Hot Tub Chemical Calculator

About this calculator

Hot tub water needs a steady free chlorine (or bromine) residual, typically 3–5 ppm, to keep bacteria in check at spa temperatures, which are warm enough to let pathogens multiply quickly. Granular dichlor (sodium dichloro-s-triazinetrione) is the standard sanitizer for hot tubs rather than liquid or gas chlorine, because it dissolves fast, is nearly pH-neutral, and comes pre-stabilized with cyanuric acid to resist breakdown from heat and use.

This calculator estimates how many ounces of granular dichlor you need to raise your spa from its current chlorine reading to your target ppm, based on your spa's water volume. Getting the dose right avoids both under-sanitized water that risks bacterial growth and over-shocking that causes the sharp chlorine smell and skin irritation spa owners associate with bad water balance.