What does BTU stand for? It stands for British thermal unit, a unit for measuring heat energy. One BTU is about the heat needed to raise one pound of water by one degree Fahrenheit, roughly the heat given off by a single kitchen match burning all the way down.
In the United States, the term shows up on air conditioners, furnaces, water heaters, space heaters, gas grills and even utility bills. On most appliances, the number really means BTUs per hour: how much heat the machine can add to a room, or pull out of it, in one hour of running.
Quick facts
Stands for
British thermal unitType
InitialismMeasures
Heat energyEqual to
About 1,055 joulesOn appliances
Usually BTU per hour (BTU/h)Also written
Btu, especially in engineeringRelated terms
Therm, ton of cooling, HVAC
What does BTU stand for, and what does it measure?
Each letter stands for one word:
| Letter | Word | Meaning |
|---|---|---|
| B | British | The unit came from the old British system of pounds and degrees Fahrenheit |
| T | Thermal | Having to do with heat |
| U | Unit | A fixed amount you can count and compare |
Most of the world measures energy in joules or kilowatt-hours. The United States still uses BTUs for heating, cooling and fuel, so the term stays on boxes, labels and spec sheets here. A BTU is a small amount of energy, which is why appliance ratings run into the thousands.
What does BTU stand for in AC units?
On an air conditioner, the BTU rating tells you how much heat the unit can remove from a room in one hour. A higher number means more cooling power. A small window unit might be rated around 5,000 BTU, enough for a bedroom, while a central system for a whole house is much larger.
Central air systems are often sized in tons. One ton of cooling equals 12,000 BTU per hour, a figure that comes from the heat needed to melt one ton of ice over a full day. So a 3-ton system moves about 36,000 BTU per hour. Learn more about the whole field on the HVAC page.
On a window AC box: "8,000 BTU, cools up to 350 sq. ft." The maker is saying the unit can pull about 8,000 BTUs of heat out of the air each hour, which suits a room of that size.
Choosing the right size
A rule of thumb often quoted for cooling is about 20 BTU per hour for each square foot of living space. A room of 300 square feet would need around 6,000 BTU. Adjust for real conditions:
- Add capacity for a sunny room, a kitchen or a room where many people gather.
- Lower it a bit for a shaded room.
- Do not oversize. A unit that is too large cools quickly and shuts off, leaving the air damp.
For a whole house, a heating and cooling contractor should run a proper load calculation, which looks at insulation, windows and climate.
BTUs on heaters, grills and gas bills
- Furnaces and boilers: rated by input BTUs (fuel burned) and output BTUs (heat delivered). The difference shows the efficiency.
- Space heaters: a typical plug-in electric heater gives about 5,100 BTU per hour at its high setting.
- Gas grills and stoves: burner ratings in BTUs tell you how much fuel they can burn per hour, not how evenly they cook.
- Utility bills: natural gas is often billed in therms. One therm is 100,000 BTU.
Contractors also use MBH, which means thousands of BTUs per hour. A 60 MBH furnace is a 60,000 BTU per hour furnace.
Writing and saying BTU
Say it letter by letter, "B T U." The plural is BTUs, though product labels often skip the s: "a 10,000 BTU unit." Engineering and government energy reports frequently write Btu with only the B capitalized. Both spellings are correct. Use a before it, since B begins with a consonant sound: "a BTU rating."
Do not confuse BTU with CFM, cubic feet per minute, which measures how much air a fan moves, not how much heat. You can find more everyday terms like these in our famous acronyms list.
Questions people ask
How many BTUs do I need for my room?
A common starting point for cooling is about 20 BTU per hour per square foot. Add more for sunny rooms or kitchens, and ask a contractor for a load calculation when sizing a whole-house system.
Is a higher BTU better?
Only up to the size of your space. Too few BTUs and the room stays warm; too many and the unit cycles on and off and leaves the air humid.
What is the difference between BTU and BTU/h?
BTU is an amount of energy. BTU/h is a rate, BTUs per hour. Appliance labels usually say BTU when they mean BTU per hour.
How many BTUs are in a ton of cooling?
One ton of cooling equals 12,000 BTU per hour. A 2-ton AC system can remove about 24,000 BTU of heat each hour.
Why is it called British?
The unit was built on the British imperial measures of the pound and the degree Fahrenheit. Britain itself now uses metric units for most energy measurements.