How much bromine to add to your hot tub

Start a fresh tub on bromine by adding sodium bromide for a reserve of about 30 ppm (2.1 oz (59 g) in 400 gallons), then a small oxidizer dose to turn some of it into bromine: 0.57 oz (16 g) of 45% MPS brings a 400 gallon tub to about 5 ppm. Keep bromine at 4 to 6 ppm with tablets in a floater.

Hot tub bromine calculator

Add

0.45oz

of Non-chlorine shock (MPS 45%)

13 gabout 1¾ tsp by volume

Raises bromine from 1 ppm to 5 ppm in 400 gal, if the tub has a bromide reserve.

MPS reads as combined chlorine on DPD tests for about a day. Test CC the next morning.
Run the jets with the lid open and wait at least 15 minutes (20 after pH decreaser) before anyone gets in.
A small, measured dose of MPS or chlorine turns some of the bromide reserve into active bromine. Not a full shock.
No reserve (fresh water)? Build it first: switch “What you need” to new water.

Check the product label. Strengths vary by brand, so enter the % printed on yours. Add one chemical at a time, into the water, and never mix chemicals.

Save this test and get a reminder before your next soak.

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How bromine works

Bromine sanitizes, turning back into bromide as it works. An oxidizer (MPS or chlorine) turns that bromide back into bromine. So a bromine tub needs a reserve of bromide, built once per fill with sodium bromide, plus regular shock and a tablet floater for a steady supply.

On a fresh reserve, don’t start with a full label shock: in a 400 gallon tub the full MPS dose (3 oz (85 g)) would read about 27 ppm bromine, far above the 8 ppm maximum. The calculator sizes the start-up dose to about 5 ppm.

Reading bromine

Bromine is measured on the same test as chlorine, multiplied by 2.25. If your kit reads 2 on the chlorine scale, that is 4.5 ppm bromine. Use the converter.

Questions

How much bromine should be in a hot tub?

4 to 6 ppm. CDC guidance is at least 4 ppm; never soak above 8 ppm.

Can I switch from bromine to chlorine?

Only after draining. Any bromide left in the water turns chlorine into bromine.