Specific Energy Converter
Convert specific energy between J/kg, kJ/kg, MJ/kg, kWh/kg, BTU/lb, and kcal/kg.
Used in fuel energy density, battery capacity, and thermodynamics calculations.
Type in any field — the others update instantly.
Specific energy is the amount of energy stored per unit mass of a substance. It is sometimes called gravimetric energy density and is expressed in joules per kilogram (J/kg) or equivalent units.
Why specific energy matters:
When comparing fuels, batteries, or food, we care about how much energy is packed into each kilogram — not just the total energy. A fuel tank full of gasoline has more energy than a tank of water because gasoline has much higher specific energy.
Key reference values:
| Material | Specific Energy |
|---|---|
| Hydrogen (gas) | ~142 MJ/kg |
| Gasoline | ~44 MJ/kg |
| Diesel | ~43 MJ/kg |
| Coal | ~24–35 MJ/kg |
| TNT (explosive) | ~4.6 MJ/kg |
| Lithium-ion battery | ~0.5–0.9 MJ/kg |
| Wood | ~15–20 MJ/kg |
| Food fat | ~37 MJ/kg (≈ 9 kcal/g) |
| Food carbohydrates | ~17 MJ/kg (≈ 4 kcal/g) |
Unit conversions:
- 1 kJ/kg = 1,000 J/kg
- 1 MJ/kg = 1,000,000 J/kg
- 1 BTU/lb = 2,326 J/kg
- 1 kcal/kg = 4,184 J/kg
- 1 kWh/kg = 3,600,000 J/kg
Applications:
Specific energy is used in aerospace engineering (fuel selection for rockets and aircraft), battery technology (comparing lithium-ion vs. solid-state vs. hydrogen fuel cells), nutrition science (caloric density of foods), and military applications (explosive yield comparisons). The higher the specific energy, the more “bang per kilogram” you get.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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