Definition
A unit of heat energy equal to the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
Plain English
A way of measuring heat. One Btu is roughly the heat given off by burning a single wooden match.
Context Anchor
Seen in powerplant and maintenance discussions about fuel heat content, engine heat, heaters, and cooling systems.
Derivation
Called 'British' because it uses pounds and Fahrenheit (Imperial units), as opposed to the metric 'calorie' or 'joule.' 'Thermal' comes from the Greek 'therme,' meaning heat. So a British thermal unit is simply a heat measurement built on the British/Imperial system.
Why Pilots Care
Fuel energy and engine heat output are often expressed in Btu. Knowing what a Btu represents helps a technician understand fuel ratings, heat transfer in engines, and the performance of cabin heating systems.
Grounding Statement
Picture one pound of water (about a pint) sitting in a cup. The heat needed to warm it by just one degree Fahrenheit is one Btu.
Intuition Check
A Btu is not a temperature. It is an amount of heat energy that can cause a temperature change.
Example Sentence 1
Aviation gasoline contains roughly 20,000 Btu of energy per pound, which is why even a small amount of fuel can produce significant power.
Example Sentence 2
Maintenance records may list heater output in Btu to verify proper operation of cabin combustion heaters.