An honest comparison of the two main hot tub sanitisers, with the four questions that will decide which suits your household best.
For most UK hot tub owners, bromine is the better sanitiser: it is more stable at spa temperatures, smells less and is kinder on the skin, winning on three of the four counts that matter. Chlorine only edges ahead on cost and on weekend-only, dose-and-forget use. Both kill the same bugs; you hold chlorine at 3 to 5 ppm and bromine at 4 to 6 ppm.
The choice really comes down to four things: smell, skin sensitivity, the temperature you run at, and how often you can be at the tub to test. Here is how they compare on each.
Chlorine is faster to act and faster to drop off. It oxidises organic matter quickly and then dissipates, especially in warm water. You typically dose little and often. Whichever you choose, the dosing calculator gives the gram dose for your tub, and the water chemistry reference covers the target ranges for both sanitisers.
Bromine works in a slower, steadier way. It tolerates higher temperatures (it is the same active species at 30C and 40C, whereas chlorine becomes less effective above about 35C) and tends to live in floating feeders or cartridges that release it gradually, usually loaded with bromine tablets that dissolve slowly.
Bromine smells less. The compounds it forms after killing organics are less volatile than the chloramines chlorine produces, which is why bromine tubs feel less harsh on the eyes.
For sensitive skin, bromine is usually kinder. Eczema and dermatitis sufferers often switch to bromine and stop reacting.
Bromine is more stable at the temperatures hot tubs run at, which is the single biggest reason most UK hot tub specialists recommend it for a domestic tub.
If you only use the tub at weekends and want to dose-and-forget all week, chlorine in slow-dissolving tablets in an in-line feeder works well.
If you have a pool that you want to share chemicals with, chlorine across both makes life simpler.
If cost matters more than convenience, chlorine granules are noticeably cheaper per kilo than bromine tablets at retail, though fast-dissolving bromine granules remain a handy way to top up a bromine tub quickly.
Not cleanly. The two reseed each other's depleted cells in the water and you spend weeks chasing readings. Switch at a refill, following our guide to switching from chlorine to bromine.
They use different test strips and different drops. Both are simple but you must use the correct test for the sanitiser you have, or the readings are meaningless.
This guide is written and maintained under the ClearSpa editorial policy. Dose rates and target ranges mirror the product labels current at the time of the last review.