Hot Tub Chemical Calculator
Hot tubs hold far less water than pools, so doses are small and easy to overshoot. Pick what you're fixing and get the amount in ounces and grams.
Quick answer: Raising free chlorine from 1 to 4 ppm in a 400-gallon hot tub takes about 0.29 oz of 56% dichlor. Hot tub doses are small, so add in stages and retest.
Checked against 6 published sources (see them). Updated October 2026.
- Also adds stabilizer (CYA)
- 2.7 ppm
- Stabilizer only comes down by draining, so keep an eye on it.
- Raises free chlorine by
- 3.0 ppm
How it works
Hot tub chemistry is the same as pool chemistry, just in far less water. The calculator uses the same published dosing rules as the pool calculators and scales them to your tub's gallons.
Chlorine: about 0.083 lb of pure chlorine raises 10,000 gallons by 1 ppm, divided by your product's strength. Dichlor is the usual hot tub chlorine; it also adds a little stabilizer (CYA).
pH uses the county health department rule of thumb (per 0.2 pH per 10,000 gallons: 6 oz soda ash, 12 fl oz muriatic acid or 1 lb dry acid). Baking soda raises alkalinity about 10 ppm per 1.4 lb per 10,000 gallons.
Tips
- A kitchen scale that reads grams makes small doses easy.
- Add chemicals with the jets running and the cover open, one at a time.
- The CDC recommends at least 3 ppm free chlorine in hot tubs, and recommends not using stabilizer (CYA) in hot tubs. Dichlor adds some, so draining and refilling regularly helps.
Sources
We checked the math on this page against:
- CDC: Home pool and hot tub water treatment and testing
- Indiana Department of Health: Adjusting Chemical Levels in a Swimming Pool
- Lucas County (Ohio) Health Department: Pool Chemistry Adjustments
- Yakima County (Washington): Chemical Dosing
- Wojtowicz, "Swimming Pool and Spa Water Chemical Adjustments," Journal of the Swimming Pool and Spa Industry (2004)
- AQUA Magazine: Dichlor, the sanitizing stalwart
Frequently asked questions
How much dichlor do I add to a 400-gallon hot tub?
About 0.29 oz (8 g) of 56% dichlor raises free chlorine by 3 ppm in 400 gallons.
What should hot tub chlorine be?
The CDC recommends at least 3 ppm free chlorine in hot tubs. Don't use the tub above the level on your product's label.
Why are hot tub doses so small?
A 400-gallon hot tub holds 1/25th of the water in a 10,000-gallon pool, so it needs 1/25th of the chemical. Small mistakes swing levels fast, so add in stages.