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Hot tub chemical dosage calculator

Enter your tub's volume and a test reading, get the exact amount of chemical to add — chlorine or bromine, pH up/down, alkalinity and calcium. Free.

The most common hot tub question is how much chlorine to add — and the honest answer is "it depends on your volume, your product, and how far below target you are." These calculators do that arithmetic. Enter your tub volume in gallons and your current reading, and each one returns a dose in grams/ml (or oz/fl oz). The doses are scaled from the same 10,000-gallon industry reference figures (TroubleFreePool tables) that the Soak app uses. Target ranges are the standard spa ranges: free chlorine 1–3 ppm, bromine 2–4 ppm, pH 7.4–7.6, alkalinity 80–120 ppm, calcium 150–250 ppm.

1. Sanitizer dose — how much chlorine (or bromine) to add

2. pH dose (up or down)

Enter your pH — below 7.5 the calculator raises it with soda ash, above 7.5 it lowers it with the acid you pick. Ideal spa pH is 7.4–7.6.

3. Alkalinity & calcium hardness dose

Safety note. Always add chemicals to water, never water to chemicals, and add one chemical at a time — never mix chemicals together or pre-dissolve chlorine and acid in the same bucket. Follow the dose on your product's label; concentrations vary by brand. These figures are estimates from reference tables — add with the jets running, wait, and re-test before adding more. Do not enter the tub until sanitizer and pH are back in range.

Balancing everything at once?

This page doses one chemical at a time. Soak: Hot Tub Assistant takes all your readings together and returns a single ordered plan — add this, wait, then that — with out-of-range warnings, an after-soak top-up that scales to how many people used the tub and for how long, weekly shock dosing, and a saved history so you're not re-typing your volume every time. The single-chemical math here is free; the full balancer is the app.

Get Soak on the App Store

How to use these numbers

  1. Read your water with a hot tub test kit or strips and note free chlorine (or bromine), pH, alkalinity and calcium.
  2. Fix alkalinity first, then pH, then sanitizer — alkalinity buffers pH, so doing it first stops pH bouncing.
  3. Dose with the right sanitizer granules, run the jets, wait a few hours, and re-test before adding anything else.

FAQ

How much chlorine do I add to a hot tub?
To raise free chlorine by 1 ppm in a 400-gallon tub: about 2.7 g dichlor, 2.3 g cal-hypo, or 24 ml liquid chlorine to move 1→3 ppm. Use calculator 1 for your exact volume and reading. Keep free chlorine at 1–3 ppm.

How much pH down / baking soda?
Aim pH at 7.4–7.6. Dropping 8.0→7.5 in 400 gallons is ~2.8 g dry acid or ~38 ml muriatic. Raising alkalinity 20 ppm is ~54 g baking soda per 400 gallons. Calculators 2 and 3 give your numbers.

Chlorine or bromine — which needs more?
Bromine targets a higher band (2–4 ppm) and doses differently, so a bromine start-up uses more granules than a chlorine top-up. Pick your sanitizer in calculator 1 and it switches the math and target for you.

Related: How much chlorine to add to a hot tub · Hot tub water balance cheat sheet · How to lower pH & alkalinity · New hot tub startup: fill & balance · Cloudy, foamy or green water

Sources

  • TroubleFreePool — chemical dosing reference tables (amount per 10,000 gal per unit change), the basis for every dose here
  • Industry pool/spa chemistry guides — standard spa target ranges (FC 1–3 ppm, bromine 2–4 ppm, pH 7.4–7.6, alkalinity 80–120 ppm, calcium 150–250 ppm)

Doses are scaled linearly from 10,000-gallon reference figures to your volume. They are starting points — product concentration varies by brand, so follow the label and always re-test after adding.