pH and Alkalinity Calculator
pH and alkalinity are joined at the hip: almost nothing you add moves one without moving the other. This calculator gives you a starting dose for each, and tells you what it will do to the other number.
High alkalinity resists pH change, so the dose is scaled to it.
Acid into water, never water into acid. Dilute in a bucket, pour slowly into the deep end with the pump running, and keep acid well away from any chlorine product.
Result
Enter your pool volume
The fields start on ordinary values so you can see what each one wants. Nothing is worked out until you put your own numbers in.
pH and alkalinity are joined at the hip: almost nothing you add moves one without moving the other. This calculator gives you a starting dose for each, and tells you what it will do to the other number.
Getting an accurate answer
- Add about two-thirds of any calculated acid dose, circulate for a few hours, then retest. Overshooting pH downward is far more annoying to fix than undershooting.
- Never pour acid into the skimmer. Dilute it into a bucket of water — acid into water, never the reverse — and pour slowly into the deep end with the pump running.
- Aerating the water (fountains, a spa spillway, returns angled up) raises pH without adding anything. That is the free way to bring pH up after an acid addition drops it too far.
Why moving one number moves the other
This is the part that makes people feel like they are going round in circles, and it is worth understanding before you add anything.
Every acid lowers both pH and total alkalinity. There is no product that lowers only alkalinity — what is sold as "alkalinity reducer" is dry acid with a markup. So when you dose to bring alkalinity down from 150, pH comes down with it, often further than you wanted.
It works the other way too. Sodium bicarbonate raises alkalinity and nudges pH up slightly. Soda ash raises pH hard and alkalinity a little. Nothing in the aisle moves one axis cleanly.
That is why the calculator shows you what a dose does to both numbers rather than just the one you asked about. The relationship in full is worth ten minutes if pH is something you fight constantly.
Where these doses actually come from
Worth showing, because a dose you can check is a dose you can trust, and pool chemistry is full of numbers repeated without a source.
Acid on alkalinity. Muriatic acid at 31.45% — the 20 degrees Baumé strength sold for pools — is about 1.16 g/mL, so 26 fluid ounces contains roughly 280 g of hydrogen chloride. Each mole of it neutralizes one equivalent of alkalinity, which works out at 10.2 ppm of alkalinity removed per 26 fl oz in 10,000 gallons. That is where the figure comes from, and it is close enough to the published 10 ppm that the two agree.
Bicarbonate on alkalinity. Sodium bicarbonate is 84.0 g/mol against an alkalinity equivalent weight of 50.0, so raising 10,000 gallons by 10 ppm takes about 1.4 lb. The industry figure of 1.5 lb is the same number with a margin for the fact that nobody weighs it.
Soda ash on alkalinity. Sodium carbonate is 106.0 g/mol and carries two equivalents of alkalinity per mole, so 6 oz in 10,000 gallons — about the dose that moves pH 0.2 — brings 4.2 ppm of alkalinity with it. Worth knowing before you use it on water whose alkalinity is already high, which is exactly when aerating is the better move.
Acid on pH is the loose one, and deliberately so. How far pH moves for a given dose depends on how much buffer is in the water, which is what alkalinity is. At a normal 80 to 120 ppm, roughly 12 fl oz per 10,000 gallons moves pH about 0.2. At 200 ppm it will move less; at 50 ppm it will move considerably more.
Lowering alkalinity without wrecking pH
The technique that solves the circle above, and the reason it takes a few days rather than an afternoon.
Add acid to bring alkalinity down and let pH fall with it — down to 7.0 or a little below is fine and expected. Then aerate: angle the returns up, run a fountain or a spa spillway, switch on a waterfall. Aeration drives carbon dioxide out of the water, and losing carbon dioxide raises pH while leaving alkalinity where it is.
That is the whole trick. Acid takes both down, aeration brings only pH back up, and repeating the cycle walks alkalinity down a step at a time. Two or three rounds over several days is normal.
It is also free. The same mechanism is why a salt pool climbs in pH on its own — the cell aerates the water constantly — and why a pool with a waterfall needs more acid than one without.
When the answer looks wrong
You have high cyanuric acid. Some of what a test kit reports as total alkalinity is cyanurate rather than carbonate, and only the carbonate part does the buffering this calculator assumes. Subtract about a third of your CYA reading from the alkalinity figure before you dose. A pool testing 100 ppm alkalinity with 60 ppm CYA has a carbonate alkalinity nearer 80, and dosing against 100 will overshoot.
You tested too soon. A dose needs a full turnover to distribute. Testing two hours after adding acid measures the water near the returns, not the pool.
You are using strips for alkalinity. Strips are acceptable for a rough pH check and poor for alkalinity, which is exactly the number this calculation is most sensitive to. A drop-based kit is the fix.
The pool genuinely resists. Very high alkalinity water takes more acid than feels reasonable, and that is not the calculator being wrong. It is what a buffer does.
Handling the acid
Short, because this is the part that sends people to hospital rather than back to the pool store.
Acid into water, never water into acid. Dilute the dose into a bucket of water first, then pour that into the deep end with the pump running. Concentrated acid meeting a small amount of water releases enough heat to flash it to steam and spit back.
Never down the skimmer. Concentrated acid drawn straight through the pump, filter, heater and salt cell does real damage to all four.
Wear the gear — chemical-resistant gloves and proper eye protection, not sunglasses — and stand upwind. The full handling rules cover storage and what to do if it goes wrong.
Questions about this calculator
How much muriatic acid do I need to lower pool pH?
For a 10,000 gallon pool with alkalinity in the normal 80–120 ppm band, roughly 12 fl oz of 31.45% muriatic acid drops pH by about 0.2, which is about 94 mL per 10,000 liters. The exact amount depends heavily on your total alkalinity — high alkalinity water resists pH change and needs noticeably more acid.
How do I raise total alkalinity?
Sodium bicarbonate — the same thing as baking soda. About 1.5 lbs per 10,000 gallons, or about 180 g per 10,000 liters, raises total alkalinity by roughly 10 ppm. It nudges pH up slightly as well, which is usually welcome since low alkalinity and low pH tend to arrive together.
Can I lower alkalinity without lowering pH?
Not directly — every acid lowers both. The standard technique is to add acid to bring alkalinity down, then aerate the water to walk pH back up without touching alkalinity. Repeat that cycle until alkalinity lands in range.
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