Muriatic acid calculator
Updated:
Enter your current and target pH, your total alkalinity, and pool volume to get the exact amount of muriatic acid to add, plus how much alkalinity it will also lower.
How much muriatic acid to lower pH?
The amount depends on how far pH has to fall and on your total alkalinity, which buffers the water. In 10,000 gallons, taking pH from 8.0 to 7.5 takes about 9 fl oz of 31.45% acid at TA 70, 12 fl oz at TA 100, and 19 fl oz at TA 150. The calculator models the carbonate buffer to get this right across the whole range, and it tells you how much alkalinity the dose also removes.
Acid lowers alkalinity too
Muriatic acid lowers both pH and total alkalinity. If your alkalinity is also high, that’s convenient. If alkalinity is already fine, add acid to the deep end and then aerate (fountains, jets, returns pointed up) to bring pH back up while alkalinity stays down. The MAHC Annex notes that alkalinity itself pushes pH up as carbon dioxide leaves the water, and aeration speeds that up.
Safety first
Always add acid to water, never water to acid. Pour slowly into the deep end with the pump running, wear gloves and eye protection, and never mix acid with chlorine, which produces toxic chlorine gas. Re-test before adding more.
How the dose is calculated
A single constant cannot do this, because alkalinity resists the change. The calculator works through the carbonate buffer in four steps.
1. Remove the CYA share of alkalinity
carbonate alkalinity = TA − cyanurate alkalinity
Cyanuric acid registers in a total alkalinity test. At pH 7.5 the calculator subtracts about 0.31 ppm per ppm of CYA, close to the MAHC Annex rule of thumb of dividing CYA by 3 (5.7.4.4.1).
2. Solve the carbonate system
pKa₁ = 6.35 · pKa₂ = 10.33
From your pH and carbonate alkalinity it works out the total carbonate in the water, then finds how much strong acid moves pH to your target. These are the carbonic acid constants the calculator uses.
3. Convert to product
liters = equivalents ÷ 10.0 (31.45%) · ÷ 4.25 (14.5%)
The calculator counts 10.0 equivalents of acid per liter of 31.45% muriatic acid and 4.25 per liter of 14.5%. Dry acid is sodium bisulfate at 93.2% strength and 120.06 g per equivalent.
4. Report the alkalinity drop
TA drop (ppm) = equivalents per liter × 50,044
Each equivalent of acid removes one equivalent of alkalinity, reported as calcium carbonate. That is the drop shown under the dose.
Ideal ranges
| Level | Target | Notes |
|---|---|---|
| pH | 7.0–7.8 | CDC; MAHC 5.7.3.4.1. For pools the app uses 7.2–7.8, from the Hayward and Pentair manuals |
| pH when balancing water | 7.2–7.6 | MAHC Annex 5.7.4.6 |
| Calculator’s default target | pH 7.5 | Inside both ranges |
| Total alkalinity | 60–180 ppm | MAHC 5.7.4.4.1. The app uses 80–120, from the Hayward and Pentair manuals |
Frequently asked questions
What strength muriatic acid does this assume?
You can pick 31.45% (full strength), 14.5% (low-fume), or dry acid in the calculator. The default is 31.45%.
Why does my pH keep rising?
Usually high alkalinity or aeration, which is common in salt pools and spas. The MAHC Annex describes alkalinity as a source of rising pH as carbon dioxide leaves the water. Keeping alkalinity at the low end of its range steadies pH.
Sources
- CDC, Home Pool and Hot Tub Water Treatment and Testing (May 10, 2024). pH range for home pools and hot tubs
- CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Sections 5.7.3.4.1 (pH) and 5.7.4.4.1 (alkalinity)
- CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Sections 5.7.4.4.1 (CYA correction, pH rise from carbon dioxide) and 5.7.4.6 (pH 7.2–7.6 for balance)
- Hayward, AquaRite 900 Series Electronic Chlorine Generator manual (092623 Rev D). Add acid to water, never water to acid
- Pentair, IntelliChlor IC15/IC20/IC40/IC60 Installation and User’s Guide (P/N 520589 Rev. R, 2023). pH 7.2–7.8 and total alkalinity 80–120 ppm
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