Hot tub & spa calculator
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
Spa volumes are tiny (often 300–500 gallons), so a “normal” pool dose can wildly overshoot. These calculators default to a small volume and give amounts in the small units you actually measure — fluid ounces, teaspoons, and capfuls. Always start low and re-test.
Hot tubs run hot, so chemistry moves fast
Heat and aeration drive pH up and consume sanitizer quickly. Test more often than a pool — often before each use — and keep alkalinity on the lower side to steady the pH.
Refresh the water on schedule
Body oils, lotions, and contaminants build up fast in a small volume. Drain and refill a hot tub roughly every 3–4 months (sooner with heavy use), then re-balance from scratch.
Ideal ranges
| Level | Target | Notes |
|---|---|---|
| Free chlorine | 1–3 ppm | Or bromine 3–5 ppm |
| pH | 7.2–7.8 | |
| Total alkalinity | 50–90 ppm | |
| Calcium hardness | 150–250 ppm |
Frequently asked questions
Can I use bromine instead of chlorine?
Yes — bromine is popular in spas because it’s stable in hot water. The DoseMyPool app supports bromine systems directly.
How often should I test a hot tub?
More often than a pool — ideally before each use, and at least twice a week.
Related calculators & guides
Get exact doses, saved and on schedule
DoseMyPool remembers your pool, gives the safe order to add things, and reminds you to test — free on the App Store.