How to use the hot tub calculator
- Enter your gallons. Your owner’s manual or the maker’s spec sheet lists how much water your hot tub holds.
- Pick your sanitizer. Choose chlorine or bromine, the way your tub is set up.
- Test the water. Enter sanitizer, pH, total alkalinity and calcium hardness. Leave a box empty to skip it.
- Add one chemical at a time. Run the jets so it mixes, wait, then retest before the next one. Fix alkalinity before pH.
Why hot tub doses are small
A hot tub holds a few hundred gallons, so a dose meant for a pool can overshoot by a lot. The calculator uses the same published per 10,000 gallon amounts as our pool calculator and scales them down to your tub. Measure with a kitchen scale or a marked cup, not by eye.
Targets the calculator uses
- Chlorine: at least 3 ppm, per the CDC. Do not use the tub above 10 ppm.
- Bromine: 4 to 8 ppm, per the CDC.
- pH: 7.4 to 7.6 is ideal; the CDC range is 7.0 to 7.8.
- Total alkalinity: 80 to 120 ppm.
- Calcium hardness: 150 to 250 ppm. Some makers list a lower range, so check your manual.
- Water temperature: never above 104 F, per the CPSC.
Stop. Never mix chemicals with each other, and never mix chlorine with acid. Add each one to the water with the jets running, keep the cover open for a while after a shock, and stay out until sanitizer is back in range.
Common questions
How much chlorine do I put in a hot tub?
Enough to hold at least 3 ppm, which the CDC sets as the minimum for hot tubs. In a 400 gallon tub, about 1 oz of dichlor granules raises chlorine by roughly 10 ppm, so small steps matter. Enter your test result and the calculator gives the amount.
How do I start bromine in a new fill?
Add a bromide reserve first, then keep bromine up with tablets in a floater or feeder. The calculator shows how much sodium bromide gives about 30 ppm bromide for your gallons.
Why does my hot tub pH keep going up?
Running jets push carbon dioxide out of the water, and LaMotte, a test kit maker, notes that this raises pH. Keep alkalinity in range first, then lower pH in small steps and retest.
How often should I test hot tub water?
Test before each use. Warm water and a small volume use up sanitizer fast, especially with several people in the tub.