How Much Bromine to Add to a Hot Tub

The quick answer: a bromine tub is run in two parts.

1. Bromide bank (once per fill): add sodium bromide for about 30 ppm of bromide, then a small oxidizer dose (MPS or chlorine) sized to bring bromine to about 5 ppm. In a 400 gallon tub that is 2.1 oz (59 g) of sodium bromide, then 0.57 oz (16 g) of 45% MPS.
2. Maintenance: keep bromine at 4–6 ppm with tablets in a floater, or with granules, and shock regularly to regenerate it.

CDC guidance is at least 4 ppm and no higher than 8 ppm (CDC). Above 6, wait and retest before soaking. The calculator turns your reading and volume into amounts for your products.

How bromine works (and why the bank matters)

Bromine is recycled. Here's the cycle:

1. Sodium bromide dissolves into bromide ions. These don't sanitize yet.
2. An oxidizer converts bromide into bromine (hypobromous acid), the sanitizer. The oxidizer can be MPS non-chlorine shock, chlorine, or the small amount of chlorine inside BCDMH tablets.
3. Bromine kills germs and oxidizes contaminants, then turns back into bromide.
4. The next shock converts that bromide back into bromine.

So the bromide bank is the reservoir and shock is the "recharge". Skip the bank after a refill, and tablets alone struggle to hold a reading. That's the most common reason for "zero bromine with a full floater".

Step by step: starting a fresh fill with bromine

1. Fill and heat. Acrylic tubs: fill through the filter housing to avoid air locks.
2. Balance first. Bring total alkalinity to your target (often 50–80 ppm), then pH to about 7.4–7.8. Bromine tolerates the upper part of that pH range better than chlorine.
3. Add sodium bromide for about 30 ppm of bromide (2.1 oz (59 g) in 400 gallons, 1.6 oz (44 g) in 300). Check your product's label, and run the jets for a few minutes to dissolve it.
4. Add a small oxidizer dose to convert some of the bromide into bromine: 0.57 oz (16 g) of 45% MPS in 400 gallons (0.42 oz (12 g) in 300) brings it to about 5 ppm. Don't use the full label shock dose here: on a fresh bank, the full dose for 300 gallons (2.2 oz (64 g)) would read about 27 ppm, far above the 8 ppm maximum. The bromine calculator sizes this dose for your tub.
5. Set up maintenance: load the floater (or dispenser) with BCDMH tablets, or plan to add bromine granules as needed.
6. Test after 15 to 30 minutes and before the first soak. Aim for 4–6 ppm.

Maintenance: tablets vs granules

Tablets in a floater dissolve slowly and continuously.

• Start with the tablet count the label suggests for your volume.
• Set the floater's vent to a middle setting.
• Over the first week, open or close the vents (or change the tablet count) until bromine holds steady from day to day.
• Floaters keep dissolving even when nobody uses the tub. If the reading climbs on quiet weeks, close the vents down.
• Keep the floater away from the shell surface when the cover is on. A concentrated pocket of bromine under a closed lid can damage the cover and fade the acrylic. Some owners remove the floater during long idle periods.

Granules are faster-acting and better for small corrections or very small tubs. Use the calculator for a dose by volume and target.

Shock once a week, and after heavy use, to regenerate bromine and oxidize waste. MPS is the most common choice for bromine tubs, with a short wait before soaking (see the label).

Reading bromine on your test kit

• Most hot tub DPD kits read bromine directly on a bromine scale.
• If yours reads chlorine: bromine = chlorine reading × 2.25. Example: a chlorine reading of 2 ppm is 4.5 ppm bromine.
• Test strips are convenient but less precise. Use a drop kit to set up the floater, then strips for daily checks if you prefer.

Bromine too low?

• New fill? You may have skipped the bromide bank. Add sodium bromide, then the small oxidizer dose.
• Just had heavy use? Shock to regenerate bromine from bromide. Bromine used up by bathers needs an oxidizer to come back.
• Floater vents closed, or tablets used up? Open the vents a notch, or refill the floater.
• pH out of range? Check it. Very high or low pH affects readings and comfort.
• Still low after shocking? The water may be old and loaded with contaminants. Check how long it's been since your last drain.

Bromine too high?

• Remove or close the floater.
• Leave the cover off and run the jets (with the air controls open if your tub has them). Bromine dissipates faster with heat, aeration and light.
• Don't soak until it's back to 6 ppm or less, and never above 8 ppm. High bromine can irritate skin and eyes.
• If it's very high, partially drain and refill. Then re-balance. Diluting also lowers your bromide bank, so top it up if you replaced a lot of water.

Switching away from bromine

Once bromide is in the water, adding chlorine just converts it back into bromine. To switch to chlorine, you have to drain, clean and refill. See bromine vs chlorine.

Common mistakes

• No bromide bank after a refill.
• Never shocking. Tablets alone don't regenerate the bank well under heavy use.
• Overfilling the floater "to be safe". The level creeps up for days and is slow to bring down.
• Mixing bromine tablets with other chemicals in the same container or dispenser, or storing them together. Store them separately, dry and cool.
• Using pool trichlor tablets in a bromine floater. Never do this.

Bromine made simple in Spa Calc

Choose bromine in Spa Calc and it:

• reminds you to build the bromide bank after every refill;
• converts chlorine-scale readings automatically;
• sizes the start-up oxidizer dose so bromine lands at about 5 ppm, not a full shock;
• tracks when you changed your floater setting.

Test before a soak and see whether your readings are in the soak range: soak, wait, or add this first.

When can you soak? Soak when free chlorine is 3 to 5 ppm and at least 7.5% of your CYA (bromine 4 to 6) and pH is 7.2 to 7.8, at least 15 minutes after adding anything (20 after pH decreaser); after chlorine, bromine or a shock, test again first. Above 5 ppm chlorine or 6 ppm bromine, wait and retest in an hour or two; never soak above 10 ppm chlorine or 8 ppm bromine, or with pH below 7.0 or above 7.8. Below 3 ppm chlorine (4 bromine), add sanitizer first; at pH 7.0 to 7.2, raise pH first; with CYA above 40 ppm, drain and refill. Hot tub limits based on CDC guidance and EPA product labels.

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Safety

• Keep chemicals out of reach of children and pets, and keep them out of the water until it is back in range.
• Never mix chemicals. Never add acid and chlorine together: mixing them releases toxic chlorine gas.
• Add chemicals to water, never water to chemicals.
• Store chemicals sealed, cool and dry, apart from each other.
• Read and follow the product label. It always comes first.