Two definitions, one kind of energy
A BTU is defined by water and Fahrenheit: the energy needed to raise one pound of water by one degree Fahrenheit. A (small) calorie is the same idea in metric dress: the energy to raise one gram of water by one degree Celsius. Because a pound is 453.6 grams and a Fahrenheit degree is five-ninths of a Celsius degree, the two definitions land close together but not on top of each other — hence 252, not a round number. The food calorie, written with a capital C or as kcal, is 1,000 small calories, which is why a single BTU is only a quarter of one food calorie.
The conversion table that matters
In practice three conversions cover nearly every question. One BTU is 1,055 joules, the SI unit that underpins both; one kilocalorie is 4,184 joules, which is why 1 kcal equals 3.968 BTU; and one kilowatt-hour is 3,412 BTU, the figure that connects a heater's BTU rating to the electricity meter. A 24,000 BTU-per-hour air conditioner, for example, moves about 7 kilowatts of heat — 24,000 ÷ 3,412 — and a 500 kcal lunch, at 1,984 BTU, is roughly the heat a 2 kW heater gives off in 35 minutes. For quick conversions between energy units, use the BTU to kWh converter or the joules to calories converter.
Why heating kept BTUs and food kept calories
Neither industry had a reason to switch. Boiler, furnace and air-conditioner makers inherited the BTU from the steam age, when fuel was sold by the pound and water by the gallon; their catalogues, installer training and building regulations all speak BTU per hour, and a typical home boiler at 24,000–30,000 BTU per hour reads more naturally than 6,000–7,500 kcal per hour. Food labelling went the other way: nutrition science standardised on the kilocalorie (and the kilojoule alongside it in the UK and EU), because diets are counted in thousands, not in BTU. The split is a historical accident preserved by habit, not a difference in the physics — a joule is a joule whatever you call it.
The trap to avoid is mixing the two calories. If a workout tracker says you burned 252 calories, that is 252 kilocalories — a full 1,000 BTU, not one. And if a heater is rated at 5,000 BTU per hour, it is delivering about 1,260 kcal per hour of heat, not 1.26 million. Keep the small calorie and the food calorie apart, and the BTU falls neatly into place between them.