Hot tub calcium hardness in hard and soft water areas

Hot tub calcium hardness is the amount of dissolved calcium in your spa water, and most manufacturers recommend keeping it between 150 and 250 ppm. In UK hard water areas the level is often too high, which causes scale and cloudy water. In soft water areas it is often too low, which can lead to foaming and corrosion.

The fix depends on what comes out of your tap. This guide explains how to test calcium, how to raise it safely, how to control scale with a sequestrant, and how a pre-filter at fill time fits in.

What is hot tub calcium hardness?

Calcium hardness is a measure of the calcium dissolved in your water. Test kits report it in parts per million (ppm), which is the same as milligrams per litre (mg/l). It is usually expressed as calcium carbonate, so readings from different kits and water companies can be compared directly.

Calcium matters because water tries to reach a balance with the surfaces it touches. Water with too much calcium drops it out as scale. Water with too little tries to pull minerals and metal from the shell, the plumbing and the heater.

Calcium hardness and total hardness

Total hardness includes calcium and magnesium together. Calcium hardness counts calcium only, so a calcium hardness test will often read a little lower than the total hardness figure your water company publishes. For hot tub care, calcium hardness is the number to work with.

Calcium is the mineral that drives scale in a hot tub. Magnesium is far less likely to come out of solution at spa temperatures, so it rarely forms deposits. That is why calcium hardness, rather than total hardness, is the figure to manage.

Water companies in Europe may quote hardness in degrees rather than ppm. As a guide, 1 German degree (°dH) equals about 17.8 ppm, 1 French degree (°f) equals 10 ppm, and 1 Clark degree (the old English unit) equals about 14.3 ppm, all as calcium carbonate.

The ideal range and where advice differs

The widely accepted range for hot tubs is 150-250 ppm. Some manufacturers set a slightly different range, particularly for acrylic shells or specific heater types, and some recommend a lower minimum. Check your owner's manual and follow it if it differs from the general figure.

Acrylic and fibreglass shells are often said to tolerate calcium up to about 275-300 ppm before scale becomes visible. Treat that as a general observation, not a target. If your owner's manual gives a figure, that figure takes priority.

Inflatable hot tubs with vinyl liners are best kept at the lower end of the range, around 150-200 ppm, to help protect the liner. If the manual for your inflatable tub gives its own figure, follow that instead.

How does UK tap water affect your hot tub?

Hardness varies a great deal across the UK. Much of south and east England, including London and East Anglia, has hard water that can arrive well above 250 ppm. Large parts of Scotland, Wales, Northern Ireland and north west England have soft water that can arrive well below 100 ppm.

You do not need to guess. Most water companies have a postcode checker on their website that shows the hardness of your supply. The Drinking Water Inspectorate regulates drinking water quality in England and Wales and can point you to your supplier. Treat the published figure as a starting point and confirm it with your own test.

If your home is on a private water supply, such as a borehole or a well, your water company's postcode checker does not apply. Private supplies can be very hard, so test the fill water yourself before you fill the tub, ideally with a liquid test kit.

In a soft water area, expect to add calcium increaser after every refill. Fresh fill water starts below the recommended range each time, so plan for it as a normal part of your refill routine rather than an occasional fix.

What happens when calcium is too high or too low?

Both problems develop slowly, so the first signs are easy to miss. Knowing what to look for helps you act before parts are damaged.

Signs of high calcium (hard water)

• A white or grey crust at the waterline, on jets or on the cover underside.
• Cloudy or milky water, especially after the water heats up or the pH rises.
• Cloudiness that does not clear after shocking, which helps tell it apart from a sanitiser problem.
• Rough patches on the shell that feel gritty to the touch.
• Filters that clog faster and feel stiff even after rinsing.
• Weaker jets and lower flow over time, as scale builds up inside the plumbing and jets.
• Slower heating, because scale on the heater element insulates it from the water.
• Heater faults, because a scaled element can run hotter than designed, which can lead to overheating and early heater failure.
• Leaks or failed seals, because scale forming on and around O-rings and seals can crack them.

Heat makes scale worse. Calcium becomes less soluble as water warms, so a hot tub at 37-40°C (99-104°F) scales far more readily than the same water in a cold tap. The heater element is the hottest surface, which is why it often scales first.

Scale on the heater element acts as a layer of insulation between the element and the water. The heater has to run for longer to bring the water up to temperature, so it uses noticeably more electricity and your running costs rise. The extra heat trapped in the element also raises the risk of the heater failing early.

Scale is a hygiene concern as well as an equipment one. Deposits create rough surfaces where bacteria and biofilm can shelter from the sanitiser, which makes the water harder to keep clean.

Signs of low calcium (soft water)

• Persistent foam that returns soon after you treat it.
• Green or brown staining from metal that the water has dissolved.
• Dull, etched or pitted patches on the shell over time.
• Corrosion on the heater element, pump seals or metal fittings.
• pH that drifts and is hard to keep steady.
• A shell that feels slippery to the touch.
• A faint metallic taste or smell in the water as metal parts corrode.

Soft water lets soaps, lotions and body oils lather more easily, which is why soft water tubs often foam. If foam is your main problem, our guide on how to get rid of hot tub foam covers the other causes as well.

Is it calcium, pH or sanitiser?

Owners often mistake calcium problems for pH or sanitiser problems, because foam, haze and scale can each have several causes. If foam, haze or scale keeps coming back while your pH and sanitiser readings are correct, test calcium hardness next.

Calcium and your warranty

Many hot tub manufacturers state a required calcium hardness range in their warranty terms. Damage to the heater, shell or seals caused by long-term chemical imbalance is usually not covered. Check your warranty and keep a dated record of your readings, so you can show the water was looked after if you ever need to make a claim.

How do you test calcium hardness?

Many basic test strips measure only pH, alkalinity and sanitiser, so check that yours includes a calcium hardness or total hardness pad. If it says total hardness only, the reading will include magnesium and may sit a little higher than true calcium.

Test strips or a liquid kit?

Test strips are quick and cheap, and fine for routine checks. Their colour scale jumps in large steps, so they show whether you are low, in range or high rather than giving an exact figure. Strips usually cost a few pounds per tub of fifty.

A liquid drop test (a titration kit) counts drops until the colour changes, which gives a more precise number. It is worth using one when you first fill, before dosing a calcium increaser, and whenever your strips give a borderline reading. Digital photometers are another option, at a higher price.

A liquid titration kit that includes a calcium hardness test typically costs roughly £20 to £40. That is more than a tub of strips, but one kit covers many tests, so the cost per test stays low.

The difference in accuracy is large. A calcium pad on a test strip can be out by around 50 ppm either way, which is half the width of the 150-250 ppm range. A liquid titration kit is typically accurate to about 10 ppm. Use strips to spot changes, but do not use a strip reading alone to calculate a dose.

A digital photometer reads the colour of a sample electronically. It needs regular calibration, as set out in its instructions, to stay accurate. It suits owners who test often or look after more than one tub.

A digital photometer typically costs from around £100 upwards, which is much more than strips or a liquid kit. You will also need to buy its reagents separately as you use them up.

A digital photometer is typically accurate to roughly 5-15 ppm for calcium hardness. That is similar to, or slightly better than, a liquid titration kit, so the main benefit is speed and an easy-to-read result rather than a big gain in precision.

Whichever method you use, wait at least 30 minutes after adding any chemical before you take a sample. Fresh additions that have not yet mixed through the water distort the reading.

How to test with a strip

1. Run the pump for a few minutes so the water is mixed.
2. Dip the strip about 45 cm (18 inches) below the surface, away from the jets, for the time stated on the pack.
3. Remove it and hold it level so the pads do not run into each other.
4. Wait the stated time, usually 15 seconds or so, then compare the calcium pad with the chart in good daylight.
5. Write down the reading with the date, along with pH and alkalinity.

How to test with a liquid kit

1. Run the pump for a few minutes so the water is mixed.
2. Rinse the test tube with spa water, not tap water, because tap water residue can affect the result.
3. Take water from about 45 cm (18 inches) below the surface, away from the jets, and fill the tube to the marked line.
4. Add any preparation drops the instructions call for, then swirl gently.
5. Add the reagent drops one at a time as directed, swirling gently after each one, and count the drops until the colour changes.
6. Convert the drop count to ppm using the kit's instructions.
7. Rinse the tube, close the bottles and write down the reading with the date.

Look after your test kit

Liquid reagents and test strips degrade with age, heat and light. Expired or badly stored reagents can give readings tens of ppm out, which is enough to lead you to the wrong dose. Check the expiry date, keep the lid closed and store the kit in a cool, dark place.

How often to test

Test calcium every time you refill, because that is when it changes most. After that, once a month is usually enough, since calcium only rises slowly as water evaporates and you top up with tap water. In a hard water area with frequent top ups, test every two weeks.

Test more often in summer, when evaporation and top ups are higher, and after a period of heavy use. Test again around 24 hours after a major chemical treatment such as a shock.

Logging your calcium readings over time shows you trends. For example, a steady rise each week points to hard top up water, and it lets you plan a partial drain before scale appears.

How do you raise low calcium hardness?

Calcium increaser is calcium chloride, sold as flakes, granules or powder. Before you add it, test and correct alkalinity and pH first, because calcium readings only make sense once the rest of the water is balanced. Our guide to hot tub water balance in the right order explains the sequence.

Use harder mains water instead of a product

If your tub water has dropped below range, for example after filling with softened water, test your mains water too. If the mains water is harder than the tub water, replacing part of the tub water with mains water raises calcium gently without adding any product.

To work out how much to replace, use this formula: fraction to replace = (target calcium - current calcium) / (mains calcium - current calcium). For example, with the tub at 60 ppm, mains water at 180 ppm and a 150 ppm target, the sum is 90 / 120, so replace about three quarters of the water. Retest and rebalance pH and alkalinity afterwards.

Work out your tub's volume

To calculate a dose you need your tub's volume in litres. The owner's manual usually gives it. If not, for a square or rectangular tub, multiply length × width × average water depth, all in metres, then multiply by 1,000 to get litres.

For a round tub, multiply 3.14 × radius × radius × average water depth, all in metres, then multiply by 1,000 to get litres. Seats and footwells take up space, so treat the result as an estimate and dose in stages.

How much calcium increaser to add

As a rough guide, about 11 g (0.4 oz) of anhydrous calcium chloride per 1,000 litres (about 260 US gallons) raises calcium hardness by around 10 ppm. Flake calcium chloride (the dihydrate form) is weaker, so you need about 15 g for the same rise. Products vary in strength, so always follow the dose on your product label.

For example, a 1,500 litre (about 400 US gallon) tub at 100 ppm needs to rise by 50 ppm to reach 150 ppm. That works out at roughly 80 g of anhydrous calcium chloride, or 110 g of flakes. Add half of the calculated amount, retest, then add the rest if needed.

If you need to raise calcium by more than about 50 ppm, do not add the whole correction in one treatment. Split it into several smaller doses and retest between each one, so you do not overshoot the range.

The table below uses the same rough guide. Each cell gives the amount of anhydrous calcium chloride first, then the amount of flakes in brackets. Check your product label and add in stages.

Choosing a calcium increaser

Calcium chloride products vary in purity, from roughly 77% to 96%. A higher purity product needs less per dose and dissolves faster, so check the percentage on the label when you compare products, not just the pack size.

As a guide to purity, a 96% calcium chloride product needs roughly 20% less product than a 77% product for the same rise in ppm. Factor that into the price per dose when you compare packs.

Finer granules dissolve faster and more completely than coarse ones. That reduces the risk of undissolved particles settling on the shell floor.

Liquid calcium increasers are also available. You pour them slowly around the edge of the tub with the pump running, without dissolving them first. They are easier to dose accurately, but they cost more per treatment than flakes or granules.

Some increasers are combined with a scale inhibitor or a pH buffer. These can change two readings at once and usually contain less calcium chloride, so you need a larger dose and the result is less predictable. Plain calcium chloride gives you the most control.

How to add calcium increaser

1. Put on chemical-resistant gloves and eye protection.
2. Fill a clean plastic bucket about three-quarters full with water from the hot tub.
3. Add the measured calcium increaser to the water, never water to the chemical, and stir with a plastic or wooden stirrer until it dissolves. Keep that stirrer for hot tub chemicals only, and do not use it for anything else.
4. Let the solution cool for a few minutes until the bucket is no longer hot.
5. With the pump running and the cover open, pour the solution slowly around the edges of the tub, away from the skimmer and not directly into the jets.
6. Let the water circulate for at least two hours with the cover open.
7. Retest calcium, pH and alkalinity, and add more only if the reading is still below range.
8. For the most accurate reading before you decide on a second dose, retest again the following day, once the water has circulated fully, rather than relying only on the first retest.

Calcium chloride gives off heat as it dissolves, and the bucket can become hot quickly. Always add the chemical to water, in small amounts, and never mix it with any other product. Store it dry, sealed and out of reach of children, and read the safety information on the label. The HSE guidance on COSHH and hazardous substances explains safe handling in general terms.

Never sprinkle calcium chloride granules straight into the tub. Undissolved granules can sit on the shell, give off heat in one spot and cause local damage, etching or cloudy patches. Always dissolve them in a bucket of spa water first.

A brief cloudiness after dosing is normal and should clear as the water circulates. If it does not clear, the pH or alkalinity may be high, so retest and correct those before adding more calcium.

Keep calcium steady once it is in range

Once calcium is in range, it is one of the most stable readings in hot tub water. It does not swing from day to day like pH and sanitiser. Small regular corrections of about 5-10 ppm are easier and cheaper than fixing a large drift later.

In a soft water area, keeping the water in good condition for longer between full drains means fewer refills. Good sanitising and regular oxidising (shocking) help the water last, so you have to rebuild calcium from scratch less often.

Enzyme treatments help here too. They break down body oils and other organic waste, and used alongside regular oxidising they can extend the life of the water by a month or two. Each refill you avoid is one less time you need to rebuild calcium from scratch.

Why baking soda does not raise calcium

Baking soda is sodium bicarbonate, and it raises total alkalinity, not calcium hardness. Adding it to fix a low calcium reading will push alkalinity out of range and can cloud the water. Only calcium chloride raises calcium hardness.

Baking soda also nudges pH up slightly. If you keep adding it in the hope of raising calcium, both pH and alkalinity can end up out of range while calcium stays exactly where it was.

Soda ash (sodium carbonate) does not raise calcium hardness either. It raises pH, and it also pushes up alkalinity. Neither soda ash nor baking soda can take the place of calcium chloride.

How do you control scale in a hard water area?

In a hard water area you are rarely trying to remove calcium. You are trying to stop it coming out of the water as scale. A sequestrant, steady pH and alkalinity, and sensible water changes do most of the work.

Use a scale control sequestrant

A sequestrant (often sold as scale and stain control) binds calcium and metals so they stay dissolved instead of forming scale. It does not lower your calcium reading, but it makes high calcium far less troublesome. Add it after every refill and then at the regular dose on the label, since sanitiser slowly breaks it down.

When comparing products, check whether the label covers calcium scale, metal staining or both, and whether it suits your sanitiser. A bottle usually costs somewhere in the region of £10 to £25, and the dose per week matters more than the bottle price.

A sequestrant works well as a holding measure when calcium is moderately high, at about 250-350 ppm. At around 400 ppm or more, a sequestrant alone is not enough, and you need to dilute the water by draining.

Some scale control products can also gradually loosen light existing scale over several weeks of regular dosing, as well as preventing new scale. Check the label to see whether yours makes that claim. Heavy scale still needs cleaning or a descaler.

Keep pH and alkalinity in check

Scale forms much more easily when pH and alkalinity are high. In hard water, aim for the lower half of the ranges: pH around 7.2-7.4 and total alkalinity around 80-100 ppm. If alkalinity keeps climbing, our article on what causes high alkalinity in a hot tub explains why and how to bring it down.

Partial drain and refill

Draining part of the water and topping up only lowers calcium if your fill water is softer than the tub water. In a hard water area this is often the case, because evaporation leaves calcium behind and the level climbs well above the tap figure. Replacing a quarter to a third of the water can bring it back towards your tap level.

To work out how much water to replace, use this formula: fraction to drain = (current calcium - target calcium) / (current calcium - fill water calcium). For example, at 400 ppm with a 200 ppm target and 100 ppm tap water, the sum is 200 / 300, so replace about two thirds of the water.

When your mains water is itself very hard, a partial drain cannot take the tub below the tap figure, so it will not reach the target on its own. Mixing some softened water into the refill can bring calcium down further, as long as the final reading stays within 150-250 ppm. Test after refilling and add calcium increaser if the reading falls too low.

1. Test the calcium in the tub and in your fill water, then work out how much to drain.
2. Turn off the heater and switch off the tub at the isolator before draining.
3. Drain the calculated amount through the drain valve or with a submersible pump.
4. Refill to the normal level, ideally through a hose pre-filter.
5. Restore power and run the jets for about 15 minutes to mix the water.
6. Retest calcium, and repeat the partial drain if the reading is still above 250 ppm.
7. Rebalance pH, alkalinity and sanitiser, and add a dose of sequestrant.

A partial drain also lowers total dissolved solids along with calcium. That freshens the water as a whole, not just the calcium reading.

If calcium, total dissolved solids and general water quality are all poor, a full drain is usually the better choice. Our guides on hot tub TDS and when to change water and how often to change hot tub water help you decide.

When a full drain makes more sense

If calcium hardness is above around 500 ppm, a full drain and refill is usually quicker and cheaper than repeated partial drains or extra scale control product.

A full drain is also the better choice when total dissolved solids reach about 2,500 ppm. At that level chemical balance becomes unreliable, and treatments stop working as expected.

Many spa professionals recommend a full drain and refill about every three to four months with regular use, whatever the calcium reading, to reset the water. Advice varies with tub size and how much the tub is used, so check your manual.

Use a full drain as a chance to clean the filter, descale the shell and the heater area with a suitable descaler or plumbing flush product, and inspect O-rings and seals for early signs of damage. Follow the product instructions and rinse well before you refill.

Calcium remover pillows

Ion-exchange calcium remover pillows sit in the filter compartment or the skimmer and capture calcium from the water that flows through them. They can slowly lower a mildly high reading, of about 250-350 ppm, over one to two weeks.

Pillows stop working once they are saturated, and then they need replacing. In a hard water area, where every top up brings more calcium back, a partial drain is often the cheaper way to bring the level down.

Pre-filters at fill

A hose pre-filter screws onto your hose and filters water as you fill. Most contain carbon and a sediment layer, which remove chlorine taste, fine particles and some metals. Some include media designed to reduce scale formation. Very few remove a meaningful amount of calcium, so read the claims carefully.

A pre-filter is still worth using in both hard and soft water areas, because it gives you cleaner fill water and fewer metal stains. Replace the cartridge as the maker advises, usually after a set number of fills. Expect to pay roughly £15 to £40 for a filter.

Is water from a home softener a good idea?

A domestic water softener swaps calcium for sodium, so fully softened water can have almost no calcium. Filling only with softened water can leave you with the same foaming and corrosion risks as a soft water area. Many manufacturers advise against it or suggest mixing softened and unsoftened water, so check your manual before using it.

Scale absorbers and other add-ons

Absorbent pads and filter inserts can catch oils and some fine particles, and some are sold for scale. They may help a little, but they will not bring a high reading down on their own. Treat them as an extra, not a replacement for a sequestrant and good water balance.

What mistakes should you avoid?

• Adding calcium increaser or scale control without testing first, which wastes product and can push other readings out of balance.
• Adding calcium increaser before correcting pH and alkalinity.
• Adding the full calculated dose at once instead of in stages.
• Trying to make a large calcium correction in one go, for example from 80 to 200 ppm, which can cause temporary cloudiness and uneven mixing as well as overshooting the range.
• Sprinkling calcium chloride granules straight into the tub instead of dissolving them first.
• Using baking soda to raise calcium.
• Testing with out-of-date or badly stored reagents or strips, which gives wrong readings and wrong doses.
• Assuming your water company's total hardness figure equals calcium hardness in the tub.
• Forgetting to add sequestrant after a refill in a hard water area.
• Filling only with fully softened water without checking your manual.
• Testing only at fill time, then not again until scale appears.

Calcium hardness at a glance

Keep calcium hardness between 150 and 250 ppm, unless your manual says otherwise. Find out your tap water hardness from your water company, then confirm it with a test each time you refill.

In soft water areas, balance pH and alkalinity first, then raise calcium in stages with calcium chloride. In hard water areas, use a sequestrant, keep pH and alkalinity in the lower half of their ranges, and dilute with a partial drain when calcium creeps up. Use a pre-filter at every fill.

Frequently asked questions

Can I use my hot tub if calcium is out of range?

Yes, as long as your sanitiser and pH are in range, because calcium hardness is about protecting the tub rather than bathers. Free chlorine or bromine should be 3-5 ppm and pH 7.2-7.6. Correct calcium soon, though, because scale or corrosion builds up quietly. If you notice skin irritation after bathing, stop using the tub and speak to a pharmacist or GP.

Does calcium hardness go down on its own?

No. Calcium does not evaporate or get used up, so the level only stays the same or rises. Each top up adds more calcium from the tap while evaporation removes pure water. That is why hard water tubs often read higher than the tap figure after a few weeks. The only ways to lower it are dilution with softer water or a full drain.

Can I put a tennis ball in the hot tub to remove calcium?

A tennis ball floating in the skimmer or the water can soak up a small amount of surface oil, which some owners use to reduce scum. It does nothing to calcium hardness and will not prevent scale. If you try it, use a new, clean ball and remove it if the felt starts to shed, as fibres can clog the filter.

Why is my calcium reading different from my water company's figure?

Water companies usually report total hardness, which includes magnesium, so your calcium test may read lower. Hardness also varies with season and the source your supply comes from that day. In the tub, evaporation concentrates calcium over time. Use the published figure as a guide and rely on your own test for dosing decisions.

Is high calcium harmful to my skin?

Hard water is not considered harmful to health, and the NHS does not advise against drinking it. In a hot tub, high calcium affects the equipment more than bathers. Some people with eczema find hard water drying, so rinse off after bathing and moisturise. If your skin becomes sore or irritated, stop using the tub and speak to a pharmacist or GP.

How long should I wait to use the tub after adding calcium increaser?

Let the water circulate for at least two hours with the cover open, then retest calcium, pH and alkalinity. Once those readings and your sanitiser level are in range, the tub is ready to use. If your product label gives a longer waiting time, follow the label. Never get in while undissolved granules are still visible.