Short answer: 1 watt = 3.412 BTU per hour, so 1 kW = 3,412 BTU/hr. Going the other way, divide BTU/hr by 3.412 to get watts. A radiator sold as 5,000 BTU/hr delivers about 1,465 W, or 1.47 kW.
That is the whole conversion. The rest of this guide is a ready-reckoner, where the number comes from, and the two confusions that lead to the wrong radiator being ordered.
Ready-reckoner
Radiator catalogues to watts:
| BTU/hr | W | kW |
|---|---|---|
| 1,000 | 293 | 0.29 |
| 2,000 | 586 | 0.59 |
| 3,000 | 879 | 0.88 |
| 4,000 | 1,172 | 1.17 |
| 5,000 | 1,465 | 1.47 |
| 6,000 | 1,758 | 1.76 |
| 8,000 | 2,345 | 2.34 |
| 10,000 | 2,931 | 2.93 |
Watts and kilowatts to BTU/hr:
| kW | W | BTU/hr |
|---|---|---|
| 0.5 | 500 | 1,706 |
| 1 | 1,000 | 3,412 |
| 1.5 | 1,500 | 5,118 |
| 2 | 2,000 | 6,824 |
| 3 | 3,000 | 10,236 |
| 5 | 5,000 | 17,061 |
For arithmetic in your head: BTU/hr × 0.3 ≈ watts (a little under), and kW × 3,400 ≈ BTU/hr (a little under). Both errors are on the side of slightly understating, so check against the table before ordering.
Where 3.412 comes from
A British thermal unit is the heat needed to raise one pound of water by one degree Fahrenheit. In modern units that is 1,055.06 joules. A watt is one joule per second, so:
- 1 BTU per hour = 1,055.06 J ÷ 3,600 s = 0.2931 W
- 1 W = 1 ÷ 0.2931 = 3.4121 BTU/hr
The BTU Calculator carries the constant as 3.412142 and applies it once, at the end, after the room’s heat loss has been worked out in watts. Everything before that step is metric, because U-values and areas are metric; the BTU figure is a translation for the catalogue.
Why both units are still in use
The UK heating trade never finished going metric. Boilers, heat pumps, immersion heaters and everything electrical are rated in kilowatts. Radiators, towel rails and a good deal of installer conversation are still in BTU per hour, because the catalogues were written that way decades ago and nobody wanted to reprint them.
So a heating engineer sizes a boiler in kW, sizes each radiator in BTU/hr, and converts in their head at the merchant’s counter. The BTU Calculator shows the room requirement in all three units at once so the conversion never has to happen by hand: compare the BTU/hr figure to the radiator catalogue and the kW figure to the boiler or heat pump output. Our guide on how many BTUs a room needs shows the calculation that produces those figures.
Two mix-ups that cause wrong orders
BTU is energy; BTU/hr is power. Strictly, a BTU is an amount of heat, like a joule or a kilowatt-hour. A radiator “rated at 5,000 BTU” is really rated at 5,000 BTU per hour: the rate it gives out heat, like a watt. The trade drops the “per hour” universally and it does no harm as long as you know it is there. Where it does harm is when someone compares a radiator’s BTU/hr to an energy figure in kWh and gets a nonsense answer.
kW is a rate; kWh is a quantity. The same distinction in metric. A 2 kW radiator delivers heat at 2 kW; running for three hours it delivers 6 kWh, which is what the gas or electricity bill counts. Radiator sizing is entirely about the rate, kW or BTU/hr. The bill is about the quantity. A radiator’s rating tells you nothing about running cost on its own; that depends on how long it runs and what the thermostat does.
The same number on an air conditioner
Air conditioners and portable coolers are also labelled in BTU, meaning BTU/hr of cooling. The conversion is identical: a 9,000 BTU/hr unit moves about 2.64 kW of heat. The electrical input is much lower than that, which is the point of a heat pump in either direction, so do not read the BTU figure as the plug load.
One more: the radiator’s own rating
A radiator’s catalogue output, in either unit, is measured at a standard temperature difference between the water and the room, written ΔT50. On a system running cooler than a conventional boiler, such as a heat pump, the real output is well below the catalogue figure and the shortfall is not a fixed percentage. The conversion between BTU/hr and watts is unaffected; the number you are converting is what changes. The BTU Calculator’s heating system setting corrects for it.
The conversion constant is as implemented in BTU Calculator (3.412142 BTU/hr per watt) and follows from the International Table definition of the BTU, 1,055.06 J. Table values are rounded to the nearest whole watt or BTU/hr. Plan CD is not affiliated with any heating manufacturer or certification body.