Pool Chemical Dosing Calculator

What a pool chemical calculator works out

A pool chemical calculator converts test-kit readings into the amount of each product to add. Enter your pool volume in gallons or litres, then the current and target figure for free chlorine, pH, total alkalinity, calcium hardness, stabilizer and salt. This one answers all six from a single set of readings and prints them as one ordered sheet — alkalinity, pH, calcium, chlorine, stabilizer, salt last — rather than six separate answers to tally by hand.

Every figure is a pure function of what you typed this session. Nothing is saved — no pool profile, no chemistry log, no account — and the label on the container you actually bought is the authority on everything the sheet does not compute.

Which pool calculator do you actually need?

The all-in-one earns its keep when several readings are off at once and you want them on one page. When a single reading is off, the narrow calculator for it asks fewer questions and shows more of its working:

Borates are the one additive this sheet does not carry, because they are dosed once in bulk rather than balanced visit to visit and the pounds depend entirely on which product is in the bag: that job belongs to the pool borate dose calculator, which also sizes the acid the tetraborate route needs to hold pH.

If you are not sure which of those a job needs, the pool water chemistry hub is the map of the whole set.

Reading the sheet in litres and kilograms

The volume field takes litres — switch the unit toggle beside it — and the result line then echoes your litres next to the gallon figure the dose factors run on. The doses themselves print in US pourable units, because that is how the products come packaged. Every factor the engine uses converts cleanly, and one of them is unusually convenient: a ratio of product volume to water volume is unitless, so one litre of 12.5% liquid chlorine per 10,000 litres raises free chlorine by 12.5 ppm, exactly as one gallon per 10,000 gallons does.

Two worked examples against those factors. A 50,000-litre pool (13,209 US gallons) with free chlorine at 0.5 and a target of 3 needs 2.5 ppm, which is 0.26 gallons of 12.5% liquid — the sheet prints 1.06 qt, and the metric column above gives the same answer as 1.0 litre. A 15,000-gallon pool with total alkalinity at 70 against a target of 100 needs 30 ppm, which is 6.75 lb of baking soda, or 3.06 kg on a pool of 56,781 litres.

That salt row is the one place this sheet and a sibling disagree slightly. The 40 lb per 10,000 gallons bag rule is the packaged-product convention, and it lands about 4% under the exact mass balance that the pool salt calculator runs. When the bag count is what you are buying against, use that calculator — it is built for the question and it rounds bags deliberately.

How much stabilizer to add, and how much your chlorine already adds

Stabilizer is the reading most often moved by accident, so it is worth splitting into two questions. Granular cyanuric acid runs about 1 lb per 10,000 gallons per 10 ppm, or 0.12 kg per 10,000 litres. But dichlor and trichlor are stabilized chlorines, so every chlorine dose from them carries stabilizer in with it:

• Dichlor: about +0.9 ppm CYA per 1 ppm of free chlorine.
• Trichlor: about +0.6 ppm CYA per 1 ppm of free chlorine.

Those are not two estimates of one number. Each molecule carries exactly one cyanurate ring, and trichlor releases three chlorines where dichlor releases two, so the same ppm of free chlorine drags half again as much stabilizer in with dichlor. The ratio falls straight out of the molar masses — cyanuric acid at 129.07 g/mol against chlorine’s 70.906 g/mol as Cl₂ — giving 0.91 for dichlor and 0.61 for trichlor. Aquatics International publishes the matching trichlor figure from APSP: 6 ppm of CYA per 10 ppm of available chlorine. Hydrated or anhydrous dichlor makes no difference; the water cancels out of the ratio.

Read the other way, that arithmetic is how a pool gets over-stabilized with nobody adding stabilizer: reaching 40 ppm of CYA takes roughly 44 ppm of accumulated free chlorine from dichlor, or about 67 ppm from trichlor. Select either product above and the sheet reports the cyanuric acid that chlorine dose carries with it. Once CYA is high, the FC/CYA ratio calculator is what tells you the free chlorine that level now demands, and the drain-and-dilution calculator is the only thing that brings it back down.

The math the sheet runs

Every dose is the operator mass-balance rule: the product amount scales with your pool volume and inversely with the product’s strength, against published per-10,000-gallon factors.

amount = Δppm × (volume ÷ 10,000) × (1 ÷ factor_per_10k)

Pick the product you actually bought — liquid chlorine at 10% or 12.5%, cal-hypo at 65% or 73%, dichlor, trichlor; muriatic acid at 14.5%, 31.45% or 34.6% — and the factor changes with it. Couplings are flagged rather than silently folded into other lines: muriatic acid also lowers total alkalinity, stabilized chlorine also raises CYA, and cal-hypo nudges calcium up. Quietly auto-correcting one reading off another dose compounds error, so the sheet shows the coupling and leaves the next reading to your test kit.

pH, acid and calcium doses are buffer-dependent, so they carry an approximate tag; total alkalinity, chlorine, stabilizer and salt are straight mass balances and do not. A reading already inside its ideal band gets “in range” and no dose. A reading above its band with no chemical that lowers it gets the dilution route instead of an invented number.

What it is not

It is a quantity estimate, not a water-safety record and not a substitute for the product label — the container you bought carries the strengths, the handling terms and the warnings that govern its use, and they win over any figure on this page. Print the sheet as a poolside checklist with a tickable box per line, or export CSV, PDF, or copy-as-text, all ungated and without an account.