Hot Tub Chemical Dose Calculator
How much dichlor chlorine granules to add to a hot tub or spa to reach a target free-chlorine level, in teaspoons and grams.
How it works
Grams = ΔFC(ppm) × liters ÷ 0.56 ÷ 1000 · Teaspoons = grams ÷ 4.5 · liters = gallons × 3.785
Dichlor granules are the usual spa sanitizer because they dissolve fast and are pH-neutral. The dose follows the same chemistry as a pool — the chlorine needed is the target rise times the water volume in liters, divided by dichlor’s 56% available-chlorine fraction — but spa volumes are small, so the answer lands in teaspoons rather than pounds. Add granules with the jets running and the cover off, and wait for the level to fall before soaking.
Worked example
To raise free chlorine by 3 ppm in a 400-gallon spa: 400 × 3.785 = 1,514 L, × 3 = 4,542 mg, ÷ 0.56 ÷ 1,000 ≈ 8.1 g. At about 4.5 g per teaspoon that is roughly 1.8 teaspoons.
Frequently asked questions
How often should I dose a hot tub?
Add a small maintenance dose after each use and test before soaking, since the hot water and bather load burn through chlorine fast. Frequent small doses keep the level steadier than one large weekly addition.
When can I get in after adding chlorine?
Wait with the cover off and jets on until free chlorine falls to about 1–3 ppm, then test. Soaking at shock levels can irritate skin and eyes and bleach swimwear.
Does dichlor build up in the water?
Yes — dichlor adds cyanuric acid (stabilizer) with every dose, which slowly accumulates in the small spa volume. When it gets high the chlorine becomes less effective, and the fix is to drain and refill.
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Estimate only — always test spa water and follow the product label; never mix spa chemicals together.