How much bromine to start a hot tub?
Short answer
To start a 400 gallon bromine hot tub, add about 6.4 fl oz of bromide booster (sodium bromide) to start, then load the floater with 4 tablets by one label’s rate (3 per 300 gallons). Top up the bromide with about 1.6 fl oz a week.
How to work it out
- In short: bromine is dosed by the label, scaled from the label’s gallons to your tub’s.
- Bromide booster (one label): 4 fl oz per 250 gal × 400 ÷ 250 = 6.4 fl oz
- Tablets (one label): 3 per 300 gal × 400 ÷ 300 = 4 → 4 tablets
A bromine hot tub starts with a reserve of bromide salt, which an oxidizer (shock) turns into active bromine; the tablets in the floater then keep it topped up. Because bromine is dosed by label rates rather than ppm arithmetic, the calculator scales the published rates to your tub.
Adjust for your project
More options: Users
For the drain-and-refill rule of thumb
Add6.4 fl oz of bromide booster
- Bromine tablets in the floater
- 4 tablets
- Weekly bromide top-up (label rate)
- 1.6 fl oz
- Drain and refill about every (rule of thumb)
- 67 days
- The drain interval uses a common spa-trade rule of thumb — gallons ÷ 3 ÷ people using it per day = days between drains — not a standard. Your manual’s schedule comes first.
- Bromine is dosed by the label, not by ppm: the bromide booster builds a reserve, and the tablets in a floater keep releasing bromine. Aim for the label’s range (one brand: 3–6 ppm); the CDC recommends 4–8 ppm of bromine in hot tubs.
- Shock a bromine tub with the oxidizer its label calls for (often a non-chlorine shock) to turn the bromide back into active bromine.
- Never put bromine tablets and chlorine products in the same floater or feeder, and don’t switch between them without draining.
Chemical safetyFollow the product label. Add chemicals to water, never water to chemicals. Never mix chemicals — especially chlorine and acid, or different types of chlorine. Add one at a time with the pump running, keep them away from children, and don’t swim until levels are back in range. This is an estimate: add it, let it circulate, then retest before adding more. Pool and spa water only — never use these doses for drinking water. Keep hot tub water no hotter than 104°F (40°C), and the CDC advises no hot tub use for children under 5.
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Bromine tablets (BCDMH)Bromine tablets in a floater release sanitizer steadily between soaks; the bromide booster gives them a reserve to work from.
View on Amazon (opens in a new tab)Bromine start-up by tub size (label rates)
| Tub | Add | Bromine tablets in the floater | Weekly bromide top-up (label rate) |
|---|---|---|---|
| 250 gal | 4 fl oz of bromide booster | 3 tablets | 1 fl oz |
| 300 gal | 4.8 fl oz of bromide booster | 3 tablets | 1.2 fl oz |
| 350 gal | 5.6 fl oz of bromide booster | 4 tablets | 1.4 fl oz |
| 400 gal | 6.4 fl oz of bromide booster | 4 tablets | 1.6 fl oz |
| 450 gal | 7.2 fl oz of bromide booster | 5 tablets | 1.8 fl oz |
| 500 gal | 8 fl oz (1 cup) of bromide booster | 5 tablets | 2 fl oz |
Bromine start-up, step by step
- Fill the tub, balance alkalinity and pH first.
- Add the sodium bromide booster with the jets running — one label gives 4 oz per 250 gallons, about 6.4 fl oz for 400 gallons.
- Shock with the oxidizer your bromine products call for, to activate the bromide.
- Load the floater: one label gives 3 tablets per 300 gallons, so 4 tablets here. Adjust the vent so bromine stays in range between soaks.
- Test before each use. The CDC recommends 4–8 ppm of bromine in hot tubs; product labels give their own range (one says 3–6 ppm).
- Weekly. The same booster label suggests about 1 oz per 250 gallons a week — about 1.6 fl oz here — plus a shock after heavy use.
- Don’t mix sanitizers. Never put chlorine products in a bromine floater, and drain before switching between chlorine and bromine.
- Read your products’ labels. Brands differ in strength; these are scaled from the published rates cited on this page.
Using chlorine instead? See how much chlorine to add to a hot tub on first use. Tub size unknown? Try how many gallons in a 7x7 hot tub.
Different size? Use the full Hot Tub Chemical Calculator (Chlorine, Bleach, Bromine).
What you’ll need
- Sodium bromide (bromine booster)Budget price
Builds a bromide reserve in fresh hot tub water so an oxidizer can turn it into bromine.
See options on Amazon (opens in a new tab) - Bromine tablets (BCDMH)Budget price
The usual hot tub bromine for a floater or feeder. Bromine holds up better than chlorine in hot water.
See options on Amazon (opens in a new tab) - Hot tub test strips (chlorine or bromine)Budget price
Hot tubs move faster than pools; a strip before each soak tells you whether the sanitizer is still there.
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Frequently asked questions
How much bromine to start a 300 gallon hot tub?
About 4.8 fl oz of bromide booster, and 3 tablets in the floater.
How many bromine tablets for a 500 gallon hot tub?
About 5 tablets by one label’s 3-per-300-gallon rate.
Other common sizes
- How much chlorine to add to a hot tub on first use — 1.6 fl oz
- How much bleach to put in a hot tub — 2.6 fl oz
Sources
- CDC Healthy Swimming — Staying healthy in hot tubs (opens in a new tab) — No hotter than 104°F (40°C); chlorine at least 3 ppm, bromine 4–8 ppm, pH 7.0–7.8; no children under 5.
- CDC Healthy Swimming — Home pool and hot tub water treatment and testing (opens in a new tab) — pH 7.0–7.8; free chlorine at least 1 ppm in pools (2 ppm with cyanuric acid) and 3 ppm in hot tubs; no CYA in hot tubs; test twice a day.
- CDC Healthy Swimming — Operating public pools, hot tubs and splash pads (opens in a new tab) — Hot tubs: at least 3 ppm free chlorine or 4 ppm bromine; water no hotter than 104°F (40°C).
- CDC — Model Aquatic Health Code, 5th edition (2024), code (opens in a new tab) — Free chlorine minimum 1.0 ppm (2.0 with CYA, 3.0 in spas); pH 7.0–7.8; CYA at or below 90 ppm; CYA:FC above 45:1 is an imminent hazard; TA 60–180 ppm; turnover tables.
- Leisure Time — Spa 56 chlorinating granules (dichlor) label (opens in a new tab) — 99% sodium dichlor dihydrate, 55% available chlorine; ½ oz per 500 gallons for a 2–3 ppm residual; 1 oz per 500 gallons to shock.
- Leslie’s — Chlor Brite spa shock (dichlor) label (opens in a new tab) — Half an ounce per 500 gallons; 1.5 oz per 500 gallons after use.
- Leisure Time — Reserve (sodium bromide) label (opens in a new tab) — Sodium bromide 32.18%; 4 oz per 250 gallons at start-up; 1 oz per 250 gallons weekly.
- Leslie’s — Bromine Tabs Plus label (opens in a new tab) — 98% active ingredient; 3 tablets per 300 gallons in a floater or feeder.
- SpaGuard — Brominating Tabs (opens in a new tab) — BCDMH 98%; target active bromine 3–6 ppm.
- OxyChem — Soda Bleach Solutions Handbook (re-hosted copy) (opens in a new tab) — Trade percent = grams per litre of available chlorine ÷ 10; weight % NaOCl ≈ 1.05 × weight % available chlorine; strength and specific gravity table.
- US EPA reg. 42177-80 — sodium dichlor label (ProPools copy) (opens in a new tab) — Dihydrate, 56% available chlorine; 2–3 oz per 10,000 gallons daily; 1 lb per 10,000 gallons to superchlorinate.
- CDC — Pool chemical injuries press release (2014) (opens in a new tab) — Never mix pool chemicals, especially chlorine products with acid; add chemical to water, never water to chemical.
- CDC — Pool chemical safety: storage poster (opens in a new tab) — Separate acid and chlorine; don’t mix different types of chlorine; keep chemicals cool, dry and in original containers.