Hot tub chemical calculator

Spa doses are tiny and easy to overshoot. Get them exactly, for your tub's gallons and test results.

In short: The math is the same as a pool, at spa volumes. Raising chlorine by 3 ppm in a 400-gallon tub takes about 0.3 oz of 56% dichlor; raising alkalinity by 30 ppm takes about 2.7 oz of baking soda.

Formulas reviewed November 1, 2026 · How we calculate

Your pool

4 chemicals to add

Add them one at a time, a few hours apart, with the pump running. Never mix chemicals together.

Pool size
400 gal
Chlorine target
4 ppm
Stabilizer target
30 ppm

What to add

  1. 2.7 ozBaking soda (sodium bicarbonate) to raise alkalinity by 30 ppm
  2. 5.9 ozCalcium chloride (dihydrate, 77–80%) to raise hardness by 75 ppm
  3. 1.6 ozStabilizer (cyanuric acid) to raise CYA by 30 ppm. Dissolves slowly: use a sock in the skimmer
  4. 0.29 ozDichlor 56% (granular) to raise free chlorine by 3 ppm

What would change your result

  • Alkalinity under 60 ppm lets pH swing with every rain and dose. Add the baking soda first, then retest pH.
  • Using dichlor 56% (granular) for 3 ppm a day adds about 19 ppm of stabilizer a week. Liquid chlorine adds none.

Note: this chlorine also adds about 2.7 ppm of stabilizer. Read every product label and follow its safety directions.

Hot tub chemical chart

Doses for common spa sizes. Add half, wait 15 minutes with the jets on, test, then add the rest.

Spa sizeDichlor 56%: +1 ppm chlorineBaking soda: +10 ppm alkalinityCalcium chloride: +10 ppm hardness
300 gal0.07 oz0.67 oz0.59 oz
400 gal0.1 oz0.9 oz0.78 oz
500 gal0.12 oz1.1 oz0.98 oz

Getting it right

A wooden hot tub with a towel, surrounded by trees.
Photo: BlisswoodB&B, CC BY-SA 4.0

Small water, fast changes. A few hundred gallons shared by several bathers at 100 °F+ uses chlorine much faster than a pool. Test before every soak if you can.

Dichlor is the usual spa chlorine. It dissolves quickly and holds pH steady, but it adds stabilizer every time. Most owners drain and refill every three to four months, which also resets stabilizer.

Add after the soak. Dose chlorine at the end of the day with the jets running, lid open for a few minutes, then closed.

Halve your doses to start. In small volumes it is easy to overshoot. Add half, wait 15 minutes, test, then add the rest.

Key facts

  • A 400-gallon spa needs about 1/25th of the dose of a 10,000-gallon pool for the same change.
  • Dichlor adds about 0.9 ppm of stabilizer for every 1 ppm of chlorine, so stabilizer climbs fast in a spa.
  • Hot water speeds up scaling: keep pH near 7.4 and calcium moderate.

How it's calculated

Every dose starts from the same formula: pool gallons × ppm change × 8.34 ÷ 1,000,000 gives the pounds of pure chemical. That is then adjusted for what the product really contains.

  • Liquid chlorine: 1 gallon of 10% raises 10,000 gallons by 10 ppm.
  • Dry chlorine: pure pounds ÷ strength (0.65 for 65% cal-hypo).
  • Baking soda: × 1.68, because alkalinity is measured as calcium carbonate.
  • Calcium chloride dihydrate: × 1.47.
  • Stabilizer: × 1.

The list is in a sensible order: alkalinity and calcium first, stabilizer next, chlorine last.

Questions people ask

How much chlorine do I put in a hot tub?

For a 400-gallon tub, about 0.1 oz of 56% dichlor raises free chlorine by 1 ppm, so roughly 0.3 oz for 3 ppm. Most owners keep 3–5 ppm.

How often should I add chemicals to my hot tub?

Chlorine after every use or every day or two; check pH and alkalinity weekly; balance calcium when you refill.

How often should I drain my hot tub?

Every three to four months for most home spas, sooner with heavy use. Draining resets stabilizer and dissolved solids.

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