A small, old unit of heat
A British thermal unit is the heat needed to raise one pound of water by one degree Fahrenheit. In metric terms that is about 1,055 joules, which makes a single BTU a tiny amount of energy, roughly what a small candle gives off in a few seconds.
The naming has aged badly. Britain specifies heating in watts and has done for decades. The unit is now mostly an American one, and it survives here on the sides of radiator boxes and air conditioners because that is what the trade got used to quoting.
The bit that trips people up: it is a quantity, and it is used as a rate
A radiator described as 5,000 BTU is not holding 5,000 BTU. It is producing them, at 5,000 an hour. The correct unit is BTU per hour, and essentially nobody writes the per hour, including the people printing the boxes.
It is the difference between miles and miles per hour, and it is the whole reason a unit this simple feels slippery to somebody comfortable with watts. A watt is already a rate, so nothing has been dropped. Read every appliance BTU figure as BTU per hour and the unit stops misbehaving.
Converting it, in the two directions you will need
- Watts to BTU per hour: multiply by 3.412. A 1,500W radiator is 5,118.
- BTU per hour to watts: divide by 3.412. A 10,000 BTU unit is about 2,930W.
- Kilowatts to BTU per hour: multiply by 3,412. Near enough 3,400 in your head.
- Tons to BTU per hour: multiply by 12,000. A three ton system is 36,000.
Where the ton comes from, because it sounds made up
It is about ice, and it is one of the better surviving fossils in engineering. Before mechanical cooling, buildings were cooled with delivered ice, so capacity was naturally described by how much ice a machine replaced. Melting a short ton of ice over twenty four hours needs about 288,000 BTU, which is 12,000 BTU per hour, and that became one ton of refrigeration.
It is worth knowing because it is the unit that makes American and Australian air conditioning impossible to compare at a glance. A three ton American unit and a 10.5kW Australian one are the same machine.
What size you need is a different sum
Knowing the unit does not tell you how many of them a room wants, and that calculation is where the real trap sits. The figure printed on a radiator is measured under standard test conditions with the water considerably hotter than most systems actually run, so the output you get in your house is lower than the box says, sometimes by a lot.
The radiator output tool is entirely about that gap and what to multiply by, which matters enormously if there is a heat pump in the picture. For the room itself, the heat loss calculator works out what it needs in the first place, and the boiler sizing tool adds those room figures up into the number that decides the appliance.
Common questions
What does BTU actually stand for?
British thermal unit, which is a small quantity of heat: roughly the energy needed to raise one pound of water by one degree Fahrenheit, or about 1,055 joules. The name is a historical joke at this point. Britain went metric decades ago and specifies heating in watts, while the unit that carries its name is now used mainly in the United States, and survives here almost entirely on radiator boxes and air conditioner labels.
How many BTU is a kilowatt?
One watt is 3.412 BTU per hour, so a kilowatt is about 3,412 BTU per hour and a 1.5kW radiator is roughly 5,118. Going the other way, divide a BTU figure by 3.412 for watts. The rough mental version that is close enough in a showroom is to divide BTU by 3,400 for kilowatts, or multiply kilowatts by 3,400 to get BTU.
Is a higher BTU always better for a radiator?
No, and oversizing has real costs. A radiator too large for its room either overshoots the temperature or spends its life cycling on and off, which is uncomfortable and wasteful, and it takes wall space you wanted. It also stops being a straightforward win once a heat pump is involved, because output figures are quoted at a flow temperature much higher than a heat pump runs at, so the number on the box is not the number you will get.
What is a ton of air conditioning?
Twelve thousand BTU per hour, and the name is genuinely about ice. The unit dates from when cooling was measured against the rate at which a ton of ice melts over a day, which works out at 12,000 BTU per hour, and it stuck. So a three ton system is 36,000 BTU per hour, or about 10.5kW. It is an American convention: Australian and British units are sold in kilowatts, which is why an Australian air conditioner and an American one of the same capacity have completely different numbers on the box.
Why do British radiators use an American unit?
Trade habit, and it has outlasted every reason for it. The heating industry here quoted output in BTU per hour long before metrication, merchants and installers learned the numbers in that unit, and catalogues kept printing them because that is what buyers recognised. Modern specification sheets carry watts, often alongside the BTU figure, and building calculations are done in watts. So the BTU number on the box is now essentially a legacy label for the retail customer.
Is BTU a measure of energy or of power?
Energy, which is exactly why it causes confusion, because almost every use of it you will meet is a power figure with the units abbreviated. A radiator or an air conditioner has a RATE of output, properly BTU per hour, and everybody including the manufacturers writes it as BTU. Treat any BTU figure on an appliance as BTU per hour. Where the distinction genuinely matters is gas, which is billed by the actual quantity of energy delivered.