Hot Tub Running Cost Calculator: Cost Per Day and Year

A hot tub has three separate bills hiding inside it: the big first heat-up from cold tap water, the steady trickle of power it takes to hold the water at temperature, and the extra you add every time the cover comes off and the bubbles go on. Most "it costs a pound a day" claims only count one of them. Put your own tub's figures and your tariff in below and the calculator adds up all three, plus the water and power for every drain and refill.

What the calculator counts

The first heat-up is pure physics. Warming one litre of water by one degree takes about 1.16 watt hours, so the energy is the volume times the temperature rise times that figure. The heat-up time shown is the shortest it could be with every kilowatt going into the water; in practice some heat escapes while it warms, especially outdoors in winter, so expect it to take longer. That is why inflatable tubs with a small heater can take the best part of a day or more from a cold fill.

Keeping it warm is the cost that runs every day whether you get in or not. A spec-sheet standby figure is the easy starting point, but it is measured under test conditions with the cover on. On a cold, windy night your tub will lose more heat than that, so the most honest number is your own: plug the tub into a smart plug with an energy reading, or note your meter at the same time on two days nobody uses it, and switch the calculator to the measured option.

Soaks add the blower or jet pump running time. They also add the heat that escapes while the cover is off, which is not in this figure and is hard to predict without measuring. If you read your meter over a normal week of use, dividing by seven and entering that as your keep-warm figure captures both at once; just set the bubble minutes to zero so nothing is counted twice.

Refills repeat the first heat-up every time, plus the water itself if you are on a meter. Water is charged per cubic metre, which is 1,000 litres, and most companies bill wastewater on top of the supply charge.

Where the money actually goes

• The cover. Nearly all the keep-warm cost is heat leaking out, and the top is where most of it goes. A cover that is waterlogged, split or not strapped down will push the daily figure up more than anything else you can change.
• The set temperature. Heat loss grows with the gap between the water and the air. Dropping a degree or two between uses costs less than holding it at the top setting all week, though very large drops just mean a bigger reheat later.
• Your tariff. Every figure moves in proportion to the unit rate. If you have an off-peak or time of use tariff and your tub can be set to heat in that window, run the calculator at the cheaper rate to see the difference.

If you are weighing up a hot tub against other ways to unwind at home, the sauna running cost calculator does the same sum for a home sauna and the ice bath calculator works out how much ice a cold plunge needs. For a warm soak without the upkeep, our round-up of private spa hire across the UK lists places you can book a hot tub by the hour.