How to raise alkalinity in a hot tub

To raise alkalinity in a hot tub, test the water, then add alkalinity increaser (sodium bicarbonate) at about 17 g per 1,000 litres for every 10 ppm you need. Circulate with the jets running for at least an hour, then retest. That, in short, is how to raise alkalinity in hot tub water without overshooting.

Below you will find a dosing table for common tub sizes, what happens to pH when alkalinity goes up, and why owners in soft water areas see low readings after almost every refill.

What does total alkalinity actually do in a hot tub?

Total alkalinity is a measure of the alkaline substances dissolved in your water, mostly bicarbonate and carbonate. It is measured in parts per million (ppm). These substances act as a buffer: they absorb acids and bases so that your pH does not swing every time something is added to the water.

When total alkalinity is too low, the buffer is weak. Sanitiser, sweat, cosmetics and even a small dose of pH adjuster can push pH up or down quickly. Technicians often call this pH bounce, and it is one of the most common reasons owners say their water is hard to keep balanced.

Low alkalinity also makes your sanitiser less effective. Chlorine and bromine work properly only within a fairly narrow pH range, and a weak buffer lets pH drift out of it. The result is water that can be less hygienic even when your chlorine or bromine reading looks fine.

When total alkalinity is too high, the opposite happens. The buffer is so strong that pH tends to drift upwards and resists every attempt to bring it down. The aim is a middle ground where pH is steady but still easy to adjust.

Alkalinity is one part of a wider routine. If you want the full picture of how it fits with pH and sanitiser, see our guide to hot tub water balance and the order to fix things in.

What reading should you aim for?

The widely accepted range for hot tubs is 80-120 ppm total alkalinity. Some manufacturers and test kit makers quote a slightly different target, so check your owner's manual and follow it if it differs.

Some manufacturers recommend a tighter target of 100-120 ppm for newer acrylic shells. Check whether your shell maker specifies this in the manual or warranty terms. If it does, aim for the upper part of the general range and use that as your target when you calculate a dose.

• Total alkalinity: 80-120 ppm
• pH: 7.2-7.6
• Calcium hardness: 150-250 ppm
• Free chlorine: 3-5 ppm, or bromine: 3-5 ppm

Alkalinity and pH work as a pair, so it helps to understand both. Our article on hot tub pH levels and the range to aim for explains what happens when pH sits outside 7.2-7.6.

What to do at each alkalinity reading

• Below 80 ppm: add alkalinity increaser, following the steps and dosing table below
• 80-120 ppm: no action needed, keep testing as normal
• Roughly 121-180 ppm: lower it gradually with small doses of pH decreaser, combined with aeration to hold pH up
• Above about 180 ppm: a partial drain and refill is usually the most practical fix

Why does hot tub alkalinity keep running low?

Low alkalinity is rarely random. It usually comes from the water you fill with, or from chemicals that slowly use up the buffer over time. Finding the cause stops you from dosing the same problem every week.

• Filling or topping up from a soft water supply that contains very little bicarbonate
• Regular use of pH decreaser (an acid), which lowers alkalinity as well as pH
• Non-chlorine shock (potassium monopersulfate), which is acidic and gradually lowers both readings
• Bromine tablets, which are mildly acidic and slowly pull pH and alkalinity down
• Dichlor (sodium dichloroisocyanurate) chlorine granules, which are mildly acidic and gradually use up alkalinity in the same way as bromine tablets and non-chlorine shock
• Overcorrecting a high pH reading with too much acid in one go
• Topping up with collected rainwater, which has almost no natural buffer
• Heavy or frequent bather use, because sweat, body oils and other bather waste are mildly acidic and gradually use up the buffer

How escaping carbon dioxide plays a part

Hot water held under jets and air bubbles constantly loses carbon dioxide to the air. You will sometimes read that this lowers alkalinity directly. In fact, carbon dioxide leaving the water raises pH and leaves the total alkalinity reading unchanged.

The knock-on effect is what lowers alkalinity. As pH creeps up from outgassing, you add pH decreaser to bring it back, and each acid dose takes some alkalinity with it. Over weeks, a heavily used tub with the jets and blower running often can lose alkalinity steadily this way, even if you never add an acidic sanitiser.

Why soft water areas run low

The minerals in tap water come largely from the ground it has passed through. Water that flows through chalk or limestone picks up calcium and bicarbonate, so it is hard and usually has plenty of alkalinity. Water from granite, upland moors and peat catchments picks up very little, so it is soft and low in alkalinity.

In the UK, much of Scotland, Wales, north-west England and south-west England tends to have soft water, while large parts of the south-east and east of England are hard. Other European countries vary in the same way. Your water supplier publishes hardness information for your postcode, usually on its website.

If you live in a soft water area, a freshly filled tub can test well below 80 ppm. That is normal, not a fault. Plan to raise alkalinity every time you refill, and check it more often in the first week while the water settles.

If alkalinity is low after every refill, test a sample of your tap water with your own kit. Run the cold tap for a minute, fill a clean container and test it as you would the tub. This confirms whether the fill water is the cause and tells you roughly how much increaser to plan for at each refill.

Once you know your tap water tests low, you can pre-treat the fill water instead of correcting the whole tub afterwards. Work out the dose for your tub's full volume, dissolve it in a jug of water, and add it in stages while the tub is filling, keeping well away from the filter. Once the tub is full, run the pumps and retest before adding anything more.

Water from a household water softener is a separate issue. Many hot tub manufacturers advise against filling with artificially softened water, because it can be very low in calcium and can make balancing harder. Check your manual before using it.

Owners in hard water areas often have the reverse problem, with alkalinity starting high after a refill. If that sounds like your tub, read how to lower alkalinity in a hot tub instead of adding increaser.

What are the signs of low alkalinity?

The only reliable way to know is to test. That said, low alkalinity tends to show itself through a handful of familiar symptoms.

• A total alkalinity reading below 80 ppm on a test strip or drop kit
• pH that reads low, or that jumps around from one day to the next
• pH that will not stay where you put it after a correction
• Chlorine or bromine that seems to underperform, or needs topping up more often than usual
• Cloudy or hazy water, because the pH swings that low alkalinity allows upset the water balance and reduce how well your sanitiser works
• A sharp chemical smell from the water, which can be caused by the low pH that usually goes with low alkalinity
• Stinging eyes or dry, itchy skin after a soak, which can follow low pH
• Dull or pitted metal fittings, or early wear on heater parts, over the longer term
• Roughening, dulling or etching of the acrylic shell, a longer-term sign of persistently low alkalinity and the acidic water it allows

Several of these symptoms can have other causes, such as a sanitiser level that is too high. Test before you treat, so you fix the actual problem rather than a guess.

If your eyes or skin stay sore after you leave the water, rinse with fresh water and stop using the tub until it is balanced. If the irritation continues, speak to a pharmacist or your GP.

How to raise alkalinity in hot tub water, step by step

Allow roughly 30-45 minutes of hands-on time for the whole correction, covering testing, weighing, dissolving, dosing and the final retest. On top of that, plan for the circulation period of at least an hour before you retest, and longer if you need a second dose.

What you need before you start

• A reliable test kit (liquid drop kits are usually more precise than strips)
• Alkalinity increaser, or pure sodium bicarbonate
• Kitchen scales that weigh in grams
• A clean plastic jug or bucket kept only for hot tub chemicals
• Chemical-resistant gloves and eye protection
• Your tub's water volume in litres, from the manual or the manufacturer

1. Find your tub's water volume. It is listed in the owner's manual or on the manufacturer's website. If you have to estimate, round down rather than up, because underdosing is easier to correct than overdosing.
2. Run the pumps for about 10 minutes, then take a sample from elbow depth, away from the jets. Test total alkalinity and pH and write both readings down.
3. If you use test strips, dip the strip at elbow depth for the time stated on the pack. Hold it flat without shaking off the water, then read it against the chart at exactly the time the pack specifies. Reading too early or too late gives a false result.
4. Work out how far you are from your target. Aiming for around 100 ppm gives you room on either side. Use the table in the next section to find your dose.
5. If the gap is large, plan to add about half the calculated dose first. You can always add more after retesting, but you cannot easily take it out.
6. Cap any single treatment at roughly 100 g of sodium bicarbonate in a standard-size tub. If your calculation comes to more than that, split the correction into separate sessions, with full circulation and a retest between each one.
7. Weigh the dose. Fill your bucket or jug with roughly 4 litres (about 1 US gallon) of warm water scooped from the tub, then add the granules to the water and stir until they are fully dissolved. Always add chemical to water, never water to chemical. Pre-dissolving matters because undissolved granules can sink and sit on the shell floor, where they dissolve unevenly and can mark the surface.
8. With the cover open and the jets running, pour the solution slowly across the surface, away from the filter housing. Avoid pouring directly in front of a jet or near the skimmer opening as well, because the solution mixes unevenly in those spots.
9. Let the water circulate for at least an hour. Some product labels suggest a longer wait, so follow yours if it says so.
10. When the circulation time is up, run the jets for a few minutes before you take a sample, so the reading reflects fully mixed water.
11. Retest total alkalinity first, then pH. If alkalinity is still below 80 ppm, repeat the dose in a smaller amount. Once alkalinity is in range, correct pH separately if it needs it.
12. Finally, check your sanitiser level and bring it back to 3-5 ppm before anyone uses the tub.

Heat is another reason to dissolve the increaser fully and add it gradually. Hot tub water is very warm, and a concentrated patch of undissolved increaser can cause minerals to come out of solution and form scale on the heater element and the shell. A fully dissolved dose, poured slowly with the jets running, spreads out before this can happen.

Safety first: wear gloves and eye protection, add one chemical at a time, and let the water circulate between doses. Never mix chemicals together, even in a dry state. If any product gets in your eyes, rinse with plenty of clean running water and get medical help or call NHS 111.

How much alkalinity increaser per 1,000 litres?

As a working rule, about 17 g of sodium bicarbonate per 1,000 litres raises total alkalinity by about 10 ppm. That is roughly 0.6 oz per 1,000 litres (about 264 US gallons). Branded increasers can vary slightly in strength, so where the label gives a different dose, follow the label.

These figures are approximate starting points for pure sodium bicarbonate. Your test kit, your real water level and any other chemicals in the tub will all affect the final result, which is why the retest matters.

Working out your own dose

If your tub size is not in the table, use this method. Subtract your current reading from your target, divide by 10, multiply by 17, then multiply by your volume in litres divided by 1,000. The answer is your dose in grams.

For example, a 1,200 litre tub reading 60 ppm with a target of 100 ppm has a gap of 40 ppm. That gives 4 multiplied by 17, which is 68 g per 1,000 litres, and 68 multiplied by 1.2 is about 82 g. On a gap this size, add around 40 g first, circulate and retest.

Weigh your dose rather than using spoons or scoops. The amount a spoon holds depends on how fine the granules are and how tightly they are packed, so the same spoonful can vary a lot.

Why your result may not match the table

• Your actual water volume is lower or higher than you assumed
• Test strips are old, damp or read at the wrong time
• You tested before the chemical had fully mixed
• An acidic sanitiser or shock is already lowering alkalinity again

What happens to pH when you raise alkalinity?

Sodium bicarbonate is mildly alkaline, so adding it nudges pH upwards a little. The effect is much smaller than a pH increaser, because bicarbonate mainly adds buffer rather than pushing pH up hard.

In practice, a low pH often comes up part of the way when you correct alkalinity. Sometimes that is enough on its own. Often you will still need a small pH adjustment afterwards, which is why you retest pH once alkalinity is in range.

Should you raise alkalinity or pH first?

Adjust alkalinity first. Because alkalinity controls how pH responds, any pH correction you make while alkalinity is low tends not to last. Once alkalinity sits in the 80-120 ppm range, pH becomes far easier to set and hold.

When pH goes above 7.6 after raising alkalinity

If pH has risen above 7.6 after your alkalinity dose, bring it down with a small dose of pH decreaser, added dissolved with the jets running. Keep the dose small, because the decreaser will also lower alkalinity slightly. Circulate, then retest both pH and alkalinity before doing anything else.

The usual acid in hot tub pH decreaser is sodium bisulfate. Look for it in the ingredients when you buy pH down, whether you need it to lower pH or to bring alkalinity down.

When both alkalinity and pH are low

Raise alkalinity with sodium bicarbonate, circulate for an hour and retest. If pH is still below 7.2, use a pH increaser (usually sodium carbonate) in small doses. Our guide on how to raise pH in a hot tub covers that step in detail.

Never add bicarbonate and pH increaser at the same time, even though both readings are low. If you add them together, you cannot tell which chemical caused any change in the readings, so your next dose becomes a guess. Add them separately, with circulation and a retest in between.

If pH is severely low, below about 7.0, bicarbonate alone will probably not bring it back into range. Expect to need a small dose of pH increaser once alkalinity has been corrected and retested.

Sodium carbonate raises alkalinity a little as well as pH. Add it in small steps and retest between doses, so that you do not push alkalinity back over 120 ppm.

When alkalinity is fine but pH is low

Do not add alkalinity increaser here, as you would push alkalinity too high. Use a small dose of pH increaser, or try aeration first.

Aeration means running the jets with the air controls open and the cover off. Carbon dioxide escapes from the water, which raises pH without changing total alkalinity. It is slower than a chemical, often taking several hours, but it adds nothing to the water.

A practical aeration schedule is to take the cover off, open the air controls or switch on the blower, and run the jets on high for 30-60 minutes, two or three times a day. Expect a noticeable rise in pH over roughly 12-24 hours. Test pH before each session so you stop once it reaches 7.2-7.6.

When alkalinity is high but pH is low

This combination is less common but does happen. The usual fix is to lower alkalinity with careful acid doses, then use aeration to bring pH back up without raising alkalinity again. Never add increaser in this situation.

Should you use baking soda or a branded alkalinity increaser?

Most alkalinity increasers sold for hot tubs are sodium bicarbonate, the same compound sold as bicarbonate of soda for baking. Check the ingredients on the label to confirm what you are buying.

Pure bicarbonate of soda from a supermarket works the same way, as long as it contains nothing else. Do not use baking powder, which contains acids and starch. You can read more in our guide to using baking soda to raise hot tub alkalinity.

Alkalinity increaser works with every common sanitiser system, including chlorine, bromine, biguanide, ozone and salt systems. It is also used in exactly the same way in swim spas as in hot tubs. You do not need to change the dose for your sanitiser type: only the water volume and the size of the gap change the amount.

Branded spa products have some practical advantages. They come with dosing instructions written for hot tub volumes, the granules are often sized to dissolve easily, and the tubs are usually resealable and labelled for safe storage. Prices are typically a few pounds to around £20, depending on size.

On cost, plain bicarbonate of soda is usually the cheaper option per dose. Branded increaser costs more, but some products come in pre-measured sachets or with a measuring cap, which lowers the risk of adding too much. If you choose plain bicarbonate, weighing each dose on kitchen scales gives you the same control.

When comparing products, look at these points:

• The active ingredient, which should be sodium bicarbonate for an alkalinity increaser
• Dosing instructions given per 1,000 litres, not only for large swimming pools
• A container that reseals properly, because damp granules clump
• Clear safety information and storage advice on the label

Swimming pool alkalinity increaser is usually the same chemical, but its instructions are written for tens of thousands of litres. If you use it, calculate the dose for your tub's volume yourself instead of following the pool directions.

Following pool instructions in a hot tub is a serious overdose. A dose meant to raise a 50,000 litre pool by 20 ppm is about 1.7 kg of bicarbonate. Added to a 1,000 litre tub, that would raise alkalinity by roughly 1,000 ppm, far beyond anything acid doses can sensibly correct. The only practical fix at that point is a full drain and refill.

Some pool-labelled alkalinity products also contain binders or anti-caking additives. These are harmless in a large pool but can cloud the much smaller volume of water in a hot tub. Choose a product labelled for hot tubs, or plain pure bicarbonate, rather than a pool product.

What are the common mistakes and safety points?

• Guessing the tub's volume instead of checking the manual
• Adding the full dose for a large correction in one go
• Testing straight after dosing, before the water has mixed
• Using pH increaser to fix low alkalinity, which can push pH too high and cloud the water
• Correcting pH before alkalinity, then chasing pH for days
• Following pool dosing instructions in a much smaller hot tub
• Treating the reading but ignoring the cause, such as soft fill water or regular acid use
• Using test strips that are out of date or have been stored in a damp place
• Leaning over the jug or tub while measuring and pouring, so that dust or splashes reach your face
• Letting children or pets near the tub while you add chemicals, or soon afterwards
• Adding a second chemical straight after the first, instead of leaving at least 20 minutes of circulation between them
• Pre-mixing two chemicals in the same bucket or jug, which can cause a reaction and, with some sanitisers, release harmful gas

If you need to add more than one chemical in the same session, leave at least 20 minutes of circulation with the jets running between each addition. That gap is a minimum for safe mixing only. Before you decide on a second dose of the same chemical, still allow the full hour of circulation and retest.

Never pre-mix chemicals in the same bucket or jug, even if each one is already dissolved. Dissolve each product separately, rinse the bucket thoroughly with clean water between products, or keep a separate bucket for each type.

Keep your face away from the powder and from splashes when you measure and pour. Fine dust can irritate the eyes and skin, even with plain bicarbonate. Pour slowly, from a low height, and turn your head away if the wind picks the dust up.

Keep children and pets away from the tub while you are adding chemicals and for at least 30 minutes afterwards. Put the cover back on during circulation if your product label allows it, and do not leave open containers unattended beside the tub.

Store hot tub chemicals in their original, closed containers in a cool, dry place, out of reach of children and pets. Keep acids, alkalis and sanitisers apart. The Health and Safety Executive's guidance on controlling hazardous substances is written for workplaces, but its principles of safe storage and protective equipment apply at home too.

Keep chemicals out of direct sunlight, which can warm containers and degrade some products. In particular, store alkalinity increaser away from acids such as pH decreaser, because the two react if they come into contact. A separate shelf or a sealed box for each type is a simple way to keep them apart.

Test strips degrade once the pot is opened. Old or damp strips can read alkalinity 20-30 ppm lower than it really is, which can lead you to add increaser the water does not need. Check the expiry date, keep the lid tightly closed, and replace an opened pot after a few months (often 3-4).

The same water balance ranges are used in commercial spa pools. The Pool Water Treatment Advisory Group publishes guidance for pool and spa operators in the UK, which is a useful reference if you want to understand the reasoning behind the targets.

The single most useful habit is to retest after every adjustment. Small doses with a retest in between will always get you closer to the right reading than one large dose.

Summary: raising hot tub alkalinity

Aim for total alkalinity of 80-120 ppm, or the range in your manual. Use about 17 g of sodium bicarbonate per 1,000 litres for each 10 ppm you need, add it dissolved with the jets running, circulate for at least an hour and retest.

Fix alkalinity before pH, then correct pH separately if it is still low. If you live in a soft water area or use acidic chemicals, expect alkalinity to drop and test it at least weekly and after every refill.

Frequently asked questions

How often should I test alkalinity in my hot tub?

Test total alkalinity at least once a week, and after every refill. Test more often if you live in a soft water area, use bromine or non-chlorine shock regularly, or have had several busy sessions. Alkalinity changes more slowly than sanitiser, so it does not need daily testing, but a weekly check catches a falling reading before pH starts to drift.

Can I use the hot tub straight after adding alkalinity increaser?

Wait until the chemical has circulated for at least an hour, then retest alkalinity, pH and sanitiser. If all three are in range, the tub is ready to use. If you still need to adjust pH or sanitiser, do that first and allow further circulation time. Always follow any waiting time given on the product label, as it may be longer.

Why does my alkalinity drop again a few days after I raise it?

Something in your routine is using up the buffer. Common causes are regular pH decreaser, non-chlorine shock, bromine tablets, dichlor granules and topping up with soft water or rainwater. Track your readings for a few weeks alongside what you added. If the drop always follows a particular product or a top-up, that points to the cause, and smaller, more frequent corrections may suit your tub better.

Will raising alkalinity make my hot tub water cloudy?

Sodium bicarbonate added in sensible doses should not cloud the water. Cloudiness is more likely if you overdose, which can push pH too high and cause calcium to come out of solution, particularly in hard water. If the water turns hazy after a dose, retest pH, alkalinity and calcium hardness, run the filter and check the cartridge is clean.

Does alkalinity increaser affect calcium hardness?

No. Sodium bicarbonate adds sodium and bicarbonate, not calcium, so it does not change your calcium hardness reading. Calcium hardness is a separate measurement with its own target of 150-250 ppm. In soft water areas both readings are often low after a refill, so you may need to correct each one with its own product.

What if I add too much and alkalinity goes above 120 ppm?

A reading slightly above 120 ppm is not an emergency, but bring it back down before it pushes pH up. Do not try to cancel it out by guessing. Lowering alkalinity normally means careful doses of pH decreaser followed by aeration, done over a day or two with retests between steps. A partial drain and refill also works if the reading is well above range.