Hot Tub Dose Calculator to Raise Spa Free Chlorine

Scale sanitizer dosing to your hot tub's actual gallons — the dichlor or cal-hypo amount that brings free chlorine to target — then verify with a test strip.

Inputs
gal
ppm
ppm
Results update as you type — no submit
ResultsLIVE
Sanitizer to add
0.3 oz
≈ 8 g
FC change3 ppm
ProductDichlor 56%
Formula
raise FC by 3 ppm in 400 gal
sanitizer = 0.3 oz ≈ 8 g
Add chemicals to water, never water to chemicals. Broadcast over the deep end with the pump running, then retest before adding more.
Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.
MeasureShedmeasureshed.com/calculator/hot-tub-dose-calculator
Hot tub sanitizer — dose sheetUNITS: US GAL · ppm
Inputs
Spa volume400 gal
Current FC0 ppm
Target FC3 ppm
ProductDichlor 56%
Results
sanitizer to add0.3 oz
Other units8 g
FC change3 FC ppm
Safe dosing: add product to water (never the reverse) · broadcast with the pump running · wait and retest before re-dosing · keep chemicals apart and dry.
Shopping list
☐ 0.3 oz (8 g) — Dichlor 56%
Formula
raise FC by 3 ppm in 400 gal
sanitizer = 0.3 oz (8 g)
Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.SHEET 1 OF 1

How it works

A hot tub is a chlorine (or bromine) pool in miniature, and its small volume is exactly why careful dosing matters — a 400 gal spa reacts to a scoop of sanitizer the way a 15,000 gal pool reacts to a bag. The calculator scales the dose to your spa's actual gallons so you add grams, not guesses.

The math is the same mass balance as a pool: volume × ppm change × product strength. Dichlor at 56% available chlorine is the usual spa granule because it dissolves fully and is pH-neutral; cal-hypo at 65% also works. Because spa water is hot and heavily used, sanitizer is consumed fast, so test and top up frequently rather than dosing high once.

Always add the sanitizer to the water with the jets running and the cover off, and wait before you get in — spa sanitizer levels climb and fall quickly. Test with a kit before each soak. Never mix products, and add chemical to water, never water to chemical.

Worked example
Spa volume 400 gal · free chlorine 0 → 3 ppm (+3 ppm) · dichlor 56%
dichlor = 3 × 400 × 8.34 ÷ 1,000,000 ÷ 0.56 = 0.018 lb = 8 g (≈½ tbsp)

Questions about hot tub dosing

Very little. Raising free chlorine by 3 ppm in a 400 gal spa takes only about 8 g of 56% dichlor — roughly half a tablespoon. That is why measuring by your spa's real volume beats eyeballing a capful.

SOURCES — Dichlor 56% / cal-hypo 65% available chlorine (labels) · ppm = mg/L
Doses are estimates from published product concentrations — always follow the product label, add chemicals to water (never the reverse), test with a kit, and retest after dosing.

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