What chemicals does a hot tub need?
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Part of our pool and hot tub water guide
The tub is full and the dealer's shelf is crowded with tubs and bottles. What does the official guidance say a hot tub needs, and how do you check it at your own tub with a 20-in-1 test strip?
Short answer: The HSE's spa-pool guidance says spa-pool water is usually 30 to 40 °C and is filtered and chemically disinfected, and that 20 to 45 °C is the range where legionella and other microorganisms can grow rapidly. So a hot tub needs a disinfectant. Products and doses are on the maker's label, and we give none.
By Andrew Tomlinson, WaterKit · Last reviewed 29 October 2026
What chemicals does a hot tub need?
A disinfectant. The Health and Safety Executive (HSE) describes a spa pool as a self-contained body of warm, agitated water that is "filtered and chemically disinfected", in its spa-pool guidance (HSG282). You can check chlorine or bromine and pH at your own tub with a 20-in-1 test strip, rather than relying on a label alone.
That guidance is written mainly for people who manage or operate spa pools, including tubs used in a business such as a holiday let. The HSE says following it is not compulsory unless specifically stated, but that following it will normally be enough to comply with the law. In the part we could read, it names no products and gives no doses. Your tub's manual and dealer set those, and we name no brands. If you want to know about pH, our guide to raising or lowering pool pH covers the basics.
Why might my hot tub water need more checking than my tap water?
Because the HSE says spa-pool water carries more organic material from bathers than a swimming pool, which makes effective disinfection harder to keep up. The area figure is an average. Your tap is one point on the network, and your tub is warm water that changes after it is filled. A figure for the area does not tell you what comes out of your own tap, or what is in the tub a week after filling.
“If your tub is going to be used regularly, I would test the water in the tub itself rather than lean on the area figure or the label. What comes out of your tap and what sits in a warm, busy tub a few days later can be quite different things.”
These are the reasons the HSE gives.
- Bathers per litre. Spa pools have a much higher ratio of bathers to water volume than swimming pools, so the water holds more organic material from bathers.
- Disinfection is harder. The HSE says that high organic content makes it difficult to maintain effective disinfection.
- Warm, bubbling water. Water is usually kept in the 20 to 45 °C range where legionellae and other microorganisms can rapidly grow, and the jets produce aerosols that can be inhaled.
- Where bacteria come from. They can be introduced from the wider environment or from the water source itself. Pseudomonas aeruginosa may also come from the bathers.
The HSE says outbreaks of infection have been linked to spa-pool systems in leisure centres, hotels, holiday homes and on cruise ships. It does not say anything about your tub. Its guidance does include testing and monitoring spa-pool water quality, and a 20-in-1 strip shows where the chlorine or bromine and pH in your own tub sit on the day. Cloudy or green water is a visible sign that something has shifted, and our cloudy water guide and green hot tub water guide go through the usual causes.
When do you add chemicals, and how much?
Follow the maker's label and the tub's manual, because the HSE guidance we read gives no doses or timings. It says a spa pool is usually drained, cleaned or refilled after a number of bathers or a maximum period of time, rather than after each bather. For how often that is in practice, see our guide to changing hot tub water.
We found no official dosing table or waiting time after adding chemicals in the sources we read, so we give none. If your fill water is hard, how hard your water is is worth knowing before you fill, and how to test water hardness at home shows how.
Can you use a hot tub without chemicals?
We found no chemical-free method in the HSE guidance we read. It describes spa-pool water as filtered and chemically disinfected, and says the high organic content makes effective disinfection hard to keep up. It says spa-pool systems must be managed carefully so that water quality does not encourage microbial growth.
On how long a tub can stand untreated, we found no official time limit in the sources we read. The HSE says systems where water is stored or recirculated, kept at 20 to 45 °C and able to support microbial growth, could potentially allow legionella and other infectious agents to grow. We do not give a number of days.
How do you dispose of hot tub chemicals?
Ask your council. The HSE guidance we read covers controlling legionella and other infectious agents, not disposal, and we found no national guidance on hot tub chemicals in the sources we read. Check your own council's website for what it accepts before putting any product in household waste.
How can I check my hot tub water at home?
With a strip, a cup and about a minute. Fill a clean glass or cup from the tub, dip one strip, wait 30 to 60 seconds and match the pads to the chart, or enter the colours into our free results checker. You can also scan the QR code on the colour chart in the pack. The strip has pads for free chlorine, total chlorine, bromine, pH, total alkalinity and hardness. A reading indicates a colour step, which you compare with the targets your tub's maker or dealer gives. We do not give pH verdicts. The table is WaterKit's own reading of the chart steps.
| Pad | What it can indicate | What it cannot resolve |
|---|---|---|
| Free chlorine | Steps at 0, 0.5, 1, 3, 5, 10 and 20 mg/l | Values between steps, for example 2 or 4 |
| Bromine | Steps at 0, 1, 2, 5, 10 and 20 mg/l | Values between steps, for example 3 or 4 |
| Total alkalinity | Steps at 0, 40, 80, 120, 180 and 240 mg/l | Values between steps, for example 100 or 150 |
| pH | A colour step to match to the chart | Values between steps, for example 7.4 sits between the 7.2 and 7.6 steps (steps are 6.2, 6.8, 7.2, 7.6, 7.8 and 8.4) |
| Hardness | Steps at 0, 25, 50, 100, 250 and 425 mg/l, for a rough comparison of fill water and tub water | We have not checked whether the pad measures calcium alone or total hardness |
Several things are outside what the kit can do.
- It has no pad for PHMB, cyanuric acid or non-chlorine shock treatments.
- We have not checked how our chlorine and bromine pads respond to the other sanitiser, so use the pad that matches yours.
- The optional rapid bacteria test is a presence screen for tap water. We do not claim the test for hot tub water, and a strip result is not a legal sample.
For the method, see how to test your water at home.
The area figure is an average, and your own tub can differ from it. Your tap is one point on the network, and the HSE says spa-pool water carries more organic material from bathers, which makes disinfection harder to maintain, and that bacteria can come from the water source or the bathers. The pads that matter here are free chlorine or bromine (whichever you use) and pH. A lower chlorine or bromine step than at your last check would indicate the disinfectant is being used up, and a pH colour that has moved away from your maker's target would indicate it needs attention. Best pack: 20 strips, enough to test your fill water, then test daily for the first fortnight after a fill and add a few weekly checks (our arithmetic), £5.99 (or £10.99 with a bacteria test), with free 2nd Class delivery on every order. See the kit
Your hot tub chemical questions
What chemicals do I need for a hot tub?
A disinfectant, according to the HSE's description of spa-pool water as filtered and chemically disinfected. Other products and doses depend on your tub's manual.
Hot tub chemicals how to use
Follow the maker's label. We found no official step-by-step instructions in the sources we read.
What chemicals to put in a hot tub after filling, and how to set them up
We found no official fill-and-set-up routine in the sources we read, so follow your manual. A strip reading of the fill water first gives you a starting point.
How to add chemicals to a hot tub
Follow the label. The HSE guidance we read does not cover adding products.
When do you put chemicals in a hot tub?
We found no official schedule in the sources we read. The HSE says spa-pool water is usually drained, cleaned or refilled after a number of bathers or a maximum period of time.
How much chemicals to put in a hot tub?
We found no official dose beyond the label, so we give none. A strip reading shows where the water sits against your maker's targets.
Can you use a hot tub without chemicals?
We found no chemical-free method in the HSE guidance we read, which describes spa-pool water as chemically disinfected.
How to maintain a hot tub without chemicals
We found no official method in the sources we read.
How long can you leave a hot tub without chemicals?
We found no official time limit in the sources we read. The HSE says water kept at 20 to 45 °C can support rapid growth of legionella and other microorganisms.
How to dispose of hot tub chemicals
Ask your council. The HSE guidance we read does not cover disposal.
Sources
- Health and Safety Executive, HSG282: The control of legionella and other infectious agents in spa-pool systems (first edition, 2017; we read the introduction, paragraphs 4 to 8 on what a spa pool is, temperatures, bather ratio, microbes and aerosols, and the start of section 1; read 29 October 2026).
- WaterKit, Water Test Kit (20-in-1 strip, 30 to 60 second read, from £3.99, free 2nd Class delivery on every order) and results checker. The pad table is WaterKit's own reading of the colour chart steps.
Contains public sector information licensed under the Open Government Licence v3.0.