Pool pH Calculator

Calculate how much muriatic acid, dry acid or soda ash moves your pool's pH to the target, taking your total alkalinity and stabilizer into account.

Units

UnitsUSMetric

Pool volume

Current pH

Target pH

Total alkalinity

CYA (stabilizer)

Acid (to lower pH)

Raising pH always uses soda ash.

Muriatic acid 31.45% to add

18.6 fl oz

Add about 18.6 fl oz of muriatic acid 31.45% to bring 15,000 gallons from pH 7.9 to 7.5.

pH change: 7.9 → 7.5
TA after: about 85 ppm
Add first (75%): 14 fl oz

Chemical safety:

Never mix pool chemicals with each other, wet or dry.
Always add chemicals to water, never water to chemicals.
Follow the manufacturer's label. It overrides any calculator, including this one.

How it's calculated

pH is not a simple dose-per-gallon problem: how far one cup of acid moves pH depends on how much carbonate buffer (total alkalinity) is in the water. The calculator models the carbonate system, the same approach used by Trouble Free Pool's PoolMath and described in standard water-chemistry texts (Stumm & Morgan, Aquatic Chemistry).

alkalinity(pH) = C_T × (α1 + 2·α2) + CYA_T × α_cy + [OH⁻] − [H⁺] acid needed = alkalinity(now) − alkalinity(target), at constant C_T soda ash = adds 2 eq of alkalinity and 1 mol of carbonate per mol

C_T is total dissolved carbonate, worked out from your TA reading and current pH.
α1, α2 are the bicarbonate and carbonate fractions at a given pH (pK1 = 6.35, pK2 = 10.33 at 25 °C).
CYA_T × α_cy is the part of your TA reading that is really cyanurate (pKa 6.88); it also buffers pH.
31.45% muriatic acid supplies 0.3 mol of acid per fl oz (density 1.16 g/mL); 93.2% dry acid supplies 0.22 mol per oz.

Temperature, borates and test-kit error shift real results by up to about 20%. That is why the result shows a 75% first dose: add it, circulate for 30 minutes, retest and finish.

Worked example

A 15,000 gallon pool reads pH 7.9, TA 90 ppm and CYA 40 ppm. You want pH 7.5 using 31.45% muriatic acid:

From TA and pH, the water holds 0.1 mmol/L of alkalinity that must be neutralized to reach 7.5.
Over 56,781 liters that is 5.5 mol of acid.
At 0.3 mol per fl oz: 5.5 ÷ 0.3 = 18.6 fl oz of muriatic acid.
Add about 14 fl oz first, retest after 30 minutes, then add the rest if needed. TA drops to about 85 ppm.

Adding acid safely

Always add acid to water, never water to acid. Pour it slowly into the deep end or in front of a return jet with the pump running, keep your face away from the fumes, and wear gloves and eye protection. Never mix acid with chlorine: together they release chlorine gas.

Acid to lower pH 7.8 → 7.5 per 10,000 gallons

With CYA at 30 ppm. Higher alkalinity means more buffer and more acid:

Total alkalinity	Muriatic 31.45%	Dry acid 93.2%
60 ppm	6.96 fl oz	9.36 oz
80 ppm	8.72 fl oz	11.7 oz
100 ppm	10.5 fl oz	14.1 oz
120 ppm	12.2 fl oz	1.03 lb

Soda ash to raise pH 7.1 → 7.4 per 10,000 gallons

Total alkalinity	Soda ash
60 ppm	15.7 oz
80 ppm	1.25 lb
100 ppm	1.52 lb
120 ppm	1.79 lb

Computed with the model above. Densities of aqueous HCl from the CRC Handbook of Chemistry and Physics (31.45% ≈ 1.16 g/mL; 14.5% ≈ 1.07 g/mL). Dry acid is sodium bisulfate, typically 93.2%.

Frequently asked questions

How much muriatic acid do I need to lower pH in my pool?

It depends on total alkalinity as much as volume. In 10,000 gallons with TA 80 and CYA 30, taking pH from 7.8 to 7.5 needs about 8.72 fl oz of 31.45% muriatic acid; at TA 120 it needs about 12.2 fl oz. Enter your own numbers above for an exact dose.

How do I raise pH without raising alkalinity?

Aerate. Running fountains, waterfalls, spa spillovers or pointing return jets at the surface drives out dissolved carbon dioxide, which raises pH while leaving total alkalinity unchanged. Soda ash raises both; borax raises pH with a smaller effect on TA.

Why does my pool pH keep going up?

The most common causes are high total alkalinity (carbon dioxide outgassing pushes pH up), a salt chlorine generator, fresh plaster, and lots of aeration. Lowering TA toward 60–80 ppm usually slows the rise.

What is the difference between soda ash and baking soda?

Both are sodium carbonates, but soda ash (sodium carbonate) is much more alkaline. Use soda ash to raise pH; use baking soda (sodium bicarbonate) to raise total alkalinity with little effect on pH.

What is the ideal pH for a pool?

Keep pH between 7.2 and 7.8, ideally 7.4–7.6. That range matches the CDC Model Aquatic Health Code. Below 7.2 water is corrosive and stings eyes; above 7.8 chlorine is slower and scale forms more easily.

Last updated: October 29, 2026 · Written and maintained by Raul Vaquerizo