Exactly how much acid to add to bring your pool’s pH or total alkalinity down — for any pool size, with full-strength, low-fume, or dry acid.
Aim for pH 7.4–7.6. Alkalinity buffers pH, so higher alkalinity needs more acid for the same pH drop.
Add this much muriatic acid
25 fl oz (≈ 3.1 cups)
to lower pH from 8 to 7.5 in 20,000 gal — also drops alkalinity to ≈95 ppm
carbonate model @ alkalinity 100 ppm: pH 8 → 7.5 removes the alkalinity that drops it to ≈95 ppm
that acid demand → 25 fl oz (≈ 3.1 cups) of muriatic acid
pH is buffered and logarithmic, so the dose is solved from the carbonate equilibrium (pK₁ 6.35, pK₂ 10.33), not a flat rule. Acid lowers pH and alkalinity together.
Always add acid TO the water — never water to acid. Pour it slowly over a return jet with the pump running, and keep it off skin and away from your face. Store it away from chlorine.
This also lowers total alkalinity (here, about 5 ppm). If your alkalinity is already low, expect to add baking soda afterward — and dose acid in stages, since pH overshoots easily.
If your cyanuric acid (CYA) is high, part of your measured alkalinity is actually from the stabilizer, so the real acid needed is slightly less than shown — another reason to add in stages and re-test.
Add about three-quarters of the dose first, run the pump 20–30 minutes, then re-test before adding the rest. Coming back up is more work than sneaking up on the target.
Muriatic acid (hydrochloric acid) is how you bring a high pool pH or high total alkalinity back down. There’s a catch worth understanding: acid lowers both at once. It neutralizes the carbonate buffer that makes up alkalinity, and as that buffer drops, pH comes down with it. So a dose aimed at pH will also shave some alkalinity, and a dose aimed at alkalinity will also drop pH.
That’s why pH dosing isn’t a flat rule — your alkalinity is the buffer. The higher your alkalinity, the more acid it takes to move pH the same amount. This calculator solves the carbonate chemistry for your exact levels rather than guessing, then shows the alkalinity you’ll land at so there are no surprises.
A 20,000-gallon pool at pH 8.0 and 100 ppm alkalinity, targeting pH 7.5:
The carbonate math works out to roughly 25 fl oz of full-strength (31.45%) muriatic acid, and it’ll pull alkalinity down to about 95 ppm along the way. Add about three-quarters of it over a return with the pump running, wait 20–30 minutes, and re-test — pH is easy to overshoot, and bringing it back up means adding soda ash or baking soda.
It depends on your pool size, how far the pH is above target, and your total alkalinity (which buffers the change). As a benchmark, lowering pH from 8.0 to 7.5 in a 10,000-gallon pool at 100 ppm alkalinity takes about 12 fl oz of full-strength (31.45%) muriatic acid. Enter your own numbers above for an exact dose — and add about three-quarters of it first, since pH is easy to overshoot.
Roughly 25.6 fl oz of 31.45% muriatic acid lowers total alkalinity by 10 ppm per 10,000 gallons. So dropping a 20,000-gallon pool from 120 to 90 ppm (30 ppm) needs about 1.2 gallons of acid. Switch to “Lower alkalinity” mode above and enter your levels for the exact amount.
Yes — acid lowers both at once, because it neutralizes the carbonates that make up alkalinity, and that pulls pH down too. That’s why this tool shows the resulting alkalinity when you lower pH. If only your pH is high but alkalinity is fine, add acid in small doses and let aeration nudge pH back up so alkalinity doesn’t fall too far.
You can’t separate them at the moment of dosing — acid drops both. The trick is the order of operations: add acid to bring alkalinity down (pH drops with it), then aerate the water (run returns pointed up, fountains, or a spillover) to drive pH back up while alkalinity stays low. Repeat until alkalinity is in range. The pH always recovers with aeration; alkalinity does not.
Wait until the acid is fully circulated and your pH is back in the 7.2–7.8 range — usually about 30 minutes to an hour with the pump running. Swimming in water with a freshly added slug of acid (low, unmixed pH) can irritate skin and eyes, so always re-test before anyone gets in.
Both lower pH and alkalinity. Liquid muriatic acid is cheapest and works instantly, but it fumes and must be handled carefully. Dry acid (sodium bisulfate) is granular, easier and safer to handle, and good for a single high-fume-sensitive spot — but it costs more and adds sulfates over time, which can build up in pools that don’t get diluted by rain or refills.
Low pH is easy to fix: aerate the water (it naturally rises as CO₂ off-gasses), or add a small amount of soda ash (to raise pH) or baking soda (to raise pH and alkalinity) to bring it back into the 7.4–7.6 range. This is exactly why you dose acid in stages and re-test — overshooting just means buying it back with another chemical.
For alkalinity, it’s linear: each 10 ppm drop per 10,000 gallons needs a fixed amount of acid. For pH, the tool models the carbonate buffer system directly — it works out how much acid (in chemical equivalents) it takes to move from your starting pH to your target at your alkalinity, then converts that to fluid ounces of your chosen product. Open “Show the math” under the result to see it.
How many gallons (or liters) your pool holds — any shape, sloped depths, spas included. The number you need before dosing anything.
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