Hot tub & spa calculator
Updated:
Hot tubs are small, hot, and turn over chemistry fast, so precise dosing matters. Enter your spa’s gallons and readings for exact chlorine, pH, and alkalinity amounts.
Chlorine (free chlorine)
pH
Total alkalinity
Dosing a hot tub is different
A hot tub holds a small fraction of a pool’s water, so a normal pool dose can overshoot badly. These calculators start at 400 gallons; replace that with your tub’s capacity from its manual. At that size, raising free chlorine by 3 ppm takes about 1.2 fl oz of 12.5% liquid chlorine, so measure carefully, start low, and re-test.
Chlorine and bromine minimums are higher in a hot tub
The CDC recommends at least 3 ppm of free chlorine in a hot tub, against 1 ppm in a pool, and recommends no cyanuric acid at all. The MAHC sets the same 3 ppm minimum for public spas, and 4.0–8.0 ppm if you use bromine.
Hot tubs run hot, so chemistry moves fast
The CDC lists hot tub jets, which throw water into droplets and mist, among the things that lower chlorine levels, alongside sweat and dirt from bathers. Its guidance is to test chlorine and pH at least twice a day, and more often when several people are using the tub.
Refresh the water on schedule
Body oils, lotions and other bather waste build up quickly in a small volume, and dissolved solids climb with every chemical you add. The MAHC Annex gives 1,500 ppm above your fill water as a generally accepted level for total dissolved solids. When a TDS test passes that, fresh water is the only way down: drain, refill, and re-balance from scratch.
How the dose is calculated
The same math as the pool calculators, applied to a small volume.
Chlorine needed
grams = 0.00378541 × (target FC − current FC) × gallons
For 3 ppm in 400 gallons that is 4.5 g of available chlorine: about 1.2 fl oz of 12.5% liquid chlorine or 1.7 fl oz of 8.25% bleach.
pH and alkalinity
the carbonate model from the muriatic acid and alkalinity calculators
Acid and baking soda doses use the same method as those calculators, scaled to your tub.
Ideal ranges
| Level | Target | Notes |
|---|---|---|
| Free chlorine | 3 ppm or more | CDC; MAHC 5.7.3.1.1.2.3 |
| Bromine (if used) | 4.0–8.0 ppm | MAHC 5.7.3.1.2.2 and 5.7.3.1.2.3 |
| pH | 7.0–7.8 | CDC; MAHC 5.7.3.4.1 |
| Cyanuric acid | None | CDC |
| Total alkalinity | 60–180 ppm | MAHC 5.7.4.4.1 |
| Calcium hardness | 100–200 ppm | MAHC Annex 5.7.4.4.3, for water above 90°F |
Frequently asked questions
Can I use bromine instead of chlorine?
Yes. Bromine is common in hot tubs. The MAHC sets a minimum of 4.0 ppm in spas and a maximum of 8.0 ppm. The DoseMyPool app supports bromine systems directly.
How often should I test a hot tub?
The CDC recommends testing chlorine and pH at least twice a day, and more often when several people are using it.
Sources
- CDC, Home Pool and Hot Tub Water Treatment and Testing (May 10, 2024). 3 ppm minimum, no CYA, pH 7.0–7.8, testing twice a day, and what lowers chlorine
- CDC, Model Aquatic Health Code, 5th edition (2024), Code Language. Sections 5.7.3.1.1.2.3 (spa chlorine), 5.7.3.1.2.2 and 5.7.3.1.2.3 (bromine), 5.7.3.4.1 (pH) and 5.7.4.4.1 (alkalinity)
- CDC, Model Aquatic Health Code, 5th edition (2024), Annex. Sections 5.7.4.4.3 (calcium above 90°F) and 5.7.4.6 (dissolved solids)
Related calculators & guides
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