Hot tub chemical calculator
Spas hold a few hundred gallons, so doses are teaspoons, not pounds, and small mistakes swing numbers fast. Enter your readings for exact small-volume doses.
Doses for 400 gallons
Chlorine (+3 ppm)
| Dichlor 56% | – |
| Liquid chlorine 10% | – |
| Household bleach 6% | – |
| Cal-hypo 73% | – |
Balance
| Lower pH 7.9 → 7.5: dry acid | – |
| Raise alkalinity 80 → 80: baking soda | – |
| Raise calcium 120 → 150: calcium chloride | – |
Teaspoon amounts are level teaspoons, estimated from typical product density. Weighing with a kitchen scale is more accurate.
Drain and refill about every 67 days (volume ÷ 3 ÷ bathers per day).
Chemical safety: read the product label first and follow it where it differs from this page. Add acid to water, never water to acid. Never mix chlorine products with acid or with each other, not even in a bucket. Add one chemical at a time with the pump running, wait at least 30 minutes (acid and chlorine: until the next test), and retest before topping up.
Hot tub vs. pool chemistry
Hot water, jets and a small volume shared by several people make spas the riskiest place for germs. CDC’s hot tub minimum is 3 ppm free chlorine (or 4–8 ppm bromine), triple the pool minimum, and it recommends against stabilizer in hot tubs. Test before every soak. Chemistry drifts fast: one adult in a 400 gallon tub is like dozens of swimmers in a backyard pool.
Questions pool owners ask
What chemical levels should a hot tub have?
CDC recommends free chlorine of at least 3 ppm (bromine 4–8 ppm) and pH 7.0–7.8 for hot tubs, and advises not using cyanuric acid (stabilizer) in hot tubs. Talk to the owner or don’t soak if chlorine is above 10 ppm. Alkalinity of about 60–100 ppm and calcium of 100–250 ppm keep pH steady and protect the heater.
How much chlorine do I add to a hot tub?
In a 400 gallon spa, one level teaspoon of dichlor granules (about 5 g) raises free chlorine by roughly 2 ppm. Liquid chlorine avoids the stabilizer buildup that dichlor causes. This calculator shows the exact amount for your volume and product.
Why does CDC say not to use stabilizer in hot tubs?
Stabilizer slows chlorine down, and hot tubs have warm water, heavy bather load and jets that make germs like Legionella and Pseudomonas harder to control. Without CYA, chlorine works at full speed. Dichlor adds CYA, so use it sparingly and drain regularly, or use non-stabilized chlorine or bromine.
How often should I drain a hot tub?
A common rule is every 3–4 months, or sooner with heavy use. One formula divides the spa volume in gallons by 3, then by the average number of bathers per day, to give days between drains.