How much muriatic acid to lower alkalinity in a 10,000 gallon pool?
Lowering total alkalinity from 150 to 100 ppm in a 10,000 gallon pool takes about 1 gal (4 qt) of 31.45% muriatic acid in total, or 25.7 fl oz (3.2 cups) for every 10 ppm. Add it in several steps.
How to work it out
- In short: alkalinity is the water’s buffer, so the more of it there is, the more acid it takes to move pH. The chemistry lines below work out how much.
- Alkalinity to remove: 150 − 100 = 50 ppm (as CaCO₃)
- 50 ppm × 37,854 L ÷ 50,044 mg per equivalent = 37.82 equivalents of acid
- 37.82 eq × 100 ml per equivalent (31.45% HCl) = 1 gal (4 qt)
High alkalinity is why pH keeps climbing back up a day after you lower it. Unlike pH, the alkalinity dose is simple arithmetic — every 10 ppm is a fixed amount of acid — but it has to go in gradually, because the same acid also drops pH.
Adjust for your project
- Add this first, then retest
- 16 fl oz (2 cups)
- Acid per 10 ppm of alkalinity in this pool
- 25.7 fl oz (3.2 cups)
- Acid lowers pH as well as alkalinity. Adding a full TA dose at once can drop pH below 7.0 — add it in parts, keep pH above 7.0, and aerate (fountains, jets aimed up) between doses to bring pH back up without raising alkalinity.
- That is more than one label allows at once (Leslie’s muriatic acid: 16 fl oz per 10,000 gal at one time; HASA: 1 qt per 20,000 gal per 24 hours). Split it into several additions with the pump running, hours apart — follow your own label.
- This is an estimate from carbonate chemistry. Borates, very high TDS, a pH test that is off by 0.1, or chlorine just added all change the real amount — which is why you add part, wait and retest.
10,000 gallons from 150 ppm, by target alkalinity
| Target TA | Acid to add (in total) |
|---|---|
| 140 ppm | 25.7 fl oz (3.2 cups) |
| 130 ppm | 51.4 fl oz (1.6 qt) |
| 120 ppm | 77 fl oz (2.4 qt) |
| 110 ppm | 102.7 fl oz (3.2 qt) |
| 100 ppm | 1 gal (4 qt) |
| 90 ppm | 1.2 gal (4.8 qt) |
| 80 ppm | 1.4 gal (5.6 qt) |
A gallon of acid, in steps
Fifty ppm of alkalinity in 10,000 gallons needs about 128 fl oz of 31.45% acid — almost exactly one gallon. Adding a gallon at once would crash pH well below 7.0, which is corrosive to plaster, metal and heaters, and stings swimmers. So the usual method alternates acid and air:
- Add enough acid to bring pH down to about 7.2 (this calculator’s pH mode gives the amount), with the pump running.
- Aerate: point return jets up at the surface, run waterfalls or a spa spillover, or use an air pump. Aeration drives off carbon dioxide, which raises pH without adding alkalinity back.
- When pH is back near 7.6–7.8, test TA and repeat. Each round takes TA down by perhaps 10–20 ppm.
- Stop at your target — most pools run 70–100 ppm for plaster, a bit higher for vinyl or salt.
Label limits still apply to each addition — one muriatic label sets 16 oz per 10,000 gallons at one time, so this job is several days of small doses. Dry acid works the same way; about 10 lb of 93.2% sodium bisulfate removes the same 50 ppm.
For a pH-only adjustment, see how much muriatic acid to lower pH in a 10,000 gallon pool. If alkalinity then ends up too low, how much baking soda to raise alkalinity in a 10,000 gallon pool brings it back.
Different size? Use the full Muriatic Acid Calculator (pH & Alkalinity Down).
What you’ll need
- Muriatic acid (31.45% HCl)Budget price
Lowers pH and alkalinity. Strong and fuming — use goggles and gloves, add it slowly to the deep end with the pump running, and store it away from chlorine.
See options on Amazon (opens in a new tab) - FAS-DPD drop test kit (chlorine, pH, TA, CH, CYA)Mid-range price
A drop-count kit reads free chlorine to 0.5 ppm (or 0.2 ppm), plus pH, alkalinity, calcium and stabilizer. Every dose on this site starts from a test, so this is the one thing worth buying first.
See options on Amazon (opens in a new tab) - Chemical-resistant glovesBudget price
Long-cuff nitrile gloves for handling acid, chlorine and calcium chloride.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much muriatic acid to lower alkalinity by 20 ppm in 10,000 gallons?
About 51.4 fl oz (1.6 qt) of 31.45% acid, added in two or more steps.
How much dry acid to lower alkalinity from 150 to 100 in 10,000 gallons?
About 10.74 lb (172 oz) of 93.2% dry acid in total, in several additions.
How much muriatic acid to lower alkalinity in a 20,000 gallon pool, 150 to 100?
About 2.01 gal (8 qt) of 31.45% acid in total.
Other common sizes
- How much muriatic acid to lower pH in a 10,000 gallon pool — 13.7 fl oz (1.7 cups)
- How much muriatic acid to lower pH in a hot tub — 0.4 fl oz
Sources
- INEOS — Hydrochloric acid 20° Bé product sheet (opens in a new tab) — 20° Baumé hydrochloric acid is 31.45–32.56% HCl, specific gravity about 1.16.
- HASA / Orenda — Muriatic acid product page (opens in a new tab) — 31.45% hydrogen chloride; label limit of one quart per 20,000 gallons per 24 hours to reduce pH.
- HASA — Acid 15 SDS (opens in a new tab) — A lower-strength pool acid: 14.5% hydrochloric acid, specific gravity 1.072.
- Leslie’s — Muriatic acid (label directions) (opens in a new tab) — “Never add more than 16 oz per 10,000 gallons at one time.”
- ProPools / Alliance — pH Down (sodium bisulfate) label (opens in a new tab) — Sodium bisulfate 93.2%; dose chart by pH; never more than 1.5 lb per 10,000 gallons at one time.
- Leslie’s — Dry Acid pH Down (opens in a new tab) — 2 lb lowers total alkalinity by 10 ppm in 10,000 gallons; 15 oz for pH 7.6–7.8, 20 oz for 7.8–8.0.
- USGS — Alkalinity calculator: methods (opens in a new tab) — Temperature-dependent equilibrium constants for water and carbonic acid (after Stumm & Morgan), with activity corrections.
- CDC — Model Aquatic Health Code, 5th edition (2024), annex (opens in a new tab) — CYA reduces chlorine’s effectiveness; correcting total alkalinity for CYA by subtracting CYA ÷ 3.
- CDC Healthy Swimming — Home pool and hot tub water treatment and testing (opens in a new tab) — pH 7.0–7.8; free chlorine at least 1 ppm in pools (2 ppm with cyanuric acid) and 3 ppm in hot tubs; no CYA in hot tubs; test twice a day.
- CDC — Pool chemical injuries press release (2014) (opens in a new tab) — Never mix pool chemicals, especially chlorine products with acid; add chemical to water, never water to chemical.
- CDC — Pool chemical safety: storage poster (opens in a new tab) — Separate acid and chlorine; don’t mix different types of chlorine; keep chemicals cool, dry and in original containers.