Why pH matters
pH controls three things you notice: comfort, chlorine strength and scale. The CDC recommends keeping pool pH between 7.0 and 7.8. Above 8.0, chlorine kills germs much more slowly. Below 7.0, water starts to corrode metal and etch plaster. Most pool owners aim for 7.4 to 7.6, which is comfortable for swimmers and keeps chlorine working well.
pH in a chlorinated outdoor pool tends to rise. Carbon dioxide gasses off, especially with waterfalls, spillovers and swimmers splashing, and that pushes pH up. Salt pools rise faster because the cell bubbles hydrogen through the water.
How the dose is worked out
Most calculators use a fixed rule like “a quart of acid lowers pH by 0.2 per 10,000 gallons.” That only holds at one alkalinity. Total alkalinity is the buffer: the more of it you have, the more acid it takes to move pH. So this calculator works from the chemistry:
- From your pH, total alkalinity, stabilizer and any borates, it works out how much carbonate is dissolved in the water.
- It finds how much acid (or soda ash) shifts that carbonate to your target pH, also counting the buffering from cyanuric acid and borates.
- It converts that amount into your product: 31.45% muriatic acid carries about 10 equivalents of acid per litre, 14.5% about 4.3, and dry acid about 7.8 per kilogram.
The equilibrium constants come from Plummer and Busenberg (1982), corrected for water temperature and dissolved solids. The same model reproduces the standard alkalinity figures, such as 25.6 fl oz of 31.45% acid lowering alkalinity by 10 ppm in 10,000 gallons.
Worked examples
Lowering pH: 10,000 gallons, pH 7.9 to 7.5, TA 90, CYA 40, 82°F
- Acid needed = 3.27 equivalents
- 31.45% muriatic acid = 3.27 ÷ 10.0 eq per litre = 11 fl oz
- Alkalinity drops slightly, to about 86 ppm
Raising pH: 10,000 gallons, pH 7.1 to 7.5, TA 80, CYA 40
- Soda ash needed = 1.45 lb
- Alkalinity rises to about 96 ppm
Acid to lower pH from 7.8 to 7.5 in 10,000 gallons
Show the numbers
| TA (ppm) | Acid (fl oz) |
|---|---|
| 60 | 6.2 |
| 80 | 7.7 |
| 100 | 9.3 |
| 120 | 10.8 |
| 140 | 12.4 |
How to add acid safely
- Wear goggles and gloves, and work upwind. Muriatic acid fumes sting and corrode metal nearby.
- With the pump running, pour slowly in front of a return jet, low over the water, so it mixes fast and doesn’t pool on the floor.
- Never mix acid and chlorine, and never add them at the same time or in the same place.
- Wait at least 30 minutes, ideally a few hours, and retest before adding more. If you’re unsure, add three-quarters of the dose first.
- Dissolve soda ash in a bucket of pool water first. Too much at once can cloud the water.
If pH keeps rising
A pool that needs acid every few days usually has alkalinity on the high side. Lowering total alkalinity toward 70 to 90 ppm slows the drift. The alkalinity calculator shows how. Waterfalls, fountains and salt cells all speed up the rise. To check whether your pH, alkalinity and calcium together will cause scale or etching, use the LSI calculator.
Frequently asked questions
How much muriatic acid do I need to lower pH?
It depends on your alkalinity. In 10,000 gallons with 90 ppm alkalinity and 40 ppm CYA, lowering pH from 7.9 to 7.5 takes about 11 fl oz of 31.45% acid. Higher alkalinity needs more.
How much soda ash raises pH by 0.2?
In 10,000 gallons at 80 ppm alkalinity, going from 7.1 to 7.5 takes about 1.45 lb, so roughly half that for 0.2. Lower alkalinity needs less.
Is dry acid better than muriatic acid?
They do the same job. Dry acid (sodium bisulfate) is easier to store and handle but adds sulfate, which builds up over time and at high levels may affect plaster. Muriatic acid is cheaper per dose.
How long after adding acid can I swim?
Once it has circulated, usually 30 minutes to an hour with the pump running. Retest pH before swimming.
Why does my pool pH keep going up?
Carbon dioxide gassing off the water raises pH. High alkalinity, aeration, waterfalls, spa spillovers and salt cells all speed it up. Lowering alkalinity is the long-term fix.
Should I lower pH or alkalinity first?
Get alkalinity into range first, then set pH. Lowering alkalinity with acid also lowers pH, so set pH last.
Sources
- Home Pool and Hot Tub Water Treatment and Testing. U.S. Centers for Disease Control and Prevention (CDC), reviewed May 10, 2024.
- Model Aquatic Health Code, 5th edition (2024). CDC.
- The solubilities of calcite, aragonite and vaterite in CO₂–H₂O solutions between 0 and 90°C (carbonate equilibrium constants). Plummer, L. N. and Busenberg, E., Geochimica et Cosmochimica Acta 46 (1982).
- Understanding LSI: The Langelier Saturation Index. Orenda Technologies.
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.