Fuel Efficiency - Mass Converter
Convert mass-based fuel efficiency between energy-per-mass units (joule per kilogram, calorie per gram, Btu per pound) and their reciprocals, mass-per-energy units such as gram per kilowatt-hour and pound per horsepower-hour. Pick two units, enter a value and read the result instantly.
Select two units to see the conversion factor.
This converter changes units of energy per mass and mass per energy only. It is not a distance-based fuel economy tool (mpg, km/L, L/100 km) and it does not convert to volume without a fuel density.
Fuel Efficiency (Mass) Formula
Every unit is related to one base unit, joule per kilogram (J/kg). A direct unit (energy per mass) is multiplied by its factor to reach J/kg and divided to leave it. A reciprocal unit (mass per energy) is the inverse: value in J/kg = factor ÷ value. For two direct units, result = value × (source factor ÷ target factor). For a reciprocal unit on either side, the value is first inverted through the base, so a larger g/kWh figure means a smaller J/kg figure.
What Is Mass-Based Fuel Efficiency?
Mass-based fuel efficiency relates energy to the mass of fuel. It can be written two ways. As energy per mass (J/kg, kJ/kg, cal/g, Btu/lb) it says how much energy each kilogram of fuel supplies, which is the same idea as specific energy or heating value. As mass per energy (kg/J, g/kWh, lb/(hp·h)) it says how much fuel is burned to produce one unit of energy or work, which is how specific fuel consumption of engines is quoted.
The two forms are reciprocals. A fuel that supplies 14.4 MJ/kg of useful work is the same as an engine using 250 g/kWh, because 3.6 MJ (one kWh) ÷ 14.4 MJ/kg = 0.25 kg. That is why this converter inverts the number when you switch between the two families instead of multiplying by a constant.
The calorie and Btu come in more than one definition. The International Table versions (cal and Btu marked IT, 4.1868 J per calorie) and the thermochemical versions (4.184 J per calorie) differ by about 0.07 %, so both are listed and labelled. Horsepower here is the mechanical horsepower (550 ft·lbf/s ≈ 745.7 W) unless “metric” is stated (735.499 W).
Mass-based efficiency is different from distance-based fuel economy (miles per gallon, km per litre) and from volume-based efficiency, which uses litres or gallons. Moving between mass and volume needs the fuel’s density, so it is not done here.
Supported Units
This converter supports the following 15 units. “Direct” units are energy per mass; “Reciprocal” units are mass per energy. The last column is the size of each unit expressed in the base unit J/kg; for a reciprocal unit it is the factor used in J/kg = factor ÷ value.
| Unit | Symbol | Type | Reference value in J/kg |
|---|---|---|---|
| Joule per kilogram | J/kg | Direct | 1 |
| Kilojoule per kilogram | kJ/kg | Direct | 1,000 |
| Calorie (IT) per gram | cal/g | Direct | 4,186.8 |
| Calorie (th) per gram | cal (th)/g | Direct | 4,184 |
| Btu (IT) per pound | Btu/lb | Direct | 2,326 |
| Btu (th) per pound | Btu (th)/lb | Direct | 2,324.444444 |
| Kilogram per joule | kg/J | Reciprocal | 1 |
| Kilogram per kilojoule | kg/kJ | Reciprocal | 1,000 |
| Gram per calorie (IT) | g/cal | Reciprocal | 4,186.8 |
| Gram per calorie (th) | g/cal (th) | Reciprocal | 4,184 |
| Pound per Btu (IT) | lb/Btu | Reciprocal | 2,326 |
| Pound per Btu (th) | lb/Btu (th) | Reciprocal | 2,324.444444 |
| Pound per horsepower-hour | lb/(hp·h) | Reciprocal | 5,918,352.502 |
| Gram per metric horsepower-hour | g/(hp(metric)·h) | Reciprocal | 2,647,795,500 |
| Gram per kilowatt-hour | g/(kW·h) | Reciprocal | 3,600,000,000 |
How to Use the Fuel Efficiency - Mass Converter
- Type the value in the box. Decimals and scientific notation such as 0.0025 or 2.5e-3 are accepted.
- Choose the From unit and the To unit.
- The result appears instantly, or press Convert Now. Very large or very small results are shown in scientific notation.
- Use Swap Units to reverse the conversion, Copy Result to copy the answer, open Show this value in all units for a full list, and Clear to start again.
Step-by-step by hand
This converter mixes direct units (energy per mass) and reciprocal units (mass per energy). Direct units are converted by multiplying, reciprocal units by dividing, through the common base unit (J/kg).
- Convert the source value to the base unit: 250 g/(kW·h) means 3,600,000,000 ÷ 250 = 14,400,000 J/kg.
- Convert from the base unit to the target: 14,400,000 J/kg ÷ 1,000 = 14,400 kJ/kg.
- Result: 250 g/(kW·h) = 14,400 kJ/kg.
Fuel efficiency values are positive quantities, so zero and negative numbers are rejected. A reciprocal unit also cannot be zero, because it would correspond to infinite energy per mass.
Common Fuel Efficiency (Mass) Conversions
- Engine consumption to energy per mass: 250 g/kWh equals 14,400 kJ/kg.
- Imperial consumption: 0.4 lb/(hp·h) equals 243.3109551 g/(kW·h).
- Heating value: 10,000 cal/g equals 41,868 kJ/kg and 18,000 Btu/lb.
- Imperial to metric: 18,000 Btu/lb equals 41,868 kJ/kg.
- Direct to reciprocal: 42,000 kJ/kg equals 85.71428571 g/kWh of fuel per kilowatt-hour at perfect conversion.
- Pound per Btu: 1 lb/Btu equals 0.0004299226139 kg/J.
Gram per Kilowatt-Hour – Common Values
Quick reference for frequently used values
| g/kWh | kJ/kg | lb/(hp·h) | kg/J |
|---|---|---|---|
| 150 | 24,000 | 0.2465980209 | 4.166666667e-8 |
| 200 | 18,000 | 0.3287973612 | 5.555555556e-8 |
| 250 | 14,400 | 0.4109967015 | 6.944444444e-8 |
| 300 | 12,000 | 0.4931960418 | 8.333333333e-8 |
| 400 | 9,000 | 0.6575947224 | 1.111111111e-7 |
| 500 | 7,200 | 0.821993403 | 1.388888889e-7 |
Where and Why This Conversion Is Used
- Engine and turbine data sheets: brake specific fuel consumption is given in g/kWh in metric documents and lb/(hp·h) in older or US ones.
- Fuel comparison: heating values appear in kJ/kg, cal/g or Btu/lb depending on the source.
- Energy balances: engineers need one consistent unit before dividing energy by mass.
- Aviation and marine: consumption per unit of power is quoted per kilowatt-hour or per horsepower-hour.
- Teaching: practising direct and reciprocal unit conversions.
Limitations
- The reciprocal relationship is exact only for the same energy. A fuel’s heating value is not the same as an engine’s consumption, because an engine converts only part of the fuel energy to work.
- Conversion to litres, gallons or miles per gallon requires fuel density or vehicle data and is not performed here.
- IT and thermochemical calories and Btu are different units; choose the one your source uses.
Important Differences Between Related Heat Quantities
- Specific heat capacity, J/(kg·K): energy needed to warm one kilogram of a material by one kelvin. It is not the total heat capacity of an object (J/K), which also depends on how much material there is, and it is not molar heat capacity (J/(mol·K)).
- Heat density, J/m²: heat or radiant energy delivered per unit area over a period. Heat flux density, W/m² is the rate of that transfer, energy per area per time. They are not interchangeable.
- Heat flux density, W/m² is heat flow per area. The heat transfer coefficient, W/(m²·K) is heat flux per kelvin of temperature difference, so heat flux = coefficient × temperature difference.
- Thermal conductivity, W/(m·K) is a material property. The heat transfer coefficient is a property of a surface or an assembly, and thermal resistance, K/W belongs to one specific object. Resistance is not conductivity or resistivity: R = L ÷ (k × A) for a slab.
- Temperature interval vs temperature: a difference of 10 °C is a difference of 18 °F. A reading of 10 °C is 50 °F. Every unit here that contains “per degree” or “K” uses intervals, so no offset is ever applied.
Each converter changes units within one quantity only. Related tools: Temperature Interval, Thermal Expansion, Thermal Resistance, Thermal Conductivity, Specific Heat Capacity, Heat Density, Heat Flux Density, Heat Transfer Coefficient, Fuel Efficiency - Mass and Fuel Efficiency - Volume.