Pool chemistry guide

How Much Muriatic Acid to Lower Pool pH?

Muriatic acid is the standard way to bring a pool's pH down, and the dose depends on more than the size of your pool. This guide gives you the working figures, explains why alkalinity changes the answer, and sets out the order to do it in safely.

By Ellyana, creator of CalcClearly · Published 2026-09-06 · Last reviewed 2026-09-06

Short answer

About 10 fl oz of 31.45% muriatic acid per 10,000 gallons lowers pH by roughly 0.2. So a 20,000-gallon pool sitting at pH 7.8 needs around 40.0 fl oz — about 5.0 cups — to reach 7.4. Add half the dose, let it circulate for half an hour, retest, and add the rest only if the reading still calls for it. If your total alkalinity is above about 120 ppm, expect the acid to move pH less than this and read the alkalinity section below before dosing.

Why There Is No Universal Dose

Four things decide how much acid you need, and a number quoted without all four is worthless.

1. Volume. Everything scales linearly off gallons. A 30,000-gallon pool needs three times what a 10,000-gallon pool needs for the same pH change. This is the input people most often estimate rather than measure, and an error here scales straight through.

2. Current pH. How far you actually have to travel. Going from 7.6 to 7.4 is one step; from 8.2 it is four times as much acid.

3. Target pH. Usually 7.4 to 7.6. Aiming at the bottom of the CDC's 7.0 to 7.8 range leaves no room for error, and overshooting downwards is harder to correct than stopping short.

4. Total alkalinity. The one that gets left out, and the reason two identical pools respond differently to identical doses. Alkalinity is the water's buffering capacity — its resistance to pH change. High alkalinity means the same acid barely moves the pH; low alkalinity means the pH swings alarmingly on a small dose.

The table below assumes ordinary alkalinity, roughly 80 to 120 ppm. Outside that band, treat it as a starting point rather than an answer, and dose in halves.

Dosing Table

Fluid ounces of 31.45% muriatic acid to bring pH down to 7.4, at normal alkalinity. If your product is a lower strength — 14.5% and 15% are commonly sold — you need roughly twice these volumes. Check the label.

Pool volume	From pH 7.6	From pH 7.8	From pH 8.0	From pH 8.2
5,000 gal	5.0 fl oz	10.0 fl oz	15.0 fl oz	20.0 fl oz
10,000 gal	10.0 fl oz	20.0 fl oz	30.0 fl oz	40.0 fl oz
15,000 gal	15.0 fl oz	30.0 fl oz	45.0 fl oz	60.0 fl oz
20,000 gal	20.0 fl oz	40.0 fl oz	60.0 fl oz	80.0 fl oz
25,000 gal	25.0 fl oz	50.0 fl oz	75.0 fl oz	100.0 fl oz
30,000 gal	30.0 fl oz	60.0 fl oz	90.0 fl oz	120.0 fl oz

For reference: 8 fl oz is a cup, 32 fl oz is a quart, 128 fl oz is a gallon. So the largest figure in this table — 120.0 fl oz for a 30,000-gallon pool at pH 8.2 — is just under a gallon of acid. A dose that size is worth splitting across two treatments a day apart rather than adding at once.

Handle It Safely

Muriatic acid is hydrochloric acid. It burns skin and eyes, and its fumes damage the airway. The CDC recorded an estimated 13,508 US emergency department visits from pool chemical injuries in 2015–2017, more than half of them at a home rather than a public pool. Almost all of that is preventable by doing things in the right order.

Read the label on your bottle first. Concentration varies by brand, and the manufacturer's directions override this page.
Wear eye protection and chemical-resistant gloves, and work outdoors upwind of the bottle. Open it away from your face — the fumes released on opening are a common cause of injury on their own.
Never mix it with anything. The CDC's guidance is to store each pool chemical separately and keep incompatible products apart. Acid mixed with chlorine produces chlorine gas. This is the single most dangerous mistake on this page, and it is usually made by adding two products to the water in quick succession, not by mixing them in a container.
If you dilute, add acid to water — never water to acid. The CDC is explicit that adding water to a pool chemical can cause a violent, potentially explosive reaction. Fill the bucket first, then pour the acid into it, slowly.
Run the pump, and pour slowly into deeper water, away from the walls, the skimmer and any metal fittings. Undiluted acid sinks, so with the pump off it sits on the floor and etches whatever it lands on.
Wait, then test. Give it 20 to 30 minutes of circulation at minimum before retesting, and do not add more on the strength of an early reading.
Do not add chlorine at the same time. Let the acid circulate and the pH settle first. Anything else goes in on a separate visit.

Store it upright, in its original labelled container, somewhere cool, dry and ventilated, and well away from any chlorine product.

Alkalinity Changes Everything

If you take one thing from this page beyond the table, take this: acid lowers alkalinity as well as pH, and alkalinity governs how much the pH moves in the first place.

High alkalinity, above about 120 ppm. The water resists pH change, so your dose moves the reading less than the table suggests and the pH drifts back up over the following days. Chasing that with more and more acid is a treadmill. The actual fix is to bring the alkalinity down first, which is a slower process involving repeated small acid additions with aeration between them — and if it is very high, partial draining and refilling is the only practical answer.

Low alkalinity, below about 80 ppm. The opposite problem: nothing is buffering the water, so pH swings sharply on small doses and will not stay where you put it. Here you want to raise alkalinity with sodium bicarbonate before touching the pH at all.

So a pool whose pH will not stay down does not usually have a pH problem. It has an alkalinity problem wearing a pH costume, and no amount of acid dosed against the pH reading will settle it.

Worked Example

A 20,000-gallon pool tests at pH 8.0 with total alkalinity 100 ppm. The target is 7.4.

From the table, that is 60.0 fl oz of 31.45% acid — 7.5 cups, or just under two quarts.

Rather than adding all of it, add 30.0 fl oz with the pump running, wait half an hour, and retest. A pool with normal alkalinity should read somewhere around 7.7. Add the remainder and retest again the following day, because pH often drifts slightly upward once the water has fully mixed and off-gassed.

If that first half dose moved the pH to 7.9 rather than 7.7, the alkalinity is doing more buffering than assumed and the total will be higher than the table says. If it dropped to 7.5, alkalinity is low and you should stop, test alkalinity, and deal with that before going further.

Common questions

It depends on your pool volume and how far the pH has to fall, so there is no single dose. As a working figure, about 10 fl oz of 31.45% muriatic acid per 10,000 gallons lowers pH by roughly 0.2. A 20,000-gallon pool at pH 7.8 therefore needs around 40 fl oz — about five cups — to reach 7.4. Add half, retest after the water has circulated, and add the rest only if it is still needed.

Because four things change the answer and only you know them: the volume of water, the current pH, the pH you want, and your total alkalinity. Alkalinity is the one people leave out. It is the water's resistance to pH change, so the same dose moves pH a long way in low-alkalinity water and barely at all in high-alkalinity water. A dose quoted without those four inputs is a guess.

The CDC recommends pH 7.0 to 7.8 for pools. Most pool operators aim for 7.4 to 7.6, which sits comfortably inside that band and is where chlorine sanitises efficiently without the water becoming corrosive or scale-forming. Above roughly 7.8, chlorine becomes noticeably less effective, which is why high pH often shows up as a pool that will not hold chlorine.

It is a poor idea. Undiluted acid is much denser than water and sinks, so with no circulation it pools on the floor and can bleach or etch the surface beneath it. Run the pump, add the acid slowly into deeper water away from the walls and any metal fittings, and leave the pump running so it disperses.

Give it at least 20 to 30 minutes of circulation before you test, and longer is better. Testing straight after adding acid samples water that has not yet mixed, which produces a reading that is either alarming or falsely reassuring depending on where you dipped. Retesting too soon and then adding more is the usual route to overshooting.

pH drops below the recommended range and the water becomes corrosive: it attacks plaster, grout, metal fittings and heat exchangers, and it is uncomfortable to swim in. It also consumes alkalinity, which makes the pH unstable afterwards. The remedy is soda ash to bring pH back up, but the better answer is to avoid it by dosing in halves and retesting.

Yes. Muriatic acid is the trade name for a technical grade of hydrochloric acid sold for pool and masonry use. Pool strength is commonly 31.45% (sometimes labelled 20 degrees Baumé), and lower-strength versions around 14.5% to 15% are also sold, often marketed as safer to handle. Strength matters for dosing: a 15% product needs roughly twice the volume of a 31.45% one for the same effect.

Sources

CDC — Home Pool and Hot Tub Water Treatment and Testing — "CDC recommends pH 7.0–7.8 and a chlorine concentration of at least 1 ppm in pools and at least 3 ppm in hot tubs." Also recommends testing pH and chlorine at least twice a day.
CDC — Pool Chemical Safety — Never mix different pool chemicals; store each separately and keep incompatible products apart; add pool chemical to water and never water to pool chemical, because a violent and potentially explosive reaction can occur.
CDC — Pool Chemical Injuries in Public and Residential Settings, MMWR 68(19) — An estimated 13,508 US emergency department visits from pool chemical injuries during 2015–2017; at least 56.3% occurred at a residence. Mixing incompatible products and lack of protective equipment are among the leading contributing factors.
The product label on the acid you bought — Concentration varies by brand and the label carries the manufacturer's own handling and dosing directions. It takes precedence over any general guidance, including this page.

About this guide. Written and maintained by Ellyana, who created CalcClearly. No pool-industry or chemistry qualification is claimed here. What can be checked is the sourcing: every figure on this page comes from the CDC guidance and the EPA-registered product labelling listed under Sources, each dosing figure is generated by the same model the calculator runs on, and a test fails the site build if the two ever disagree. It is general guidance, not a substitute for the directions on your own product.

Last reviewed 2026-09-06. General guidance for domestic pools only. The directions on the product you actually bought take precedence over anything written here, and public or commercial pools are subject to separate local health regulations.