What is being measured?
Energy is a quantity that can be transferred as work or heat. A unit conversion does not determine which physical process produced it.
Calories are thermochemical. BTU uses the international-table definition. Foot-pounds mean foot-pounds force.
Meaning of fJ
Femtojoules uses the symbol fJ in this library and measures energy. Under the stated reference convention, 1 fJ = approximately 1e-15 J. This equivalence changes how a known value is written. It does not increase measurement accuracy or establish whether a measurement is appropriate to a real application.
Conversion method
Multiply the value by the source factor (1e-15), then divide by the target factor (1). The factor ratio expresses the same energy measurement in the other unit.
Worked example
For a known value of 10 fJ, the reference conversion gives 1e-14 J. Converting that result back gives approximately 10 fJ. This reverse check helps catch a reversed factor, but matching round-trip arithmetic alone does not independently verify a source definition.
Common reference values
| Input (fJ) | Reference equivalent (J) |
|---|---|
| 1 | 1e-15 |
| 5 | 5e-15 |
| 10 | 1e-14 |
| 25 | 2.5e-14 |
| 50 | 5e-14 |
| 100 | 1e-13 |
Compare one fJ with other units
| Target unit | Equivalent of 1 fJ | Definition guide |
|---|---|---|
| Joules (J) | 1e-15 | Read definition |
| Kilojoules (kJ) | 1e-18 | Read definition |
| Kilowatt-hours (kWh) | 2.77777777778e-22 | Read definition |
| Thermochemical calories (cal) | 2.39005736138e-16 | Read definition |
| International-table BTU (BTU IT) | 9.47817120313e-19 | Read definition |
| electronvolts (eV) | 6241.50907446 | Read definition |
| foot pounds (ft·lbf) | 7.37562149277e-16 | Read definition |
| kilocalories (kcal th) | 2.39005736138e-19 | Read definition |
Precision and common mistakes
- Keep the source and target quantity consistent. A matching symbol is not enough to establish a compatible definition.
- Record the original measurement precision. Showing twelve significant digits in a table does not make a rounded input more precise.
- Check symbol capitalization, prefixes and any regional convention in the source. Use the displayed unit definition rather than a similarly named unit.
- Factors involving repeating fractions or π use finite numerical precision. Round the final answer to the precision justified by your input.
Use a working converter
- Energy converter
- Femtojoules to Joules converter
- Femtojoules to Kilojoules converter
- Femtojoules to Kilowatt-hours converter
- Femtojoules to Thermochemical calories converter
- Femtojoules to International-table BTU converter
Source and verification scope
Examples and tables are computed from the versioned unit definitions used by the linked converters. The source access dates appear below. Computandum is independent of the cited organizations; a citation does not mean that a publisher endorses this page or an actual project.
Sources
- NIST / BIPM: NIST: unit conversion factors ↗ · 2026-10-08