To lower pH from 8.0 to 7.5 in a 15,000 gallon pool with total alkalinity 100 ppm and cyanuric acid 30 ppm, pour about 21.6 fl oz of 31.45% muriatic acid: 2.7 cups or 638 mL. The same job with 14.5% acid takes 50.6 fl oz (1.5 L), and with dry acid 1.8 lb. To lower alkalinity from 100 to 80 ppm instead takes 76.9 fl oz (9.6 cups, 2.3 L). Pick your strength and your goal above.
How the acid amount is calculated
Muriatic acid is hydrochloric acid in water. Every ppm of total alkalinity (TA) it removes needs a fixed number of acid molecules, so the amount follows from strength and density.
Acid = TA removed × gallons ÷ 10,000 × acid per ppm of TA per 10,000 gal
For 31.45% acid (20 Baume, density 1.157 g/mL) that is 2.56 fl oz per ppm of TA per 10,000 gallons, or 25.6 fl oz for 10 ppm. This matches the PHTA alkalinity sheet, which gives 1.6 pints of 31% per 10 ppm per 10,000 gallons. The 14.5% acid is a bit under half as strong, so it takes 2.3 times the volume.
For a pH target the tool first finds how much TA must go for the pH to fall to the target, from your TA, pH and cyanuric acid, then converts that to acid. In the example the model removes 5.6 ppm of TA: 5.6 ÷ 10 × 1.5 × 25.6 = 21.6 fl oz. Add half, run the pump, retest, and add the rest, because it is a model of a closed jar of water.
Which strength do you have
| Strength | Also sold as | Per 10 ppm TA, 10,000 gal |
|---|---|---|
| 31.45% | 20 Baume, "pool grade" | 25.6 fl oz |
| 14.5% | about 10 Baume | 60.1 fl oz |
| Sodium bisulfate 94% | dry acid | 2.1 lb |
Swim University advises pool-grade acid of around 31%, since hardware store acid is often weaker. Read the percent on the jug, not the name: a weaker acid is safer to splash but takes more volume. The comparison list under the result shows all three for your own job.
Lowering pH or lowering alkalinity
Acid does both at once, which is why the goal matters. Aim for pH and you use the amount that reaches the pH, which lowers TA a little. Aim for TA and you use more acid, and pH will drop much further; then aerate to bring pH back up without changing TA. Swim University describes aeration as pointing the returns up or running a fountain. Do TA first, then pH, as the PHTA suggests. The pool alkalinity calculator covers the TA target, and the pool pH calculator covers the pH side.
How much at once
The Indiana State Department of Health chemical adjustment guide caps one addition of liquid acid at 1 quart per 10,000 gallons and says to retest about 12 hours later. Swim University says never more than half a gallon per session. The two differ, and we split doses at the smaller quart limit, so 76.9 fl oz in 15,000 gallons is added in two parts of 48 fl oz or less.
Handling
Swim University lists splash goggles, acid-resistant gloves, a chemical-resistant apron and covering clothing, and warns that acid burns skin in seconds and the fumes irritate the lungs. Never add water to acid. If you dilute first, put the water in the bucket first: Swim University suggests about 10 parts water to 1 part acid. Do not mix acid with chlorine of any kind, which releases chlorine gas. Keep swimmers out until the acid has circulated; Swim University waits at least an hour. Whether to run the pump while pouring differs between guides, so follow your jug.
Metric and hot tubs
In a 56.8 m³ pool the pH example is 638 mL of 31.45% acid. For a spa, doses are a fraction of an ounce, so the hot tub chemical calculator works in teaspoons.
What to do next
Enter all your readings in the pool chemical calculator, check saturation with the LSI calculator, and measure your gallons with the pool volume calculator. The rest is in the pool chemistry hub.