Pool pH Adjustment Calculator (Acid / Base)

Target pH is 7.4-7.6. Below 7.0 the pool starts dissolving metal fittings and irritating skin. Above 7.8 chlorine becomes lazy.

How lazy is worth being precise about, because the usual quoted numbers are wrong. Chlorine’s active form is hypochlorous acid, and the split between it and the inert hypochlorite ion is set by a dissociation constant of pKa 7.54. That gives 52% active at pH 7.5, 39% at 7.7, and 26% at pH 8.0. The “75%” figure you see repeated everywhere belongs to pH 7.0, not 7.5.

So the honest version is that going from 7.5 to 8.0 halves your chlorine’s effectiveness rather than knocking a third off it. The 7.4-7.6 window is also close to the pH of tears, which is the reason eyes do not sting there.

To raise pH (pH below 7.4):

• Soda ash (sodium carbonate): the standard. Also raises TA slightly.
• Borax: raises pH with less TA effect, but adds borate to the pool (some people like this, some do not).
• Aeration (no chemical): if TA is high, just running fountains for hours raises pH naturally as CO₂ outgases.

To lower pH (pH above 7.6):

• Muriatic acid (hydrochloric acid, ~31.45%): the standard. Lowers both pH and TA.
• Dry acid (sodium bisulfate): same effect as muriatic, easier to handle, but adds sulfate ions over time.
• CO₂ injection: commercial pools mostly. Lowers pH without lowering TA.

Why TA affects the dose, and why a flat multiplier gets it wrong. Total alkalinity is the pH buffer, so a pool with TA 120 genuinely does need more acid than one at TA 60. But the relationship is not a simple ratio, and neither is the relationship between acid and pH.

Pool water is a carbonate buffer. The pH is set by the ratio of bicarbonate to carbonic acid, and that ratio moves on a log scale:

pH = 6.35 + log([HCO₃⁻] ÷ [H₂CO₃])

Acid converts bicarbonate into carbonic acid, moving both sides of that ratio at once. The practical consequence is that the first 0.2 of pH costs far less acid than the next 0.2, and a calculator that multiplies a fixed dose by the size of the drop will badly overshoot on a big correction. This one solves the equilibrium instead, which is both more accurate and, since acid is the direction that damages plaster and burns skin, the direction worth being careful in.

Rough doses per 10,000 gallons at TA 100. These move with where you start, which is the whole point of the paragraph above, so treat them as the order of magnitude rather than the answer:

• Muriatic acid 31.45%: about 5 fl oz takes pH from 7.8 down to 7.6, and costs roughly 2 ppm of TA
• Soda ash: about 11 oz takes pH from 7.3 up to 7.5, and adds roughly 8 ppm of TA
• Borax: about 20 oz for the same rise, with no carbonate alkalinity added at all

Notice that the two directions are not symmetrical. Nudging pH down from 7.8 takes 5 ounces of acid; pushing it up from 7.3 takes twice that weight of soda ash. Low pH is genuinely harder to correct than high pH, which is one more reason to catch it early.

That acid figure surprises people, and it should. A quart of muriatic acid is a major correction, not a routine dose. If you have been pouring a quart into a 20,000 gallon pool to nudge pH down a notch, you have been driving your alkalinity into the floor for years without noticing, and then wondering why the pH will not hold.

The “add acid in stages” rule. When dropping pH by more than 0.4, add half the calculated dose, run the pump for an hour, retest, then add the rest if needed. Doses over-correct easily, and the buffer responds on a delay.

Safety. Muriatic acid is hydrochloric acid and it is genuinely dangerous. Wear eye protection and gloves. Always add acid to water, never water to acid. Add it slowly in front of a return jet with the pump running, never near the skimmer, and never within half an hour of adding chlorine, because acid plus hypochlorite releases chlorine gas. Do not pour it into still water and walk away.