How much muriatic acid to add to a pool
How much acid it takes to lower pH depends mostly on your total alkalinity, not just the pH reading. The same 0.4 drop in pH can take nearly twice as much acid in one pool as in another. The tables below come from our pH calculator, which you can use for your exact numbers.
Acid to lower pH from 7.8 to 7.4
For 10,000 gallons with 40 ppm stabilizer, using 31.45% muriatic acid:
| Total alkalinity | Muriatic acid (31.45%) | Alkalinity afterwards |
|---|---|---|
| 60 ppm | 10.5 fl oz | 56 ppm |
| 80 ppm | 12.9 fl oz | 75 ppm |
| 100 ppm | 15.2 fl oz | 94 ppm |
| 120 ppm | 17.6 fl oz | 113 ppm |
The dose scales with pool size: a 20,000-gallon pool needs twice as much. Smaller pH changes need proportionally less, but the relationship isn't exactly linear, so use the calculator for anything outside this table.
Worked example
A 15,000-gallon pool reads pH 7.8, alkalinity 90 ppm and stabilizer 40 ppm. To bring pH to 7.4, the pH calculator gives:
- 31.45% muriatic acid: about 21.1 fl oz (623 mL)
- 14.5% muriatic acid (often sold as "low-fume" or "safer" acid): about 49.6 fl oz
- Dry acid (sodium bisulfate): about 1.77 lb
Alkalinity ends at about 85 ppm. Add about three-quarters of the dose first (roughly 16 fl oz of 31.45% acid), circulate for at least 30 minutes, retest, and add the rest if needed.
Check your acid's strength
Muriatic acid is hydrochloric acid. Pool-store and hardware-store versions are usually sold as 31.45% (sometimes labeled 20° Baumé) or as a diluted version of about 14.5%. The weaker one needs a bit more than twice the volume for the same effect. Read the label, and choose the matching strength in the calculator.
Acid to lower total alkalinity
Every 10 ppm of alkalinity in 10,000 gallons takes about 25.6 fl oz of 31.45% acid. In a 15,000-gallon pool, lowering alkalinity from 120 ppm to 80 ppm takes about 1.2 gallons in total, according to the alkalinity calculator.
Don't add it all at once: that would push pH far too low. Lower alkalinity in rounds. Add acid to bring pH down to about 7.0–7.2, aerate to let pH rise again, and repeat. Lowering pH and alkalinity explains the method.
How to add muriatic acid safely
- Run the pump.
- Measure the acid in a clean plastic container used only for acid.
- Pour it slowly into the deep end or in front of a return jet, close to the water surface to limit splashing. Some owners pre-dilute it in a bucket of pool water first.
- Let it circulate for at least 30 minutes, longer in a large pool, before retesting.
- Keep swimmers out until the acid has mixed. Follow the label for the waiting time.
Store acid in its original container, tightly closed, away from chlorine products and metal tools. Its fumes corrode metal nearby. See pool chemical safety.
Common mistakes
- Ignoring alkalinity. A "per 10,000 gallons" rule of thumb that doesn't account for alkalinity can be off by 50% or more, as the table shows.
- Testing pH after shocking. High chlorine can make pH test reagents read falsely high. Wait until chlorine is back in range.
- Chasing pH every day. If pH rises fast, alkalinity is probably too high. Lowering alkalinity into the 60–80 ppm range usually slows the rise.
Frequently asked questions
Can I swim after adding muriatic acid?
Follow the product label. A common approach is to wait until the acid has circulated, typically 30 minutes to an hour, and to retest before swimming.
Is dry acid better than muriatic acid?
Dry acid is easier to handle and store, but it costs more per dose and adds sulfate to the water, which can build up over time. Muriatic acid is cheaper and adds only chloride, which is harmless.
How much does 1 gallon of muriatic acid lower pH?
It depends on pool size and alkalinity. In the table above, 1 gallon (128 fl oz) is far more than any 10,000-gallon pool needs for a 0.4 drop. Use the pH calculator rather than a fixed amount.