How do you raise or lower the pH in a pool?
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Part of our pool and hot tub water guide
If a 20-in-1 test strip says the pH has drifted, what do the UK pool bodies say about bringing it back, and what do they leave out?
Short answer: PWTAG, the Pool Water Treatment Advisory Group, gives 7.2 to 7.4 for chlorinated pools, lower end better. It names sodium carbonate to raise pH and acids such as sodium bisulphate or hydrochloric acid to lower it, added gradually. In the sources we read, there is no UK official range or dose for a home pool.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
What should the pH of a pool be?
For a pool dosed with chlorine, PWTAG's technical note on pH control gives 7.2 to 7.4 as best for effective disinfection, and says the bottom of that range is better. Its reason is that each extra 0.5 in pH value doubles the amount of disinfectant needed. You can see where your own pool sits by dipping a 20-in-1 test strip in a glass of the water and reading the pH pad after 30 to 60 seconds.
The same figures are not an official range for a home pool. PWTAG's code of practice is written for public pools and lists domestic pools and paddling pools as not covered. It adds that many of its principles can be applied to them and that they are dealt with in its book Swimming Pool Water, which we have not read. The code also says the pH has to be kept at the correct value for the disinfectant to act effectively. The pH of your tap water is a separate question, covered in what pH drinking water should be.
What makes pool pH rise or fall?
Some chlorine products push pH up. PWTAG says sodium hypochlorite and calcium hypochlorite both tend to raise the pH of pool water, which is why an acid is needed to bring it back down. Its troubleshooting page for pool operators lists too little acid, an empty acid tank, a fouled or faulty controller and a blocked sample flow as reasons for a high pH. For a low pH it lists over-dosing of acid, and the same controller faults.
Alkalinity is the buffer. PWTAG lists water not buffered by alkaline content as a possible reason for an erratic pH, and alkalinity that is too high as a possible reason a pH is difficult to lower. It also says a new concrete pool can show high alkalinity from the initial leaching of alkaline salts, which it says to accept for the first few months.
Which chemicals do UK sources name for raising or lowering pH?
PWTAG names sodium carbonate to raise pH and acids to lower it, in its advice to public pool operators.
| To do this | What PWTAG names |
|---|---|
| Raise a low pH | Stop dosing acid and, if necessary, consider sodium carbonate |
| Lower a high pH | An acid. PWTAG says sodium bisulphate and hydrochloric acid are the chemicals most often used, and that carbon dioxide is an alternative |
| Raise alkalinity when pH keeps swinging | Sodium bicarbonate |
| Lower a pH that is hard to lower because alkalinity is high | More acid dosing and/or dilution with fresh water |
Each choice has a trade-off in PWTAG's note. Sodium bisulphate raises the sulphate level of the water, and the only remedy it gives for too much sulphate is dilution. It says carbon dioxide is not suitable where the source water has alkalinity over 150 mg/l or calcium hardness over 300 mg/l (both as calcium carbonate), nor in leisure pools and spas.
On speed, PWTAG says some reactions in pool water take many hours, sometimes days, to complete, and that "Gentle, gradual adjustment of chemicals is best practice." We found no method for a quick change in the sources we read.
What do PWTAG's pages say about mixing chemicals?
They treat accidental mixing as a serious hazard. PWTAG's technical note says mineral acids can be mixed with liquid hypochlorite through spillage or operator error, and that this could accidentally produce chlorine gas, which it calls a significant hazard in swimming pool installations. We give no mixing instructions of our own, so follow each product's label.
Why might my pool fill water not match the area picture?
The area figure is an average. Your tap is what fills the pool, and PWTAG's code says public pool treatment should be set up to take account of an analysis of all relevant source water parameters. A figure for the area does not tell you what comes out of your own tap.
“The area picture is only an average, so I would not assume your own tap fills the pool with the same alkalinity and hardness. Testing the fill water as well as the pool is a sensible way to see what you are actually starting with.”
PWTAG's code says mains water, especially from a river or reservoir, is likely to contain some or all of a list that includes lime and other alkalis, added to prevent corrosion in the supply network. It also names phosphates, added to stop lead and copper dissolving from pipework, which it says encourage algal growth in a pool. Its conclusion is that pH, alkalinity and calcium hardness all need careful control.
Hardness is the other piece. PWTAG's troubleshooting page lists water that is too hard as a reason for scaling on surfaces, fittings and inside pipes, and suggests a Langelier or Palin balanced water test to adjust pH, alkalinity and hardness together. To see where your own supply sits, a test strip's hardness pad gives a rough band, and how hard is my water explains the bands.
How can a test strip help you adjust pool pH?
It shows which colour step each pad matches, a band rather than an exact figure. Fill a clean glass or cup with the water, dip one strip, wait 30 to 60 seconds and match the pads to the chart, or enter the colours in the free results checker. The kit has pads for pH, total alkalinity, free chlorine, total chlorine, bromine and hardness. The pH pad has six steps, at 6.2, 6.8, 7.2, 7.6, 7.8 and 8.4. The table is our reading of the chart steps, not a claim from PWTAG.
| PWTAG figure | What the pad steps can show |
|---|---|
| pH 7.2 to 7.4 (technical note, chlorinated pools) | A 7.2 step indicates the bottom of the range, 7.6 above it and 6.8 below. The chart cannot separate 7.2 from 7.4 |
| Source water alkalinity over 150 mg/l, where carbon dioxide is unsuitable | The 120 step sits below 150 and 180 above it. The unit basis of the pad is unconfirmed, so this is a rough comparison |
| Calcium hardness over 300 mg/l, same note | The 250 step sits below and 425 above. The hardness pad's unit basis is unconfirmed, so use it only as a rough guide to fill water |
Several things are outside what it can do.
- It cannot show a small correction. There is nothing between 7.2 and 7.6, so a shift of 0.1 or 0.2 may not change the colour.
- It cannot tell you how much to add, and we found no UK official dose in the sources we read.
- The free chlorine pad has coarse steps at 1 and 3 mg/l, so it cannot track a fine target.
- It has no pad for cyanuric acid, which PWTAG mentions for outdoor pools to limit chlorine loss in sunlight.
- We do not claim the rapid bacteria test for pool water, and a strip result is not a legal sample.
For the general method, see how to test your water at home. If the water is green rather than just off-balance, why is my pool water green covers that.
The area figure is an average, and your own tap may differ, because PWTAG says mains water, especially from a river or reservoir, is likely to contain lime and other alkalis added to prevent corrosion in the supply network. The pads that matter here are pH, total alkalinity and hardness. A pH pad moving from the 7.2 step to 7.6 would indicate a rise above PWTAG's range, a higher alkalinity step would fit its link between high alkalinity and a pH that is hard to lower, and a higher hardness step would fit its scaling cause. Testing the fill water and the pool before and after each adjustment shows which way things are moving. Best pack: 20 strips, enough to test before and after 10 adjustments (our arithmetic), £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your pool pH questions
How to raise pH in a pool?
For public pools, PWTAG says to stop dosing acid and, if necessary, consider sodium carbonate. We found no UK official dose for a home pool in the three PWTAG pages we read.
How to lower pH in a pool?
PWTAG names acids, most often sodium bisulphate and hydrochloric acid, with carbon dioxide as an alternative. If the pH is hard to lower, it lists more acid dosing and/or dilution with fresh water.
How to raise pH in a pool quickly?
The sources we read describe gradual adjustment, and we found no UK official method for a quick change.
What causes high pH in a pool?
PWTAG says sodium and calcium hypochlorite both tend to raise pH. It also lists under-dosing of acid and a faulty or fouled controller.
What pH should a swimming pool be?
PWTAG gives 7.2 to 7.4 for chlorinated pools, with the bottom of the range better. We found no UK official range for a home pool in the PWTAG pages we read.
How much pH plus or pH minus should I add to a pool?
We found no UK official dose in the sources we read, whatever the product is called. PWTAG's advice is gradual adjustment, so test, add a little and test again.
Does baking soda raise pH in a pool?
PWTAG names sodium bicarbonate for raising alkalinity when pH keeps swinging, and sodium carbonate for raising a low pH. Baking soda is the kitchen name for sodium bicarbonate, so in PWTAG's terms it is the alkalinity chemical, not the one it names for a low pH.
How to lower alkalinity in a pool without affecting pH?
PWTAG links high alkalinity with a pH that is hard to lower, and lists more acid dosing and/or dilution with fresh water. We found no method in these sources that leaves pH unchanged.
How often to test pH in a swimming pool?
PWTAG's code says automatic dosing with continuous monitoring is the usual method for public pools, and that manual tests on samples are still important. We found no UK official testing frequency for a home pool in the PWTAG pages we read.
What happens if the pH is too high or too low in a pool?
PWTAG says chlorine becomes less efficient as pH rises, so more is needed. Its troubleshooting page lists both a high and a low pH among the reasons for a strong chlorinous smell with eye and upper respiratory irritation.
Sources
- Pool Water Treatment Advisory Group, Code of Practice: The management and treatment of swimming pool water (July 2021, updated July 2025; pools not covered, section 3 on mains water, source water and disinfection; read 29 October 2026).
- Pool Water Treatment Advisory Group, Typical Pool Problems (pH too high, too low, erratic and hard to lower; high alkalinity in a new pool; cloudy water; strong chlorine smell; scaling; undated page; read 29 October 2026).
- Pool Water Treatment Advisory Group, Carbon Dioxide for pH Control (technical note, URL dated August 2011, so an older note; 7.2 to 7.4, hypochlorites raise pH, acids, gradual adjustment; undated page; read 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, optional bacteria test, free 2nd Class delivery on every order) and results checker. The strip-chart table and the list of limits are WaterKit's own reading of the colour chart steps.
PWTAG material is summarised from PWTAG's own publications, which remain its copyright.