British thermal unit.
BTU to Millijoules Converter — BTU to mJ
Convert BTU (BTU) to Millijoule (mJ) using the exact conversion factor (1 btu = 1,055,055.853 millijoule). See the formula, worked examples, conversion table, history, and common uses.
BTU to Millijoules 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.
Millijoule is 0.001 joule units.
Result = input x 1055.055853 / 0.001.
Energy units are compatible.
Reserved above the conversion cards and below the converter.
About Converting BTU to Millijoules
BTU and Millijoule 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. BTU is most often used for air conditioner and furnace capacity ratings in the us.
Formula
millijoules = btu × 1055055.85262
This factor comes from each unit's defined relationship to the category's base unit: 1 btu equals 1055.05585262 base units, and 1 millijoule equals 0.001 base units, so dividing one by the other gives the direct btu-to-millijoule factor of 1055055.85262.
Simple example
1 BTU × 1055055.853 = 1,055,055.853 mJ
1 btu = 1,055,055.853 millijoules.
Real-world example
100 BTU × 1055055.853 = 105,505,585.3 mJ
100 btu = 105,505,585.3 millijoules.
Conversion table
| BTU (BTU) | Millijoule (mJ) |
|---|---|
| 1 BTU | 1,055,055.853 mJ |
| 10 BTU | 10,550,558.53 mJ |
| 50 BTU | 52,752,792.63 mJ |
| 100 BTU | 105,505,585.3 mJ |
| 500 BTU | 527,527,926.3 mJ |
| 1,000 BTU | 1,055,055,853 mJ |
Reverse conversion: Millijoule to BTU
1 mJ × 9.478171e-7 = 9.478171e-7 BTU
1 millijoule = 9.478171e-7 btu.
btu = millijoules × 9.478171e-7
For a page dedicated to this direction, see Millijoule to BTU.
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.
Understanding the Millijoule (mJ)
Millijoule is 0.001 joule units.
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 millijoules are in 1 btu?
1 btu equals 1,055,055.853 millijoules, using the exact defined conversion factor rather than an estimate.
How do I convert btu to millijoule?
Multiply the btu value by 1055055.853. The converter above does this instantly to whatever precision you set.
How do I convert millijoule back to btu?
Use the reverse factor: 1 millijoule equals 9.478171e-7 btu. You can also use the swap control in the converter above to flip the direction instantly.
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.
What is a millijoule?
Millijoule (mJ) is a unit of engineering.
Is the btu to millijoule 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.
Where is the btu still used today?
United States (HVAC, heating, and appliance energy ratings)
Why do engineering unit conventions vary so much by region and industry?
Pressure, force, and stress can be expressed in metric (pascal, newton), US customary (psi, pound-force), or older and industry-specific units (atmosphere, torr, kip) depending on the country, industry, and even the equipment manufacturer, which is why engineers routinely need to convert between them when working across specs from different sources.