pH Adjustment Formula
The Formula
| Symbol | Description | Unit |
|---|---|---|
ΔpH | Desired pH decrease (current pH minus target pH) | pH units |
Volume | Pool volume in gallons | gallons |
0.0833 | Approximation constant for 100% acid per 10,000 gal per pH unit at TA ≈ 100 ppm | constant |
Acid Concentration % | Concentration of acid product as a decimal (e.g. 0.315 for 31.5% muriatic acid) | decimal |
Worked Example
Pool: 15,000 gallons. Current pH: 7.8. Target pH: 7.4. Using 31.5% muriatic acid. TA is 100 ppm (typical).
- ΔpH = 7.8 − 7.4 = 0.4
- Constant for 31.5% acid: 0.0833 ÷ 0.315 = 0.264
- Dose = 0.4 × 15,000 × 0.264 ÷ 10,000
Alternatively, a commonly used rule-of-thumb: 31.5% muriatic acid drops pH by 0.2 per 10,000 gal using approximately 10 fl oz. For 0.4 pH units on 15,000 gal: - Base dose for 0.4 on 10,000 gal ≈ 20 fl oz - Scale for 15,000 gal: 20 × 1.5 = 30 fl oz
Start with 25–30 fl oz (about 3–4 cups), wait 30–60 minutes, then retest.
How This Formula Works
pH adjustment dose depends on three factors: the amount of pH change required, the pool volume, and the total alkalinity — which acts as a buffer that resists pH change. Higher alkalinity means more acid is needed for the same pH change.
- To lower pH: use muriatic acid (31.5% or 20%) or dry acid (sodium bisulfate).
- To raise pH: use sodium carbonate (soda ash) — approximately 6 oz per 10,000 gallons per 0.2 pH unit increase.
- Pre-dilute muriatic acid in a bucket of pool water before adding.
- Add near a return jet to prevent concentrated acid from sitting on the pool surface.
Limitations & Notes
These are starting-dose approximations. Total alkalinity has a major effect on how much pH shifts per dose of acid — high alkalinity significantly increases the required dose. Pools with alkalinity above 150 ppm may need 2–3× the estimated dose for the same pH change. Always add incrementally and retest.
Last reviewed: 2026-06-01