How much chlorine (or bromine) to add to a hot tub

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:

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.