How much chlorine (or bromine) to add to a hot tub
The dosing math by product, plus after-soak top-ups and weekly shock — with a worked 400-gallon example.
Just want the dose? The hot tub chemical dosage calculator takes your tub volume and reading and returns the exact grams to add — chlorine, pH, alkalinity, calcium. Free.
The honest answer is "it depends on your tub volume and which product you use" — but the math is simple once you have those two things. Hot tub doses are small (a tub is a fraction of a pool's volume), which is exactly why eyeballing it leads to overshooting.
The base dosing rates
These are the standard amounts to raise free chlorine by 1 ppm per 10,000 gallons:
| Product | Per 10,000 gal / 1 ppm |
|---|---|
| Liquid chlorine (12.5%) | 10 fl oz |
| Cal-hypo (68%) | 2.0 oz (dry) |
| Dichlor (56%) | 2.4 oz (dry) |
For bromine, the rule of thumb is about 0.13 oz of sodium bromide per 100 gallons to raise bromine by 1 ppm.
Worked example: a 400-gallon tub
A typical home hot tub holds 300–500 gallons. Say yours is 400 gallons and you want to raise free chlorine by 1 ppm. Scale the rate by 400 ÷ 10,000 = 0.04:
- Liquid chlorine 12.5%: 10 fl oz × 0.04 = 0.4 fl oz (a bit under a tablespoon)
- Cal-hypo 68%: 2.0 oz × 0.04 = 0.08 oz (~2.3 grams)
- Dichlor 56%: 2.4 oz × 0.04 = 0.096 oz (~2.7 grams)
These are tiny amounts — which is the whole point. A "splash" of cal-hypo can be several ppm in a small tub. Use a measuring spoon or a small scale, not your hand.
After a soak
Bathers consume sanitizer. The working rule is about 1 ppm of sanitizer demand per person, per hour in the tub. So two people for an hour will burn roughly 2 ppm — top up afterward to bring chlorine or bromine back into range before the water sits overnight.
Weekly shock
- Chlorine tubs: shock up to about 10 ppm free chlorine once a week to clear combined chloramines (the source of the "chlorine smell"). Scale the dichlor/cal-hypo rate above to your volume.
- Bromine tubs: shock with 1 oz of MPS (non-chlorine shock) per 100 gallons to reactivate the bromide bank.
After shocking, let sanitizer fall back into range (1–3 ppm chlorine / 2–4 ppm bromine) before soaking. Don't stack doses on top of a high reading — more isn't better, and high sanitizer is hard on skin and equipment.
Gear this guide uses
The two things you need are your tub volume and a current reading — then the right sanitizer to dose with.
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- Hot tub test strips or a drop test kit — to read free chlorine/bromine and pH before you dose.
- Sanitizer to match the table: dichlor, cal-hypo, or liquid chlorine 12.5%.
- Running bromine instead? Sodium bromide + non-chlorine (MPS) shock.
- Measure, don't eyeball: a small digital scale or measuring spoons — tub doses are tiny.
Sources
General guidance, not a substitute for your own testing. Doses scale with your exact volume — measure carefully and re-test after dosing.