A thermochemical calorie is 4.184 joules.
Calories to BTU Converter — cal to BTU
Convert Calorie (cal) to BTU (BTU) using the exact conversion factor (1 calorie = 0.0039656668 btu). See the formula, worked examples, conversion table, unit comparison, history, and common uses.
Calories to BTU converter
Converter
Engineering Converter
Convert common stress, force, torque, power, and energy units used in engineering.
Context values
Some unit families need one extra value for a physically accurate result.
British thermal unit.
Result = input x 4.184 / 1055.055853.
Energy units are compatible.
Reserved above the conversion cards and below the converter.
About Converting Calories to BTU
Calorie and BTU both measure a quantity used in structural or mechanical engineering — commonly stress, pressure, force, torque, power, or energy — where converting between the unit conventions used in different regions, industries, or equipment specifications is a routine part of design work. Calorie is typically associated with nutrition labeling in the US and several other countries still commonly uses the kilocalorie (as 'Calorie'), while BTU is typically associated with united States (HVAC, heating, and appliance energy ratings).
Formula
btu = calories × 0.00396566683139
This factor comes from each unit's defined relationship to the category's base unit: 1 calorie equals 4.184 base units, and 1 btu equals 1055.05585262 base units, so dividing one by the other gives the direct calorie-to-btu factor of 0.00396566683139.
Simple example
1 cal × 0.003965666831 = 0.0039656668 BTU
1 calorie = 0.0039656668 btu.
Real-world example
100 cal × 0.003965666831 = 0.3965666831 BTU
100 calories = 0.3965666831 btu.
Conversion table
| Calorie (cal) | BTU (BTU) |
|---|---|
| 1 cal | 0.0039656668 BTU |
| 10 cal | 0.0396566683 BTU |
| 50 cal | 0.1982833416 BTU |
| 100 cal | 0.3965666831 BTU |
| 500 cal | 1.982833416 BTU |
| 1,000 cal | 3.965666831 BTU |
Reverse conversion: BTU to Calorie
0.0039656668 BTU × 252.1644007 = 0.9999999921 cal
0.0039656668 btu = 0.9999999921 calories.
calories = btu × 252.164400722
For a page dedicated to this direction, see BTU to Calorie.
Calorie vs. BTU
| Aspect | Calorie | BTU |
|---|---|---|
| Family | Non-SI unit (limited use with SI) | US customary unit |
| Typical use | Food energy labeling (as kilocalories, often written 'Calories') | Air conditioner and furnace capacity ratings in the US |
| Where it's used | Nutrition labeling in the US and several other countries still commonly uses the kilocalorie (as 'Calorie') | United States (HVAC, heating, and appliance energy ratings) |
Understanding the Calorie
Calorie (cal) measures energy. The thermochemical calorie used in this converter is defined as exactly 4.184 joules. It is classified as a Non-SI unit (limited use with SI).
Where it's used: Nutrition labeling in the US and several other countries still commonly uses the kilocalorie (as 'Calorie')
Calorie vs. Calorie (kilocalorie). Food labels' capitalized 'Calorie' is actually one kilocalorie (1,000 small calories) — a long-standing labeling convention that causes frequent confusion.
Uses of the Calorie today
- Food energy labeling (as kilocalories, often written 'Calories')
- Historical chemistry and thermodynamics contexts
History of the Calorie
The calorie was originally defined in the early 19th century as the heat needed to raise one gram of water by one degree Celsius, but different scientific traditions defined it at slightly different reference temperatures, producing several close but non-identical 'calorie' values historically (15 °C calorie, mean calorie, thermochemical calorie).
To resolve this ambiguity, the thermochemical calorie was fixed by convention at exactly 4.184 joules; the very similar International Table calorie is fixed at exactly 4.1868 joules. The nutritional 'Calorie' seen on food labels is actually a kilocalorie (1,000 calories).
Historical origin. Calorie was not the invention of a single named person; it developed through the process described above.
Understanding the BTU
BTU (BTU) measures energy. A British thermal unit is defined as exactly 1,055.05585262 joules (the International Table BTU). It is classified as a US customary unit.
Where it's used: United States (HVAC, heating, and appliance energy ratings)
Uses of the BTU today
- Air conditioner and furnace capacity ratings in the US
- Water heater and appliance energy specifications
- US energy industry (natural gas is often billed via BTU content)
History of the BTU
The BTU originated in 19th-century steam engineering as the heat needed to raise one pound of water by one degree Fahrenheit, paralleling the calorie's metric definition. Several slightly different BTU values existed historically before the 'International Table' BTU was standardized at exactly 1,055.05585262 joules.
Historical origin. BTU was not the invention of a single named person; it developed through the process described above.
Calorie — sources
- NIST Special Publication 811 — Documents the thermochemical calorie's exact joule equivalence.
BTU — sources
- NIST Special Publication 811 — Documents the International Table BTU's exact joule equivalence.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many btu are in 1 calorie?
1 calorie equals 0.0039656668 btu, using the exact defined conversion factor rather than an estimate.
How do I convert calorie to btu?
Multiply the calorie value by 0.003965666831. The converter above does this instantly to whatever precision you set.
How do I convert btu back to calorie?
Use the reverse factor: 1 btu equals 252.1644007 calories. You can also use the swap control in the converter above to flip the direction instantly.
What is a calorie?
Calorie (cal) measures energy. The thermochemical calorie used in this converter is defined as exactly 4.184 joules. It is classified as a Non-SI unit (limited use with SI).
What is a btu?
BTU (BTU) measures energy. A British thermal unit is defined as exactly 1,055.05585262 joules (the International Table BTU). It is classified as a US customary unit.
Is the calorie to btu conversion exact?
Yes. A British thermal unit is defined as exactly 1,055.05585262 joules (the International Table BTU). That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
What is the difference between calorie vs. calorie (kilocalorie)?
Food labels' capitalized 'Calorie' is actually one kilocalorie (1,000 small calories) — a long-standing labeling convention that causes frequent confusion.
Where is the calorie still used today?
Nutrition labeling in the US and several other countries still commonly uses the kilocalorie (as 'Calorie')