How these calculators work

Every tool here runs in your browser, on numbers you type. Nothing is sent anywhere. No price is looked up for you — if a tool needs your power rate, you enter it.

What they are for

These are planning estimates. They are good enough to compare choices. Is a better cover worth it? Should you drop the setpoint? Is 6 kW enough, or do you need 8?

They are not a site survey. They do not replace the maker's own chart, and they do not replace an electrician working out your load.

Why some answers are a range

Real heat loss depends on insulation, draughts, wind and how you use the room. A short form cannot capture all that. So where the honest answer is a range, we show a range.

Heater sizing is the clearest case. It is a trade convention, not a law of physics. We show a band of sizes you can actually buy.

Sauna running cost

We split each session in two. Heating up from cold draws close to full power. Holding the heat draws much less, because the heater cycles on and off.

You set that holding share yourself. Weeks become years at 52.18 weeks, and a month is that year divided by twelve. So the weekly, monthly and yearly figures always agree.

Hot tub running cost

Three separate costs. First, heat leaking out, worked out from how many watts escape for each degree between the water and the air. Second, the pump: watts times hours. Third, reheating, from how much energy it takes to warm water.

That heat-loss figure is a rough measure set by your cover choice, not a full model of the tub. You can change it if you know your own.

Heater sizing

Start with the room volume. Then add extra for surfaces that lose heat fast: about 1.2 m³ for each square metre of glass, and 1.5 m³ for each square metre of stone, tile or concrete.

Multiply the total by a sizing rule, usually 1 kW per cubic metre. Then we round out to real heater sizes. You can change both the rule and the extras, because makers differ.

Where these stop being reliable

• Wind is not counted. An exposed tub loses more than we show.
• Heat pump tubs use three to five times less power than the electric heating we assume.
• Sizing rules vary by brand. Check the chart for the heater you are buying, including its smallest allowed room.
• If your power price changes by time of day, use the rate for the hours you would run.

Found a mistake?

These are rules of thumb, written down so you can check them. If a number here looks wrong to you, it may well be.