Specific Energy Converter: J/kg, Btu/lb, cal/g & More
The ultimate specific energy converter for measuring fuel efficiency (mass), heating values, and thermodynamic constants.
Convert energy per mass (J/kg, Btu/lb, cal/g) and mass per energy (kg/J, lb/Btu, g/kWh) for fuel efficiency, heating value, and thermodynamics. Covers SI, imperial, and automotive units (lb/(hp·h), g/(kW·h)). Free, runs in your browser—no sign-up required.
What is specific energy?
Specific energy is energy per unit mass, usually expressed in joules per kilogram (J/kg) in SI. It shows how much energy is stored or released per kilogram of material—useful for fuels (heating value), batteries, and food. The same quantity is sometimes given as mass per energy (e.g. kg/J or lb/Btu), especially in fuel economy: “how much mass of fuel per unit energy.” This converter supports both forms so you can switch between J/kg, Btu/lb, cal/g, and inverse units like lb/(hp·h) or g/(kW·h) in one place.
High specific energy means more energy per kilogram (e.g. gasoline vs. battery mass). Engineers and chemists use it for heating values, specific enthalpy, and fuel comparison; automotive specs often use mass-per-energy for engine efficiency.
How specific energy conversion works
All energy-per-mass units are converted via the SI base J/kg. Multiply your value by the source unit’s factor (how many J/kg one unit equals), then divide by the target unit’s factor:
For mass-per-energy units (e.g. kg/J, lb/Btu, g/(kW·h)), the converter uses the reciprocal: specific energy in J/kg = 1 ÷ (value in mass-per-energy with the right scale). So 1 kg/J = 1 J/kg; 1 g/(kW·h) corresponds to 3.6×10⁹ J/kg.
Example: convert 1 Btu/lb to J/kg. 1 Btu (IT)/lb ≈ 2326 J/kg. Convert 0.5 lb/(hp·h) to J/kg: that’s about 2.96×10⁶ J/kg. The tool above handles all 15 units and both directions.
Key units explained
Energy-per-mass (to J/kg): J/kg and kJ/kg are SI. Cal/g and Btu/lb are common in heating value and fuel specs. Mass-per-energy units are used in fuel economy and efficiency.
Joule per kilogram (J/kg)
Factor: 1 (SI base)
Standard unit for specific energy. Used in thermodynamics, heating value, and battery energy density.
Btu (IT) per pound [Btu/lb]
Factor: 1 Btu/lb ≈ 2326 J/kg
Common in US fuel and HVAC specs. Heating value of fuels is often given in Btu/lb.
Calorie (IT) per gram [cal/g]
Factor: 1 cal/g ≈ 4186.8 J/kg
Used in chemistry and food energy (e.g. 4 kcal/g for fat). Cal (th)/g uses 4184 J/kg.
Pound per horsepower-hour [lb/(hp·h)]
Mass per energy (inverse)
Fuel consumption: mass of fuel per mechanical hp·h. Lower is more efficient. Common in automotive and marine.
Gram per kilowatt-hour [g/(kW·h)]
Mass per energy (inverse)
Used for battery or fuel cell efficiency: mass of fuel or electrode per kWh. 1 g/(kW·h) = 3.6×10⁹ J/kg equivalent.
Kilogram per joule [kg/J]
Inverse: 1 kg/J = 1 J/kg (reciprocal)
Mass per unit energy. Rare in practice; kg/kJ (1000× smaller) and lb/Btu are more common.
When to use specific energy vs fuel efficiency
Specific energy (J/kg, Btu/lb, cal/g) answers “how much energy per kilogram?”—ideal for comparing fuels, heating values, and battery energy density. Mass per energy (lb/Btu, g/(kW·h), lb/(hp·h)) answers “how much mass per unit energy?”—used in fuel economy and efficiency (e.g. brake-specific fuel consumption). Both describe the same physical relationship; this converter lets you switch between them and between SI, imperial, and calorie-based units so you can match textbooks, datasheets, or regional standards.
Who uses a specific energy converter?
Engineers and chemists convert heating values (Btu/lb, cal/g) to J/kg for calculations. Automotive and marine teams work with lb/(hp·h) and g/(kW·h) for fuel consumption. Battery and energy storage use J/kg or Wh/kg for energy density. Students and educators need to move between SI and imperial or between energy-per-mass and mass-per-energy. All conversions run in your browser—no data is sent to a server.
Common conversions at a glance
Handy reference for J/kg to Btu/lb, cal/g to J/kg, and inverse units.
| From | To | Formula | Example |
|---|---|---|---|
| Btu/lb | J/kg | × 2326 | 1 Btu/lb ≈ 2326 J/kg |
| cal/g | J/kg | × 4186.8 | 1 cal (IT)/g ≈ 4187 J/kg |
| kJ/kg | J/kg | × 1000 | 1 kJ/kg = 1000 J/kg |
| lb/Btu | J/kg | 2326 ÷ value | 0.00043 lb/Btu ≈ 2326 J/kg |
| g/(kW·h) | J/kg | 3.6×10⁹ ÷ value | 1 g/(kW·h) → 3.6×10⁹ J/kg |
Avoiding common mistakes
Don’t mix energy per mass (J/kg, Btu/lb) with mass per energy (kg/J, lb/Btu)—they are reciprocals. Double-check whether your source uses “energy per kg” or “kg per joule.” Calorie (IT) and calorie (th) differ slightly (≈4186.8 vs 4184 J per gram); this converter supports both. For lb/(hp·h) and g/(kW·h), mechanical hp (745.7 W) and metric hp (735.5 W) are different; we use the standard definitions so results match typical datasheets. When in doubt, convert to J/kg first, then to your target unit.
Specific Energy Conversion FAQ
? How do I convert Btu/lb to J/kg?
Multiply Btu (IT)/lb by about 2326. So 1 Btu/lb ≈ 2326 J/kg. The exact factor comes from 1 Btu (IT) ≈ 1055.06 J and 1 lb = 0.453592 kg. Use the converter above for other units like cal/g or lb/Btu.
? What is the SI unit for specific energy?
Joule per kilogram (J/kg). It’s energy (joule) per unit mass (kilogram). Kilojoule per kilogram (kJ/kg) is common for heating values and battery energy density. Mass-per-energy units like kg/J are dimensionally the reciprocal and are converted via 1/(value) with the correct scale.
? What is lb/(hp·h) and how do I convert it?
Pounds per horsepower-hour is fuel consumption: mass of fuel burned per mechanical hp·h of work. Lower values mean better efficiency. To get specific energy in J/kg, use the relationship 1 lb/(hp·h) ≈ 5.92×10⁶ J/kg (mechanically). The converter above does this and the metric equivalent g/(metric hp·h).
? What is the difference between calorie (IT) and calorie (th)?
IT uses the international steam table (1 cal ≈ 4.1868 J); th is thermochemical (1 cal ≈ 4.184 J). For specific energy, 1 cal (IT)/g ≈ 4186.8 J/kg and 1 cal (th)/g = 4184 J/kg. The difference is small; we support both so you can match your source.
? How do I convert g/(kW·h) to J/kg?
Gram per kilowatt-hour is mass per energy. 1 kW·h = 3.6×10⁶ J and 1 g = 0.001 kg, so 1 g/(kW·h) corresponds to (3.6×10⁶)/(0.001) = 3.6×10⁹ J/kg. So specific energy in J/kg = 3.6×10⁹ ÷ (value in g/(kW·h)). The converter handles this and the inverse.
? What is specific energy used for in heating value and batteries?
Heating value (e.g. of fuels) is often in kJ/kg or Btu/lb—energy released per unit mass. Battery energy density is in Wh/kg or J/kg. Specific energy is the same dimension: energy per mass. Use this converter to switch between J/kg, Btu/lb, cal/g, eV/amu, and mass-per-energy units like lb/(hp·h) or g/(kW·h) for engines and efficiency.