Hot Tub Chemical Calculator

The alkalinity logic only computes a dose when targetAlkalinity exceeds currentAlkalinity (alkIncreaserOz is set from baking soda dosing in that case) — there's no branch for the opposite direction. Lowering alkalinity typically requires muriatic acid or dry acid dosed and diluted very differently than raising it with baking soda, so that case is intentionally left out of this model rather than approximated.

Chlorine dose is computed from clDiff = Math.max(0, targetChlorine - currentChlorine), so any case where current chlorine is at or above your target is clamped to zero rather than producing a negative dose. This reflects that the calculator can only tell you how much dichlor to add to raise chlorine — reducing an already-high reading requires time, sunlight exposure, or a chlorine reducer, none of which are modeled here.

The reference dosing rates (6 oz soda ash or 3 oz dry acid per 0.2 pH-unit step) are calibrated to a 500-gallon tub, so the calculator computes a volumeRatio of your tub's gallons divided by 500 and multiplies the reference dose by that ratio. A 200-gallon tub needing the same 0.2-unit pH swing gets 0.4× the reference dose, while an 800-gallon tub gets 1.6×.

pH, alkalinity, and chlorine interact chemically — raising alkalinity with baking soda also tends to push pH upward, so dosing everything at once makes it hard to know which chemical caused which change and can lead to overshooting a target. Waiting 15-20 minutes with the pump circulating between additions lets each chemical fully mix and register on a retest before you decide whether more is needed.