Ph Adjustment Calculator
The old version of this page estimated acid and base doses from the pH change alone, and that rule is wrong in a way that matters: the same pH shift needs far more chemical in strongly buffered water than in weakly buffered water. This calculator doses from what actually determines the dose, the alkalinity of the water, plus the volume, the chemical and its strength. Enter current and target alkalinity, and it returns a measured dose instead of a rule of thumb.
Quick Answer
A 10,000 gallon pool at 120 ppm alkalinity, dosed down to 100 ppm with standard 31.45 percent muriatic acid, needs about 1,513 mL of acid (about 51 fl oz). That figure is the tool output for the default inputs, and it is a theoretical dose: add half, circulate, retest, and only then add the rest if the test still calls for it. Pool guides commonly quote roughly 1 quart per 10,000 gallons for each 10 ppm drop, which would suggest about 2 quarts for this 20 ppm move; the stoichiometric figure is a little lower because published rules round generously and real water re-equilibrates with carbon dioxide, which is exactly why retesting beats trusting either number blindly.
Why the old rule failed
The retired rule multiplied the absolute pH change by the volume in litres by 0.1 and called the result a dose in millilitres. It gave the same answer for a pH move in soft water at 50 ppm alkalinity as in hard, strongly buffered water at 200 ppm, yet buffering is precisely what resists a pH move: the same pH shift needs roughly four times the acid at 200 ppm as at 50 ppm. A dose formula that never asks about alkalinity cannot see that difference, so it was wrong by multiples, not by rounding. The replacement asks about alkalinity first, because alkalinity is the water's buffer reserve, and the dose follows from it.
How the dose is calculated
Alkalinity in ppm as CaCO3 converts to equivalents at 50 mg per milliequivalent, so the equivalents to move are the alkalinity change in ppm divided by 50, times the volume in litres. One dKH equals 17.848 ppm as CaCO3, which is the conversion the tool uses when you enter dKH. Each chemical then converts equivalents to a weighed or measured dose from its own stoichiometry: muriatic acid at 31.45 percent is about 10.0 moles per litre (31.45 percent of a 1.16 g per mL solution, divided by the 36.46 g per mol molar mass of HCl); sodium bicarbonate supplies one equivalent per 84.01 g; soda ash supplies two equivalents per 105.99 g, which is 52.995 g per equivalent; dry acid supplies one equivalent per 120.06 g. The strength input scales the result, so a weaker acid or a lower-assay dry product doses proportionally more.
Worked examples, checked against the tool
Example 1, the default inputs: 10,000 gallons, 120 ppm down to 100 ppm, muriatic acid at 31.45 percent. The change is 20 ppm, the volume is 37,854 litres, the equivalents are 20 divided by 50 times 37,854, which is 15,142 meq, and at 10.0 mol per litre that is about 1,513 mL. Example 2, an aquarium: 100 litres, 4 dKH up to 6 dKH, sodium bicarbonate at 100 percent. The change is 2 dKH, which is 35.7 ppm, the equivalents are 35.7 divided by 50 times 100, which is 71 meq, and at 84.01 g per equivalent that is about 6 g. Both examples are tool outputs.
Choosing a chemical
Use an acid when the target is below the current alkalinity and a base when it is above; the tool enforces that direction and refuses the opposite pairing. Muriatic acid is the liquid workhorse for pools, dosed in millilitres and fluid ounces. Dry acid suits small systems and spot corrections, dosed in grams and ounces, and it is easier to store than liquid acid. Sodium bicarbonate raises alkalinity gently and is the usual aquarium choice, while soda ash raises alkalinity more strongly per gram, since it supplies two equivalents per molecule against bicarbonate's one, and it also pushes pH up harder, so it suits pools correcting both numbers at once rather than a delicate tank. Whichever chemical you pick, wear gloves and eye protection, add acid to water and never water to acid when diluting, keep chemicals away from children and pets, and follow the product label, which outranks any calculator output.
What this dose does not promise
Moving alkalinity is not the same as landing on a target pH. Acid lowers both alkalinity and pH, and pH then drifts back up as carbon dioxide leaves the water, which is why pool guides describe lowering alkalinity as a cycle of acid plus aeration rather than a single pour. This tool sizes the alkalinity move and stops there, honestly: it will not convert your target alkalinity into a promised pH, because that conversion needs the starting pH, the carbon dioxide level and the rest of the water chemistry. Dose in halves, retest after circulation, and follow the product label for handling, gloves and eye protection, since muriatic acid and dry acid are corrosive. For the volume itself, the pool volume calculator measures the water first, and for solution strengths the dilution molarity calculator and the concentration calculator cover the neighbouring chemistry.
Frequently asked questions
Why does pH dose depend on alkalinity?
Alkalinity is the water's buffer reserve: it absorbs added acid or base before pH moves. High-alkalinity water resists a pH change, so the same pH shift needs much more chemical at 200 ppm than at 50 ppm. Any dose formula that ignores alkalinity misses the main variable.
What is the difference between ppm and dKH?
Both report alkalinity. Ppm as CaCO3 is the pool-industry unit; dKH is the aquarium unit. One dKH equals 17.848 ppm as CaCO3, and this tool converts between them from the unit you pick.
Can I use this tool to hit a target pH?
No, and that is deliberate. The tool sizes the alkalinity move, which is computable from chemistry. The resulting pH also depends on carbon dioxide and starting pH, so any tool promising an exact pH from a dose alone is overstating what it knows. Dose the alkalinity move, then test pH.
Why add only half the dose first?
Because real water re-equilibrates, labels vary, and test kits have error. Adding half, circulating and retesting costs a few minutes and prevents the overshoot that is hard to undo, especially with acid in a small tank.
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References
- Pool Parts To Go, What does muriatic acid do for a pool? (1 quart of 31.45 percent muriatic acid per 10,000 gallons lowers total alkalinity by approximately 10 ppm, a starting estimate to cross-check the stoichiometric dose).
- Beatbot, How Much Muriatic Acid Lowers Alkalinity by 10 ppm? (same published guideline, with the retest warning).