Muriatic acid calculator: lower pH or alkalinity

Pool pH creeps up: aeration, salt cells, fresh plaster and liquid chlorine all push it higher. Acid brings it back. How much acid a given pH change takes depends mainly on total alkalinity — the water’s buffer — so the calculator needs your TA as well as your pH.

How the acid dose is worked out

For alkalinity, the math is exact: acid neutralizes alkalinity one equivalent for one equivalent. 10 ppm of alkalinity (as calcium carbonate) in 10,000 gallons is 7.56 equivalents, which is about 25.7 fl oz of 31.45% muriatic acid or 2.15 lb of 93.2% sodium bisulfate. One dry-acid label rounds that to 2 lb per 10 ppm per 10,000 gallons.

For pH, the calculator models the carbonate buffer. It takes your TA, subtracts the part a test counts from cyanurate (stabilizer), and works out how much acid shifts the carbonate balance from your pH to the target, using temperature-dependent constants for carbonic acid published by the USGS (after Stumm and Morgan) with a correction for dissolved salts. Real pools differ — borates add buffering, and test kits read pH in 0.1–0.2 steps — so treat it as an estimate.

equivalents of acid = ppm of alkalinity removed × litres ÷ 50,044
31.45% muriatic acid ≈ 100 ml per equivalent
93.2% dry acid ≈ 129 g per equivalent
10,000 gal = 37,854 L

Typical doses per 10,000 gallons

Three worked pages cover the common cases: how much muriatic acid to lower pH in a 10,000 gallon pool, how much muriatic acid to lower alkalinity in a 10,000 gallon pool and how much muriatic acid to lower pH in a hot tub.

Adding muriatic acid safely

1. Wear chemical goggles and gloves. Work upwind.
2. With the pump running, pour the acid slowly into the deepest part of the pool, close to the surface — or pre-dilute it by adding the acid to a bucket of pool water (acid to water, never water to acid).
3. Never add acid within a few hours of chlorine, never in the same spot, and never in the same bucket or pitcher.
4. Respect the label’s limit per addition. Wait several hours with the pump running, retest pH, then add more if needed.
5. Store acid sealed, low, and away from chlorine; the CDC’s storage guidance says to keep acids and chlorine apart.

pH vs alkalinity: which to lower?

If pH is high but alkalinity is normal (60–120 ppm), lower pH and stop there. If both are high, alkalinity is the cause: lower it gradually — acid down to about pH 7.2, aerate to let pH rise, repeat — until TA is where you want it. The Model Aquatic Health Code allows 60–180 ppm in public pools; most residential plaster pools run 70–100 ppm, vinyl a little higher. The order of adjustments is explained in how to balance pool water.