Why hot tubs need extra care
A hot tub holds a few hundred gallons of water at around 100 °F (38 °C), shared by several people at once. Warm water helps germs grow and uses up sanitiser fast, and a few bathers add as much sweat and body oils to a spa as dozens would to a pool. That is why the US Centers for Disease Control and Prevention (CDC) recommends higher sanitiser levels in hot tubs than in pools, and why hot tub rash and other infections are linked to poorly maintained spas.
| Test | CDC recommendation for hot tubs |
|---|---|
| Free chlorine | At least 3 ppm |
| Bromine (if you use bromine instead) | 4–8 ppm |
| pH | 7.0–7.8 |
| Cyanuric acid (stabilizer) | CDC recommends not using it, or chlorine products containing it, in hot tubs |
Do not exceed the maximum on your chlorine product's label, and test the water before every use.
Chlorine product = ppm increase × litres ÷ 1,000 ÷ product strength
400 US gallons = 1,514 litres · 1 ppm in 400 gal = 1.5 g of available chlorine
Worked examples (400-gallon spa)
- Chlorine from 1 to 4 ppm: 7 g (0.25 oz) of 65% cal-hypo, 36 mL (1.23 fl oz) of 12.5% liquid chlorine, 72 mL (2.4 fl oz) of 6% bleach, or 8.1 g of dichlor.
- Alkalinity from 60 to 90 ppm: 76 g (2.7 oz) of baking soda.
- Alkalinity from 140 to 100 ppm: 121 mL (4.1 fl oz) of 31.45% muriatic acid or 156 g (5.5 oz) of dry acid in total, added in small steps with the jets running so pH does not fall too far.
Spa doses are small, so weigh granules on a kitchen scale reserved for pool chemicals and measure liquids with a marked cup. A heaping spoonful can easily be double the dose.
Chlorine, dichlor and bromine
- Dichlor granules are the most common spa chlorine because they dissolve fast and are pH-neutral, but they contain cyanuric acid, which builds up with every dose. Since the CDC recommends against cyanuric acid in hot tubs, many owners use non-stabilized chlorine (cal-hypo or liquid) instead, or drain and refill more often. If you use dichlor, the calculator shows how much stabilizer it adds.
- Bromine is popular in hot tubs because it stays more stable at high temperatures. It is usually fed from tablets in a floater or feeder, so the dose is set by the feeder rather than calculated; keep it at 4–8 ppm.
- Shocking after heavy use oxidises body oils and restores sanitiser. Follow the product label for the shock dose and for when the tub can be used again.
Routine
- Test sanitiser and pH before each use; alkalinity and hardness weekly.
- Correct alkalinity first, then pH, then sanitiser.
- Add chemicals with the jets running and the cover open, one at a time.
- Drain and refill regularly as the manufacturer recommends; dissolved solids build up quickly in such a small volume.
Hot water scales easily, so check the water balance (LSI) calculator at your spa temperature.
Common mistakes
- Guessing the volume. Use the manufacturer's capacity; seats and footwells make the box measurement misleading.
- Using pool doses. A pool scoop in a spa can be many times too much.
- Mixing chemicals. Never mix chlorine and acid, or chlorine and bromine products, in the same container.
Estimates for general information. Always follow your hot tub manufacturer's instructions and the product label.
Hot tub dose chart
Amounts to raise free chlorine by 1 ppm and alkalinity by 10 ppm for common spa sizes.
| Spa size | +1 ppm: cal-hypo 65% | +1 ppm: liquid 12.5% | +1 ppm: dichlor | +10 ppm alkalinity: baking soda |
|---|---|---|---|---|
| 200 gal (757 L) | 0.04 oz (1.2 g) | 0.2 fl oz (6.1 mL) | 0.05 oz (1.4 g) | 0.45 oz (12.7 g) |
| 300 gal (1,136 L) | 0.06 oz (1.7 g) | 0.31 fl oz (9.1 mL) | 0.07 oz (2 g) | 0.67 oz (19.1 g) |
| 400 gal (1,514 L) | 0.08 oz (2.3 g) | 0.41 fl oz (12.1 mL) | 0.1 oz (2.7 g) | 0.9 oz (25 g) |
| 500 gal (1,893 L) | 0.1 oz (2.9 g) | 0.51 fl oz (15.1 mL) | 0.12 oz (3.4 g) | 1.1 oz (32 g) |
| 600 gal (2,271 L) | 0.12 oz (3.5 g) | 0.61 fl oz (18.2 mL) | 0.14 oz (4.1 g) | 1.3 oz (38 g) |
Frequently asked questions
What should chlorine be in a hot tub?
The CDC recommends at least 3 ppm free chlorine in hot tubs (or 4–8 ppm bromine), with pH 7.0–7.8.
How much chlorine do I add to a 400 gallon hot tub?
To raise free chlorine by 3 ppm: about 7 g (0.25 oz) of 65% cal-hypo, 36 mL of 12.5% liquid chlorine or 8 g of dichlor granules.
Can I use stabilizer in a hot tub?
The CDC recommends not using cyanuric acid or chlorine products containing it, such as dichlor, in hot tubs.
How much baking soda to raise alkalinity in a hot tub?
About 76 g (2.7 oz) raises a 400 gallon hot tub by 30 ppm, or about 25 g per 10 ppm.
How much chlorine for a 300 gallon hot tub?
Each 1 ppm needs about 1.7 g of 65% cal-hypo or about 9 mL of 12.5% liquid chlorine.
Sources
- CDC: Preventing hot tub rash (chlorine at least 3 ppm, bromine 4–8 ppm, pH 7.0–7.8).
- CDC: Home pool and hot tub water treatment and testing (no cyanuric acid in hot tubs).
Last reviewed 28 October 2026. Results are estimates; see our disclaimer.