How to Lower pH in a Pool
When your test shows pH at 8.0 or higher, muriatic acid is the standard fix, and the dose is simple arithmetic: about 83 fluid ounces of 31.45% muriatic acid lowers pH by a full point in 10,000 gallons of water (at typical alkalinity). This guide walks through the math, the safe way to add acid, and what to retest afterward.
To lower pool pH, add 31.45% muriatic acid at roughly 83 fluid ounces per full pH point per 10,000 gallons. A 20,000 gallon pool at pH 8.2 needs about 133 fluid ounces (just over 1 gallon) to reach 7.4. Add acid diluted at a return jet with the pump running, retest in 2 to 4 hours, and never mix acid with chlorine.
The short answer: muriatic acid, dosed by the math
Pool pH should sit between 7.2 and 7.8. Above 7.8, chlorine loses strength, the water can turn cloudy, and scale forms on surfaces and equipment. The fix is muriatic acid (31.45% hydrochloric acid), sold at pool stores and hardware stores. The rule of thumb our calculator uses: about 83 fluid ounces of muriatic acid per 1.0 pH drop per 10,000 gallons, assuming total alkalinity near 100 ppm. Alkalinity buffers pH, so pools with very high alkalinity need a bit more acid than this, and pools with low alkalinity need a bit less.
Dry acid (sodium bisulfate) is the alternative if you prefer granules. It costs more per dose and adds sulfates to the water, but it is easier to handle. Roughly 1.5 pounds of dry acid does the work of 1 quart of muriatic acid. The rest of this guide uses muriatic acid, since that is what most pool owners keep on hand.
Worked example 1: 20,000 gallons, pH 8.2 down to 7.4
This is the classic summer reading after a week of aeration and high alkalinity. The drop is 0.8 pH points (8.2 minus 7.4).
0.8 x 83 fl oz x (20,000 / 10,000) = about 133 fluid ounces, which is just over 1 gallon (128 fl oz).
If your alkalinity is far above 120 ppm, treat this as a starting point and plan on retesting rather than pouring extra upfront. Add half to two-thirds of the dose if you are unsure of your exact volume, wait a few hours with the pump running, and retest before adding more.
Worked example 2: 10,000 gallons, pH 8.0 down to 7.5
Smaller pool, smaller move: the drop is 0.5 pH points.
0.5 x 83 fl oz = about 41.5 fluid ounces, or roughly 5 and a quarter cups.
Same rule applies: add it, circulate, retest. Chasing pH to two decimal places is pointless; anything in the 7.2 to 7.8 band is fine.
How to add muriatic acid safely
Muriatic acid is a strong acid, so the procedure matters as much as the dose. Wear gloves and eye protection, and work upwind so fumes blow away from you. Always add acid to water, never water to acid. Dilute the measured acid in a bucket of pool water, then pour it slowly in front of a return jet with the pump running. This spreads the acid before it can settle and etch the pool surface.
Never add acid and chlorine at the same time or mix them in the same bucket; the combination can release dangerous gas. Space chemical additions hours apart and retest between them. Keep the pump running for at least 30 minutes after dosing, and do not swim until the pH is back in range and you have retested.
What acid does to alkalinity (and how to handle it)
Acid does not just move pH. Every acid addition also knocks total alkalinity down: roughly 25.6 fluid ounces of muriatic acid lowers alkalinity by 10 ppm per 10,000 gallons. In example 1, the 133 fluid ounces in 20,000 gallons drops alkalinity by about 26 ppm. That is usually welcome when alkalinity started high, but if your alkalinity was already near 80 ppm, retest it and be ready to raise it back with baking soda later.
The one-two combination matters: lower pH first with acid, then if alkalinity ended up high anyway, keep it in check with the acid-and-aerate cycle described in our guide to lowering alkalinity.
Why pH keeps climbing back up
If you lower pH and it drifts back up within days, something is pushing it. Aeration is the usual culprit: waterfalls, fountains, spillover spas, and return jets pointed at the surface all drive carbon dioxide out of the water, which raises pH. High total alkalinity also pulls pH upward on its own. Point return jets slightly downward, trim fountain run time, and bring alkalinity into the 80 to 120 ppm band so your acid doses actually stick.
Skip the arithmetic
Rather than doing the acid math by hand, enter your pH now and your target into our free pool chemical calculator and get the exact muriatic acid dose for your pool's volume.
Lowering pool pH: common questions
How long after adding muriatic acid can I swim?
Wait until the acid is fully mixed and the pH reads between 7.2 and 7.8, which usually takes 30 to 60 minutes with the pump running. Retest before swimming, especially after a large dose. If you also added chlorine, wait until chlorine is at or below 5 ppm.
Can I add muriatic acid and chlorine at the same time?
No. Add one chemical at a time, hours apart, and retest between them. Never mix acid and chlorine together in a bucket or add them simultaneously, since the combination can release dangerous gas.
Will muriatic acid lower my alkalinity too?
Yes. About 25.6 fluid ounces of muriatic acid lowers total alkalinity by 10 ppm per 10,000 gallons. If your alkalinity drops below 80 ppm after an acid dose, raise it back with baking soda (24 ounces per 10 ppm rise per 10,000 gallons).
How much muriatic acid lowers pH from 8.0 to 7.5 in a 10,000 gallon pool?
About 41.5 fluid ounces, roughly 5 and a quarter cups. The math is 0.5 pH points times 83 fluid ounces per point per 10,000 gallons. Add it diluted at a return jet with the pump running, then retest in 2 to 4 hours.
Why does my pool pH keep rising after I lower it?
Aeration pushes pH up: waterfalls, fountains, spillovers, and return jets aimed at the surface all drive out carbon dioxide. High total alkalinity also pulls pH upward. Point jets downward, reduce fountain time, and bring alkalinity into the 80 to 120 ppm range so the pH stays put.