Pool pH Calculator
Enter your volume, current and target pH, total alkalinity, and cyanuric acid to get how much muriatic acid to lower pH, or soda ash to raise it. Unlike a fixed per-0.1 rule, this uses the carbonate buffer model, so it accounts for how your alkalinity and stabilizer actually resist the change.
Alkalinity and CYA are the buffers that decide how much chemical pH takes, so both matter. If you do not know your CYA, leave it at 0, but the result is more accurate with it.
Muriatic acid 31.45% (full strength)
23 fl oz (about 2.9 cups)
Muriatic acid 14.5%: 55 fl oz (about 1.7 quarts)
Dry acid (sodium bisulfate): 1.96 lb (31 oz)
This also lowers total alkalinity by about 6.1 ppm.
Acid is corrosive. Add it slowly with the pump running, no more than about a third of this at a time, then retest before adding more. pH often creeps back up over a day or two as dissolved carbon dioxide balances out, which is normal, so aim a touch low and recheck.
This is an estimate from the carbonate buffer model. It is far more accurate than a flat per-0.1-pH rule because it uses your actual alkalinity and stabilizer, but temperature and salinity shift it slightly. Always add in stages and retest.
pH and alkalinity move together
You cannot think about pH on its own. Total alkalinity is the buffer that decides how hard pH is to move, and acid lowers both at once. If your pH will not hold, the real fix is usually the alkalinity, not more acid. Set alkalinity first with the alkalinity calculator, and start from an accurate pool volume. The full calculator list covers chlorine, salt, stabilizer, and hardness.
Frequently asked questions
How much muriatic acid do I need to lower pH?
It depends on your pool volume, the pH change, and your total alkalinity and cyanuric acid, which buffer the water. There is no fixed amount per 0.1 pH, because alkalinity resists the change. As a rough idea, lowering pH from 8.0 to 7.5 in 10,000 gallons at 100 ppm alkalinity takes about 12 fluid ounces of 31.45 percent muriatic acid. Enter your numbers for an exact estimate.
Why can't I just use a fixed amount per 0.1 pH?
Because pH is logarithmic and buffered. Total alkalinity, and cyanuric acid in a stabilized pool, absorb acid and resist pH movement, so a pool with high alkalinity needs much more acid than one with low alkalinity to make the same pH change. A flat rule ignores this and is often wrong by a wide margin.
Does lowering pH also lower alkalinity?
Yes. Muriatic acid lowers both pH and total alkalinity together, and this calculator shows the alkalinity drop alongside the acid amount. That is useful when you are bringing both down, and it is why people aerate afterward to nudge pH back up while alkalinity stays lower.
How do I raise pH without raising alkalinity?
Aerate the water. Running spa jets, fountains, or a return pointed up off-gasses carbon dioxide and raises pH on its own, with no chemical and no change to alkalinity. Soda ash also raises pH but adds a fair amount of alkalinity, so aeration is better when your alkalinity is already in range.
What should pool pH be?
Aim for 7.4 to 7.6, which is comfortable for swimmers and keeps chlorine effective and water balanced. Anywhere from 7.2 to 7.8 is acceptable. Below 7.2 the water turns corrosive and harsh; above 7.8 chlorine weakens and scale forms.
Why does my pH keep rising?
Outgassing of carbon dioxide naturally pushes pH up, and it is faster with aeration, high alkalinity, and salt systems. This is normal. The fix is usually to lower total alkalinity a little so pH drifts up more slowly, rather than chasing it with acid every few days.
Estimate from the aquatic carbonate buffer model (carbonic acid pK1 6.35, pK2 10.33, cyanuric acid pKa 6.88), deriving total carbonate from your alkalinity, pH, and CYA. The acid demand it returns is internally consistent with the alkalinity calculator. Muriatic acid strengths and sodium bisulfate equivalents from product data. Temperature and salinity shift results slightly; add in stages and retest.