Hot tub bromine: levels, start-up and dosing
Keep a hot tub at 4–6 ppm bromine, never below 4 ppm. Enter the dose on your label, and the calculator scales it to your tub.
Always follow your product’s label directions. Pool and spa chlorine, bromine, shock and algaecides are EPA-registered pesticides: use products labeled for pool or spa use, and don’t swim or soak until levels are within the limit on your product’s label. If this page and a label disagree, the label wins.
On this page (8 sections)
Target bromine level and how the test reads it (2.25 × chlorine)
The CDC's model code recommends 4–6 ppm bromine for spas, with 4 ppm as the minimum. Our calculator uses the same range. Bromine doses depend on the product, so the calculator scales the amount and gallons printed on your label to your tub: a label dose "per 500 gallons" becomes 0.8 of that amount in a 400-gallon tub. Always follow the product label.
Bromine and chlorine are read with the same DPD chemistry. Many drop kits print only a chlorine scale and tell you to multiply by 2.25 for bromine. That factor is the ratio of the weights of bromine and chlorine (MAHC Annex), so it converts the reading, not the target.
| Kit reads (chlorine scale) | Bromine, ppm | Status |
|---|---|---|
| 1.0 ppm | 2.25 | too low: add bromine |
| 1.5 ppm | 3.4 | below the 4 ppm minimum |
| 2.0 ppm | 4.5 | in range |
| 2.5 ppm | 5.6 | in range |
| 3.0 ppm | 6.75 | above target: hold off on adding more |
Test strips usually have a separate bromine pad that reads bromine directly. Don't multiply those readings.
Starting a tub on bromine: bank, then maintain
Bromine works as a cycle. Bromine kills germs and turns into bromide, which does nothing on its own. An oxidizer (shock) turns bromide back into active bromine. So a bromine tub needs two things: a reserve of bromide in the water, and a steady supply of new bromine or oxidizer.
- Fill and balance. Adjust alkalinity to 80–120 ppm and pH to 7.4–7.6 before adding sanitizer. The first-fill checklist has the full order.
- Build the bromide bank. Add sodium bromide (sold as "bromine booster" or "spa bromide") at the amount on its label for your gallons. Use the calculator's label field to scale it.
- Activate it. Add shock (non-chlorine MPS or chlorine) as its label directs. It converts the bromide into bromine, and the test should now show bromine.
- Maintain with tablets. Load bromine tablets into a floater or the tub's built-in feeder. Tablets dissolve slowly and keep feeding bromine between soaks.
- Test and adjust for a week. Test daily at first, then two or three times a week once the level is stable.
You build the bank only once per fill. Draining removes it, so repeat step 2 after each refill.
Floater and tablets: how to set the vents
Most bromine tablets are BCDMH (bromo-chloro-dimethylhydantoin): they release bromine plus a little chlorine. How fast a floater feeds depends on how many tablets are in it, how far the vents are open, water temperature, and how much the jets run. There's no fixed "tablets per gallon" number that works for every tub, so set it by testing:
- Fill the floater with the number of tablets on the product label, and start with the vents about half open.
- Test bromine every day for the first week at the same time of day.
- Below 4 ppm two days in a row: open the vents one notch. Above 6 ppm: close one notch.
- Refill when the tablets are about half gone, not when the floater is empty, so the feed rate stays steady.
- Take the floater out while you soak, and never let it sit in one spot under a closed cover for days: the concentrated water under it can bleach and damage the shell and cover.
Bromine tablets are mildly acidic, so tubs on tablets tend to drift down in pH and alkalinity over weeks. That's the opposite of the upward drift most tubs have from jets. If yours is low, see low pH in a hot tub.
Shocking a bromine tub
Yes, a bromine tub still needs shock, and for a specific reason: shock reactivates bromide that bathers have used up. Without it, bromine readings drop even with a full floater. Either type works:
- MPS (potassium monopersulfate, a non-chlorine shock) reactivates bromide and oxidizes sweat and lotions. You can usually soak soon after, as its label directs.
- Chlorine shock (dichlor or liquid chlorine) does the same and also sanitizes directly. Keep in mind that dichlor adds stabilizer.
A weekly shock plus one after heavy use (a party, or several people for a long soak) is a common routine. The hot tub shock page has a calculator and waiting times.
Bromine vs chlorine: pros and cons
| Question | Bromine | Chlorine |
|---|---|---|
| Target level | 4–6 ppm | 3–4 ppm (up to your label's limit) |
| Active share at pH 7.5 | about 94% | about 50% |
| Active share at pH 8.0 | about 83% | about 20% |
| Stabilizer (cyanuric acid, CYA) | does not protect bromine | should not be used in hot tubs |
| Dosing | slow tablets, steady level between soaks | manual doses after each soak, easy to measure |
| Switching later | need to drain first | can switch to bromine after draining |
The active-share figures are from the CDC model code (MAHC 2024 Annex). They are the main reason bromine is popular in hot tubs: jets push pH up, and bromine loses much less of its strength when that happens. Chlorine is usually cheaper and easier to measure exactly, which suits small tubs you dose after every use.
One practical rule: once bromide is in the water, any chlorine you add turns it back into bromine, so a "chlorine" test will actually be reading bromine. If you want to switch a bromine tub to chlorine, drain and refill first. For chlorine doses, see chlorine by tub size.
Bromine too high: what to do
High bromine usually comes from a floater set too open, a floater left under a closed cover, or a shock added on top of an already high level. Signs are itchy or dry skin and a harsh smell. To bring it down:
- Take the floater out, or close the feeder.
- Open the cover and run the jets and air controls. Heat, air, and time use bromine up.
- Don't shock until it's back in range, since shock creates more bromine from the bank.
- If you need the tub soon, drain part of the water and refill with fresh water. Replacing a quarter of the water lowers bromine by about a quarter.
- Stay out until the reading is back to 6 ppm or less, or the maximum on your product label.
Safety
- Never mix bromine tablets with chlorine products, or put them in a feeder that has held chlorine tablets: the combination can react.
- Keep tablets dry and sealed, away from acids and other pool chemicals. Wear gloves.
- Keep the water at 104 °F or lower (CPSC).
- If someone swallows a tablet or gets product in their eyes, call Poison Control at 1-800-222-1222.
- Always follow the product label. It is a violation of federal law to use a pesticide product in a manner inconsistent with its labeling.
FAQ
What happens if you put too much bromine in a hot tub?
Skin and eyes can get irritated, and the water smells harsh. The level falls on its own with the cover off and the jets running. A partial drain speeds it up. Stay out until it is back to 6 ppm or your label's maximum.
Do you need to shock a hot tub if you use bromine?
Yes. Shock turns used-up bromide back into active bromine and breaks down sweat and lotions. Weekly, and after heavy use, is a common routine. MPS or chlorine shock both work.
How often should I add bromine to my hot tub?
With a floater, tablets feed continuously: refill when they are about half gone, often every week or two depending on use and temperature. Test two or three times a week and adjust the vents to hold 4–6 ppm.
Sources
- CDC, Model Aquatic Health Code 2024, Annex (spa bromine 4–6 ppm, 2.25 factor, active bromine and chlorine by pH, CYA does not stabilize bromine): cdc.gov (PDF)
- CDC, Home pool and hot tub water treatment and testing: cdc.gov
- U.S. Consumer Product Safety Commission, hot tub temperatures: cpsc.gov