Dichlor or cal-hypo for a spa?

Dichlor is the common choice: it is pH-neutral and dissolves completely, which suits hot, frequently used water. Cal-hypo works but adds calcium. Whatever you pick, dose small and retest — spa chemistry moves fast.

Why does spa chlorine disappear so quickly?

Heat, aeration from the jets, and bather load all consume chlorine rapidly. A spa can go from well-sanitized to zero within a day of use, so test before every soak and top up as needed rather than dosing a big amount up front.

What about pH in a hot tub?

Dichlor is roughly pH-neutral, but aeration tends to push spa pH up over time. Manage pH separately with the pH adjustment calculator, and always test both sanitizer and pH with a kit before you use the tub.

Hot Tub Dose Calculator

Spa-scale sanitizer dosing — the small amount of dichlor or cal-hypo to bring a hot tub's free chlorine to target from its actual gallons.

Inputs

Spa volume — gallons

gal

Current FC

ppm

Target FC

ppm

Product — 56% available chlorine — pH-neutral spa granules

Results update as you type — no submit

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.