Hot tub chemistry for beginners
A hot tub uses the same chemistry as a pool, squeezed into a few hundred gallons of hot, heavily used water. That changes three things: doses are tiny, sanitizer is used up fast, and the water needs replacing far more often. Your hot tub's manual is the authority for your model. This guide covers the chemistry behind it.
Target levels
| Test | Target | Source |
|---|---|---|
| Free chlorine | At least 3 ppm | CDC |
| Bromine (if you use bromine instead) | 4–8 ppm | CDC |
| pH | 7.0–7.8 | CDC |
| Total alkalinity | Commonly 60–120 ppm; check your manual | Common practice |
| Calcium hardness | As your manual specifies | Manufacturer |
| Water temperature | No higher than 104 °F (40 °C) | CDC |
The CDC also recommends not using cyanuric acid (stabilizer) or chlorine products that contain it in hot tubs. Sources: CDC home pool and hot tub testing and staying healthy in hot tubs.
Why hot tubs are harder than pools
- Heat and jets use up sanitizer. Hot water and the spray from jets drive chlorine off, and the CDC notes that jets creating droplets or mist reduce chlorine.
- Heavy use per gallon. Two people in a 400-gallon tub add as much sweat and body oil per gallon as dozens of swimmers in a pool.
- Doses are tiny. A teaspoon too much moves the readings a lot. Measure carefully.
- Dissolved solids build up fast, so the water needs replacing every few months, as your manual recommends.
Chlorine or bromine?
Both work. Chlorine is cheap, fast and easy to test. Bromine holds up a bit better in hot water and is often used in a floating dispenser. See bromine vs. chlorine for hot tubs for the full comparison.
A common chlorine choice for hot tubs is dichlor granules, because they dissolve quickly and are close to pH-neutral. Dichlor contains cyanuric acid, which the CDC recommends against in hot tubs. Each ppm of chlorine from dichlor adds about 0.9 ppm of stabilizer, so it builds up quickly in a small volume. Unscented bleach or liquid chlorine avoids that, though they raise pH a little when added.
Worked doses for a 400-gallon hot tub
All figures come from our calculators, which work for any volume:
- Raise free chlorine from 0 to 4 ppm: about 3.4 fl oz of 6% bleach, 1.6 fl oz of 12.5% liquid chlorine, or 0.3 oz of 65% cal-hypo (chlorine calculator).
- After a soak, top up to 10 ppm: about 8.5 fl oz of 6% bleach from zero.
- Lower pH from 7.9 to 7.5 with alkalinity at 80 ppm: about 0.4 oz of dry acid or 0.3 fl oz of 31.45% muriatic acid (pH calculator, with stabilizer set to 0).
- Raise alkalinity from 50 to 80 ppm: about 2.7 oz of baking soda (alkalinity calculator).
These amounts are small, so use measuring spoons or a kitchen scale. Many hot tub products give doses per 100 or 500 gallons on the label: follow that for the specific product.
A simple routine
- Before each use: test sanitizer and pH. The CDC suggests testing at least twice a day when a tub is in regular use.
- After each use: add sanitizer to bring it back up, with the jets running and the cover off for a while so the water mixes.
- Weekly: test alkalinity, rinse the filter cartridge, and shock or oxidize as your product directs.
- Every few months: drain, clean the shell and replace the water, as your manual recommends. Deep-clean or replace the filter on the maker's schedule.
Common problems
Cloudy water
Usually low sanitizer, high pH or alkalinity, a dirty filter, or old water with built-up body oils and products. Raise sanitizer, correct pH, clean the filter, and if the water is near the end of its life, drain and refill.
pH keeps rising
Jets and air blowers drive carbon dioxide out of the water, which raises pH, just as waterfalls do in a pool. Keeping alkalinity at the lower end of your manual's range slows the rise.
Strong chemical smell or foam
The CDC notes that a well-chlorinated hot tub has little odor. A strong smell usually means combined chlorine (chloramines), not too much chlorine. Foam usually comes from body products and soaps. Both point to too little sanitizer for the load, or water that needs replacing. See foamy water.
Frequently asked questions
Can I use pool chemicals in a hot tub?
The chemicals are the same compounds, but pool products are dosed for thousands of gallons and some contain stabilizer. Use products labeled for spas, or calculate the dose carefully for your volume.
Why does the CDC recommend against stabilizer in hot tubs?
Stabilizer holds chlorine in reserve, which slows how quickly it kills germs. Hot tubs don't get much benefit from it, since they're usually covered and the chlorine is lost mainly to heat and use rather than sunlight.
Who shouldn't use a hot tub?
The CDC advises not letting children under 5 use hot tubs, not drinking alcohol before or during use, and talking to a healthcare provider first if you're pregnant.