Hot Tub Water Change Calculator: When to Drain Next

"Every three months" is the answer most people hear, but how long hot tub water lasts depends on two things: how much water there is and how many people sit in it. Every soak leaves sweat, skin oils, lotion and the residue of the chemicals used to deal with them, and a small tub used daily fills up with that much faster than a big one used at weekends. The rule of thumb many hot tub makers print in their manuals is: the tub's volume in US gallons, divided by 3, divided by the average number of bathers a day, gives the days between water changes. Enter your tub's size and how you use it below to get the number of days and the date to drain it next.

How the calculation works

The formula treats a tub's water as having a fixed capacity for bathers. Divide the volume in US gallons by 3 and you get the number of "bather days" the water can take before it is worth replacing; divide that by how many people use it on an average day and you get the days between changes. The calculator converts litres and UK gallons to US gallons first (one US gallon is 3.785 litres), works out the average bathers a day from your sessions per week, then applies the formula.

Worked through for a typical 1,000 litre tub: that is 264 US gallons, and 264 divided by 3 is about 88 bather days. Two people using it four times a week is 8 bathers a week, or 1.14 a day, so the water lasts about 77 days, or 11 weeks. Use the same tub every evening with four people and it drops to about 22 days.

If your manual gives a maximum gap between water changes, enter it and the calculator uses whichever comes first. The formula is a planning guide, not a test result, so the water can need changing sooner, and lightly used tubs still need fresh water now and then.

Signs the water needs changing early

• Foam that keeps coming back. Body oils, lotions and detergent left on swimwear build up in the water and foam when the jets run.
• Cloudy or dull water that does not clear after you correct the pH and dose sanitiser.
• Chemicals that will not hold. If the chlorine or bromine reading drops to nothing soon after a dose, or the pH will not stay in range, the water has taken on more than the sanitiser can handle. The hot tub chlorine calculator helps you dose correctly before you decide to drain.
• A strong chemical smell. A sharp "chlorine" smell usually comes from used-up chlorine compounds, not from too much chlorine.
• A heavy party or a lot of guests. One weekend with a full tub every night can use up weeks of the water's capacity. Enter the extra sessions above to see the effect.

Holiday lets and rental hot tubs

If you let out a cottage or lodge with a hot tub, the formula above is not the standard you are held to. The HSE's guidance on spa pools, HSG282, says that for domestic-type hot tubs used as part of a business the water should be changed after each rental or each week, whichever is sooner. Choose "Holiday let or rental guests" above and the calculator applies that rule. Our guide to spa pool water safety and HSG282 covers the testing it expects too.

Before you drain

• Switch the power off at the isolator first. Running a heater or pump with no water in it can damage it.
• Many makers recommend a line flush a day or so before draining, to clear residue from the pipework so it does not foul the fresh water. Follow your manual for the product and method.
• Check where the water can go. Treated water can harm plants, ponds and wildlife. Let the sanitiser level fall before draining, and ask your water company or council if you are unsure where to send it.
• Cost it. Each refill means fresh water and a full heat-up from cold. The hot tub running cost calculator shows what each drain and refill adds to the bill.