pH and acid demand calculator
pH is the reading that decides whether your chlorine works, whether the plaster etches and whether swimmers' eyes sting. It is also the one nobody can dose by eye, because the acid it takes depends entirely on the buffer behind it.
Acid demand depends on the buffer, so this asks for total alkalinity too. WHO puts the pH band at 7.2–7.8 for chlorine-disinfected water.
Add
33fl oz muriatic acid
or 2.75 lb of dry acid (sodium bisulfate). Pour half, circulate four hours, re-test, then add the rest if pH still needs it.
Total alkalinity lands near 94 ppm afterwards — check it on the next stop, because this is the reading acid quietly eats.
pH after the correction
7.5pH
Do not swim
Add acid to water, never water to acid, never near a chlorine product, and keep swimmers out until the water has circulated and you have re-tested.
Why pH sits in a narrow band
The WHO guideline is explicit — 7.2 to 7.8 for chlorine, up to 8.0 for bromine — and the reason is chemistry, not comfort. Chlorine in water exists as two species; the killing one dominates at lower pH and gives way to a far weaker one as pH rises. At 8.0 you may hold 3 ppm of free chlorine and still have a pool that behaves like it is under-dosed.
Why this asks for alkalinity
Acid demand is not a fixed number of ounces per tenth of a point. In water buffered at 60 ppm, a modest pour moves pH a long way; at 150 ppm the same pour barely registers. This calculator scales the dose to your total alkalinity and then tells you what the correction costs in alkalinity, because that is the reading that quietly walks down every time you use acid.
It is a model, not a titration. The acid figure is anchored to a manufacturer's label (about 3 fl oz of 31.45% muriatic, or a quarter pound of dry acid, per 0.1 pH per 10,000 gallons at 100 ppm alkalinity). Pour half, re-test after four hours of circulation and add the rest only if pH still needs it — the same discipline any tech uses with an acid demand test kit. A professional pool test kit such as Taylor's K-2006 carries the acid and base demand tests. Soda ash labels run higher than the 6 oz this tool uses (one brand says a pound per 0.1 pH per 10,000 gallons), so start low and re-test before adding more.
Salt pools drift up
The electrolysis that makes chlorine in a salt pool pushes pH upward continuously. Expect acid on most stops, and check total alkalinity monthly rather than seasonally. The lower-pH guide covers the method and the retest timing; going the other way, see how to raise pool pH. Carry dry acid instead? The muriatic vs dry acid comparison has the conversion, and the saturation index calculator checks that the pH you are aiming for keeps the water balanced.
Worked example
| Pool | Reading | Dose | TA after |
|---|---|---|---|
| 22,000 gal, TA 100 | pH 8.0 → 7.5 | ~33 fl oz acid (or ~2.75 lb dry acid) | ~94 ppm |
| 22,000 gal, TA 150 | pH 8.0 → 7.5 | ~49 fl oz acid (or ~4.1 lb dry acid) | ~141 ppm |
| 22,000 gal, TA 90 | pH 7.0 → 7.4 | ~3.3 lb soda ash | ~94 ppm |