Target levels for a hot tub
| Reading | Target | Source |
|---|---|---|
| Free chlorine | 3 to 5 ppm | CDC minimum 3 ppm |
| Bromine (if used instead) | 4 to 6 ppm | Model Aquatic Health Code: 4 to 8 ppm in spas |
| pH | 7.2 to 7.6 | CDC range 7.0 to 7.8 |
| Total alkalinity | 80 to 120 ppm | MAHC allows 60 to 180 |
Hot water uses up chlorine fast and lets germs grow fast, so the CDC sets the hot tub minimum at 3 ppm, three times the pool minimum. It also advises against cyanuric acid in hot tubs, because stabilizer slows chlorine down and builds up quickly in a small volume.
How the doses are worked out
Chlorine (g) = rise in ppm × litres ÷ 1,000 ÷ product strength
pH and alkalinity doses use the same carbonate chemistry as our pool calculators, run at your water temperature. Hot water shifts the chemistry slightly, which is why temperature is an input.For a 400 gallon tub, raising chlorine by 1 ppm takes 0.1 oz of 56% dichlor, 0.41 fl oz of 12.5% liquid chlorine, or 0.62 fl oz of 8.25% bleach. Those are small amounts, so a measuring spoon or kitchen scale beats guessing.
Dichlor needed to raise chlorine from 1 to 4 ppm
Show the numbers
| Tub volume | Dichlor for +3 ppm |
|---|---|
| 200 gal | 0.14 oz |
| 300 gal | 0.21 oz |
| 400 gal | 0.29 oz |
| 500 gal | 0.36 oz |
| 600 gal | 0.43 oz |
Worked example
400 gallons at 102°F: FC 1, pH 7.9, TA 60
- Raise alkalinity 60 → 90 ppm: 2.7 oz of baking soda
- Lower pH 7.9 → 7.4 with dry acid: 0.5 oz (the calculator works this out from the pH after step 1)
- Raise chlorine 1 → 4 ppm: 0.29 oz of 56% dichlor, which also adds 2.7 ppm of stabilizer
A simple routine
- Before each soak: test chlorine and pH. Top chlorine up to 3 to 5 ppm.
- After each soak: add a small dose of chlorine with the jets running, since bathers use it up quickly.
- Weekly: test alkalinity, and shock with a larger chlorine dose if the water smells of chlorine or looks dull.
- Every few months: drain and refill. Dichlor builds stabilizer and dissolved solids build up fast in a small tub. Many owners refill every three to four months, sooner with heavy use.
Add chemicals with the cover open and the jets running, one at a time, and wait 15 to 20 minutes between them. Never mix products, and keep the cover open for a few minutes after a large chlorine dose so fumes don’t build up under it.
Not sure of your volume? The hot tub volume calculator estimates it from the tub’s size.
Frequently asked questions
How much chlorine do I put in my hot tub?
About 0.1 oz of 56% dichlor per 1 ppm in a 400 gallon tub. To go from 1 to 4 ppm, add 0.29 oz.
What should the chlorine level be in a hot tub?
At least 3 ppm according to the CDC. Most owners aim for 3 to 5 ppm.
Can I use pool chlorine in a hot tub?
Liquid chlorine and dichlor work in both. Avoid slow-dissolving trichlor tablets in hot tubs: they’re strongly acidic and add stabilizer.
How do I lower pH in a hot tub?
Use a pH decreaser, which is usually dry acid (sodium bisulfate). In 400 gallons with 80 ppm alkalinity, going from pH 7.9 to 7.4 takes about 0.5 oz.
Can I use bleach in my hot tub?
Yes, plain unscented bleach is sodium hypochlorite, the same as liquid chlorine. Avoid scented, splash-less or thickened bleach.
Why does my hot tub smell like chlorine?
A strong chlorine smell usually means combined chlorine (chloramines), not too much chlorine. A shock dose with the cover open clears it.
Sources
- Home Pool and Hot Tub Water Treatment and Testing. U.S. Centers for Disease Control and Prevention (CDC), reviewed May 10, 2024.
- Model Aquatic Health Code, 5th edition (2024). CDC.
- The solubilities of calcite, aragonite and vaterite in CO₂–H₂O solutions between 0 and 90°C (carbonate equilibrium constants). Plummer, L. N. and Busenberg, E., Geochimica et Cosmochimica Acta 46 (1982).
Reviewed November 7, 2026. We check the formulas against the sources above and re-run the worked examples whenever we update this page. Found something wrong? Tell us.