Free hot tub chemical calculator: bromine, chlorine (dichlor), pH and alkalinity doses sized for small spa volumes. No signup — US gallons + liters.
Spas hold a fraction of a pool's water, so start with an accurate volume in gallons or litres; most portable hot tubs hold 250-500 US gal (roughly 950-1,900 L), and your owner's manual usually states it. Tiny volumes mean tiny doses, so this tool works in fractions of an ounce, teaspoons, grams, and millilitres rather than pounds. Test more often than a pool, ideally before each soak and after heavy use, because the high bather-to-water ratio and 37-40C (98-104F) heat consume sanitizer and shift pH fast. Measure free sanitizer (chlorine target 1-3 ppm, or bromine 3-5 ppm), pH (7.4-7.6), and Total Alkalinity (80-120 ppm), plus calcium hardness if you use a chlorine generator or have soft fill water. Enter your sanitizer type, since dichlor, bromine tablets, and liquid chlorine each dose differently. Note current readings so the calculator solves the gap to target.
Dose math mirrors pool chemistry but scales to spa volume. To raise free chlorine, the tool uses concentration-specific factors; for granular dichlor (about 56% available chlorine) roughly 0.5 oz (14 g) raises FC about 10 ppm in 300 gal (1,135 L). Worked example: a 350 gal (1,325 L) spa needs +2 ppm FC with dichlor. Scale that factor: 0.5 oz x (2/10) x (350/300) = about 0.12 oz, roughly 3.3 g, or about three-quarters of a teaspoon. For pH, sodium bicarbonate raises alkalinity; about 1 tablespoon (14 g) lifts TA roughly 25 ppm in 300 gal, so a 350 gal spa needs a touch more. Because doses are so small, always measure with a kitchen scale or proper spoons, not a guess. Dissolve granular sanitizer in a cup of spa water first, then add it with the jets running to disperse it before anyone re-enters.
Precision is safety-critical in small water: a teaspoon of overdose in 350 gal can spike chlorine far higher than the same scoop in a 10,000 gal pool, and CDC Healthy Swimming guidance warns that hot, agitated water raises chemical exposure for soakers. Always add chemicals to water, never water to chemicals, and never mix a chlorine product with a bromine or acid product in the same cup, as the reaction can release toxic gas. Common mistakes are dosing by pool-sized scoops, soaking before sanitizer has dispersed, and ignoring that dichlor steadily adds cyanuric acid that eventually locks up your chlorine. Wait at least 15-20 minutes with jets running, then retest before entry, keeping free chlorine below about 5 ppm. Because dissolved solids accumulate fast, drain and refill every 3-4 months to reset chemistry that has become impossible to balance.
Spas have a much higher bather-to-water ratio and warmer temperatures. This causes chemicals to be used up faster and pH to drift more rapidly than in a large pool.
The ideal bromine level for a hot tub is 3-5 ppm. Bromine stays effective at the high temperatures of a spa and is more stable than chlorine in hot water, which is why most hot tubs are run on bromine; test before each use because the level drops quickly in a small, heavily used volume.
Test the water before every use, or at least every few days. Because a spa has a high bather-to-water ratio and runs hot, sanitizer and pH shift much faster than in a pool, so you will usually add bromine or chlorine and check pH and alkalinity several times a week.
The ideal hot tub pH is 7.4-7.6. Too low and the warm, aerated water irritates skin and eyes and corrodes the heater and jets; too high and the sanitizer loses effectiveness and scale forms quickly at spa temperatures. Adjust alkalinity into the 80-120 ppm range first to hold pH steady.
While the chemicals are similar, you must be very careful with dosages. We recommend using products specifically labeled for spa use to avoid damaging your equipment.