partial drain

1. Work out the share of water to replace using the formula above.
2. Switch off at the isolator, exactly as for a full change, and check that the display is dark and the jets have stopped.
3. Note the normal water level, then open the drain valve or start a submersible pump.
4. Stop draining once the water level has dropped by the share you worked out, for example about half the depth for a 50% change. Seats and the footwell make the shape uneven, so depth is only a rough guide.
5. Close the drain valve and refill to the normal level with fresh water.
6. Make sure the pump is primed and free of airlocks, then switch back on and let the water circulate.
7. Retest TDS to confirm that the rise is now back below your threshold, then rebalance the water.

Before you start

• Your test kit or test strips and your TDS meter.
• pH increaser, pH decreaser and alkalinity increaser.
• Calcium increaser.
• Sanitiser, either chlorine or bromine, or salt for a salt system.
• A few hours of free time, because a full drain and refill rarely happens quickly.

Allow roughly 2 to 4 hours for a full drain, clean and refill. This does not include the time the new water then needs to heat up and settle before it is ready to use, so plan the change for a day when you will not need the tub straight away.

Switch off at the isolator before you start draining, so the heater and pumps cannot run without water. Do not switch back on until the tub is filled above the jets and the filters are back in place.

1. If your manual recommends a pipe cleaning product, add it to the old water before draining and run the jets as directed.
2. Switch off at the isolator. Check that the display is dark and the jets have stopped.
3. Find the drain valve. Its position differs between models, so check your owner's manual before connecting a hose. Then drain the tub through the valve or with a submersible pump.
4. Remove the water left in the footwell with a wet-and-dry vacuum or a large sponge.
5. Remove and clean the filters, or replace them if they are worn.
6. Wipe down the shell with a cleaner suitable for acrylic, then rinse well so no residue goes into the new water.
7. Refill, through the filter housing if your manual suggests it, which helps push air out of the pipes. Fill to the level the manufacturer recommends, typically around the middle of the skimmer opening.
8. Test the fill water from the hose and record your new baseline figure.
9. Before switching back on, make sure the pump is primed and free of airlocks, following the method in your manual.
10. Switch back on, let the water circulate and heat, then start rebalancing.

For the full method, including where the old water can go, see our guide on how to drain a hot tub step by step. Water containing sanitiser should not go into a surface water drain, a ditch or a stream, so check your local water company's advice before you start.

Do not drain water with chemical residue onto lawns or plant beds. Sanitiser and a high mineral load can damage grass and plants.

How should you rebalance after a refill?

Fresh water gives every chemical a clean start. Balance in a set order, give each adjustment time to circulate, and test before each step.

After a partial drain, rebalance just as carefully. The fresh water you add dilutes alkalinity, pH, calcium and sanitiser as well as TDS, so retest all of them and correct them in the same order as after a full refill.

Add one chemical at a time. Let the water circulate for at least 15 to 30 minutes before you test and add the next, because adding several at once gives misleading readings.

Take the time to get the balance right the first time. Every corrective dose you add to fix poorly set chemistry puts more dissolved solids into the water, so a careful start helps each fill last longer before TDS reaches your change point.

1. Adjust total alkalinity first, aiming for 80-120 ppm, because it helps hold pH steady.
2. Adjust pH next, aiming for 7.2-7.6.
3. Check calcium hardness, aiming for 150-250 ppm, or the range in your manual.
4. Add sanitiser last: free chlorine at 3-5 ppm or bromine at 3-5 ppm, or the salt level and output setting your salt system specifies.
5. After 24 hours of circulation, take a TDS reading. This is the figure you will compare against until the next change.

Do not get in after a refill until the water has circulated for a few hours and a retest shows sanitiser and pH in range.

Which chemical adjusts what?

• To raise alkalinity, use sodium bicarbonate.
• To raise pH, use sodium carbonate.
• To lower pH or alkalinity, use sodium bisulphate.
• To raise calcium hardness, use calcium chloride.
• To lower calcium hardness that is too high, no chemical will work. Partially drain and refill with lower-calcium water.
• To lower chlorine that is too high, use sodium thiosulphate or partly drain and refill.
• To lower bromine that is too high, partially drain and refill, or leave the cover off and wait for the level to fall, rather than adding a neutraliser.

Why pH matters even when TDS is fine

pH below 7.2 can corrode equipment and make eyes and skin sting. pH above about 7.8 sharply reduces how well chlorine works. Out-of-range pH can therefore look like chemicals not working even when TDS is fine, so always check pH before blaming the water's age.

The effect is larger than many owners expect. At pH 7.5, about half of the free chlorine is in its active form. At pH 8.0, only about a fifth is. A small upward drift in pH therefore cuts sanitising power sharply, even when your free chlorine reading looks fine.

Calcium, stabiliser and sanitiser choice

In soft water areas you may need to add calcium to protect the heater and seals, while in hard water areas you may need a scale control product instead. Our guide to hot tub calcium hardness in hard and soft water areas explains how to judge which applies to you.

Low calcium hardness can contribute to foaming. Calcium above about 400 ppm adds noticeably to TDS and encourages scale on heaters and jets. Scale control products can help stop high calcium forming scale, but they do not lower the reading.

If you use stabilised chlorine granules, cyanuric acid builds up alongside TDS and can make chlorine less effective at high levels. A water change is the main way to bring it down, which is one more reason to keep an eye on your log. Our guide to hot tub cyanuric acid and when stabiliser becomes a problem covers the details.

In a chlorine tub, dichlor granules are the usual way to bring free chlorine up to 3-5 ppm after a refill. Add them in measured doses with the jets running, then retest before adding more.

Avoid trichlor tablets as your main hot tub sanitiser. They add cyanuric acid quickly at hot tub temperatures. Aim to keep cyanuric acid below about 50 ppm.

Bromine needs no stabiliser, so it does not add cyanuric acid. After a refill, bromine is usually brought to 3-5 ppm with granules and then held at that level with tablets in a floater.

Bromine also stays effective at higher water temperatures and across a wider pH range than chlorine. This is one reason many owners prefer it for hot tubs.

Keep the water no hotter than 40°C (104°F), which is the usual safe upper limit for bathing. Hotter water also speeds up evaporation and sanitiser use, and both push TDS up faster.

Water hotter than 40°C puts extra strain on the heart and circulation. This matters most for people with a heart condition or high blood pressure, and during pregnancy. If any of these apply to you, check with your GP, or your midwife if you are pregnant, before using a hot tub.

Hot tub TDS in short

Test your tap water and your freshly balanced tub water on every fill, and treat that second reading as your baseline. Check TDS every week or two with the same meter, testing the same way each time, and record the rise above the baseline.

When the rise reaches about 1,500 ppm, or the limit in your manual, plan a full water change. In a saltwater tub, set the baseline after salting and track the rise from there. Combine TDS with a calendar backstop and your normal chemical tests, and you will change the water when it needs it, not before or long after.

Frequently asked questions

Does chlorine raise or lower TDS in a hot tub?

Chlorine raises TDS slightly every time you add it. Once it has reacted with contaminants, the leftover salts and by-products stay dissolved in the water. Shock and other oxidisers break down organic matter, but they do not remove dissolved solids, so they cannot lower a TDS reading. The only practical way to bring TDS down is to replace some or all of the water.

Will my hot tub filter remove TDS?

No. A cartridge filter traps suspended particles such as hair, lint and debris, which keeps the water clear, but dissolved solids pass straight through it. Hose pre-filters used when filling mainly remove sediment and some chemicals from tap water and usually make little difference to TDS. A clean filter still matters, because it reduces the load your sanitiser has to deal with.

Can I fill with softened water to start with a lower TDS?

Softened water does not have a lower TDS. A salt-based water softener swaps calcium and magnesium for sodium, so the dissolved total stays similar. Very soft water can also be aggressive to heaters and seals, and many manufacturers advise against filling from a softener. Check your manual, fill from the mains supply and correct calcium hardness if it is low.

Why is my TDS reading lower than on the day I filled?

The usual reasons are dilution or a testing difference. Topping up after splash-out or a partial drain adds lower-TDS tap water. A dirty probe, a meter that needs calibrating or switching to a different meter can also change the figure. Clean the probe, let the sample cool, check calibration and test again before deciding anything from the result.

How often should I check TDS?

Every week or two is enough for most home tubs, alongside your usual sanitiser and pH tests. TDS moves slowly, so testing daily rarely tells you anything new. In a heavily used tub, such as a holiday let or a tub used daily by several people, check every few days as the rise approaches 1,000 ppm, so you can plan the water change in good time.

Is high TDS bad for your skin?

High TDS rarely harms skin directly, but tired water is harder to keep sanitised and balanced, and that can leave skin dry or irritated. Showering after a soak helps. If a rash or itching appears, stop using the tub and test the water. Speak to a pharmacist or GP if it does not settle within a few days, spreads, or comes with a fever.