Femtojoule is 1e-15 joule units.
Femtojoules to BTU Converter — fJ to BTU
Convert Femtojoule (fJ) to BTU (BTU) using the exact conversion factor (1 femtojoule = 9.478171e-19 btu). See the formula, worked examples, conversion table, history, and common uses.
Femtojoules 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 1e-15 / 1055.055853.
Energy units are compatible.
Reserved above the conversion cards and below the converter.
About Converting Femtojoules to BTU
Femtojoule 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. Specifically: Femtojoule is 1e-15 joule units, and british thermal unit.
Formula
btu = femtojoules × 9.478171e-19
This factor comes from each unit's defined relationship to the category's base unit: 1 femtojoule equals 1e-15 base units, and 1 btu equals 1055.05585262 base units, so dividing one by the other gives the direct femtojoule-to-btu factor of 9.478171e-19.
Simple example
1 fJ × 9.478171e-19 = 9.478171e-19 BTU
1 femtojoule = 9.478171e-19 btu.
Real-world example
100 fJ × 9.478171e-19 = 9.478171e-17 BTU
100 femtojoules = 9.478171e-17 btu.
Conversion table
| Femtojoule (fJ) | BTU (BTU) |
|---|---|
| 1 fJ | 9.478171e-19 BTU |
| 10 fJ | 9.478171e-18 BTU |
| 50 fJ | 4.739086e-17 BTU |
| 100 fJ | 9.478171e-17 BTU |
| 500 fJ | 4.739086e-16 BTU |
| 1,000 fJ | 9.478171e-16 BTU |
Reverse conversion: BTU to Femtojoule
9.478171e-19 BTU × 1.055056e+18 = 0.9999999786 fJ
9.478171e-19 btu = 0.9999999786 femtojoules.
femtojoules = btu × 1.055056e+18
For a page dedicated to this direction, see BTU to Femtojoule.
Understanding the Femtojoule (fJ)
Femtojoule is 1e-15 joule units.
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.
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 femtojoule?
1 femtojoule equals 9.478171e-19 btu, using the exact defined conversion factor rather than an estimate.
How do I convert femtojoule to btu?
Multiply the femtojoule value by 9.478171e-19. The converter above does this instantly to whatever precision you set.
How do I convert btu back to femtojoule?
Use the reverse factor: 1 btu equals 1.055056e+18 femtojoules. You can also use the swap control in the converter above to flip the direction instantly.
What is a femtojoule?
Femtojoule (fJ) is a unit of engineering.
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 femtojoule 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.
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.