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Hot tubs & spas

Hot Tub Chemical Calculator

Enter your hot tub's size and test results; get each chemical to add, in the right order, in grams and ounces.

What you'll need (3)
  • Your hot tub's capacity in gallons (from the manual)
  • Free chlorine, pH and total alkalinity results
  • A kitchen scale (small doses need accuracy)
Units
Your hot tub
Don't know it? Work it out in about a minute.
Your test results

Test with jets off and the water at its usual temperature.

Example: 1.
CDC minimum for hot tubs is 3 ppm.
Example: 7.9.
Example: 70.
Spa chlorine granules are usually 56% or 62%.

What to add, in order

Estimate

Your result will appear here

Enter your hot tub size and test results.

Order matters

The calculator gives doses in the order you should add them:

  1. Alkalinity (baking soda) if it’s below about 50 ppm. It steadies pH.
  2. pH: dry acid (“pH down,” sodium bisulfate) to lower it, soda ash (“pH up”) to raise it.
  3. Chlorine (dichlor) to bring free chlorine to at least 3 ppm, or higher if your water has stabilizer.

Each step uses the same chemistry as our pool calculators, run with hot-tub volumes.

Small water, small doses

A 400-gallon hot tub holds about 1/50th of a typical pool. A dose that’s a few ounces off in a pool is a big swing in a spa. Weighing chemicals on a kitchen scale (in grams) is much more accurate than scoops or capfuls, which vary with how the granules pack.

Why CYA matters in hot tubs too

Dichlor adds about 0.9 ppm of CYA for every 1 ppm of chlorine. In a hot tub dosed daily, that adds up within weeks. As CYA rises, you need more free chlorine for the same germ-killing power. Draining and refilling on schedule resets it. The calculator raises your chlorine target if your CYA calls for it.

Assumptions and limitations

  • Covers chlorine (dichlor) systems only; bromine systems follow their own product directions.
  • Uses a higher dissolved-solids estimate than pools when modeling pH, since hot tub water builds up solids faster.
  • Doses for a few hundred gallons are small. Weighing in grams is far more accurate than scoops.

Common questions

How much chlorine should a hot tub have?

CDC recommends at least 3 ppm free chlorine in hot tubs (versus 1 ppm in pools), with pH between 7.2 and 7.8. Hot water and several bathers in a small volume use chlorine up quickly.

Why dichlor for hot tubs?

Dichlor dissolves quickly, is close to pH-neutral, and comes in small, easy-to-measure granules. Its drawback is that it adds stabilizer (CYA) every time, which builds up. That is one reason hot tubs need regular water changes.

Does the calculator handle bromine?

Not yet. Bromine systems work differently (a bromide reserve regenerated by an oxidizer), and dosing depends on the product system. Follow the product's directions.

Sources and methodology references

  1. Home pool and hot tub water treatment and testing, US Centers for Disease Control and PreventionAt least 3 ppm free chlorine in hot tubs; pH 7.2–7.8.

Published October 2, 2026. Results are estimates. See how our calculators work.

Spotted a wrong result or an outdated source? Report a problem with this calculator.

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