Hot Tub Bromine Calculator
Bromine is the other sanitizer — softer on skin, steadier in hot water, and the standard choice for indoor spas. Target: 3–5 ppm bromine, built on a "bromide bank" of ~30 ppm sodium bromide.
How the math works
- Sodium bromide (oz) = target ppm × gallons × 8.34 ÷ 1,000,000 × 16 — bromide salt is ~100% active, no strength correction
- Example: a 350-gal spa needs 30 × 350 × 8.34 ÷ 1M × 16 ≈ 14 oz of sodium bromide to establish the bank
- Activation: bromide alone doesn't sanitize — an oxidizer (MPS or ozone) converts bromide into active bromine, which then gets recycled back after killing. That's why bromine spas "reuse" their chemical.
Bromine vs chlorine for a hot tub — honestly
- Bromine wins on: stability at 104°F (chlorine burns off 2–3× faster), gentler smell, softer feel, and the bromide bank keeps working between doses
- Chlorine wins on: price (bleach is dirt cheap), faster kill speed, and no lock-in — you can switch to chlorine anytime
- The classic setup: sodium bromide bank + MPS after each soak + an occasional dichlor top-up. Test strips for bromine read the TOTAL (free + spent), so don't panic at a 6 ppm strip reading — a DPD test separates them.
Three rules for bromine spas
- Never let the bank hit zero. If bromine reads 0, the bank is gone and you restart from sodium bromide — annoying and avoidable with weekly testing.
- MPS after every soak re-activates spent bromine. This is the maintenance rhythm: soak → shock → tomorrow it's ready again.
- Watch your pH. Bromine is more forgiving than chlorine at high pH, but 7.4–7.6 is still where it works best.
Frequently asked questions
How much sodium bromide do I need to start a 350 gallon spa?
About 14 oz (400 g) to establish a 30 ppm bromide bank. Dissolve with jets running, then activate with MPS and test before soaking.
Is bromine better than chlorine for hot tubs?
For hot water it holds better, smells milder and feels softer — at roughly 2–3× the chemical cost. Heavy users and indoor spas usually prefer bromine; budget users do fine with bleach.
Can I switch from chlorine to bromine?
Yes — just add the sodium bromide bank and start using MPS. You don't need to drain. Switching back to chlorine requires a partial drain to remove the bromide.