Fuel Efficiency - Volume Converter
Convert volume-based fuel efficiency between energy-per-volume units (joule per cubic metre, megajoule per cubic metre, Btu per cubic foot, therm per cubic foot) and reciprocal volume-per-energy units such as litre per joule and US gallon per horsepower-hour.
Select two units to see the conversion factor.
US and UK (Imperial) gallons are different sizes. Each unit here names its gallon. This converter does not convert to mpg, km/L or L/100 km, and it does not convert between mass and volume without a fuel density.
Fuel Efficiency (Volume) Formula
All units are related to the base unit joule per cubic metre (J/m³). A direct unit (energy per volume) is multiplied by its factor to reach J/m³ and divided to leave it. A reciprocal unit (volume per energy) uses the inverse: J/m³ = factor ÷ value. Exact definitions: 1 ft = 0.3048 m, 1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L, 1 Btu (IT) = 1,055.05585262 J, 1 therm (EC) = 100,000 Btu (IT).
What Is Volume-Based Fuel Efficiency?
Volume-based fuel efficiency relates energy to the volume of fuel. As energy per volume (J/m³, MJ/m³, Btu/ft³, therm/ft³) it is the energy content of a gas or liquid, such as the heating value of natural gas. As volume per energy (m³/J, L/J, gallon per horsepower-hour) it says how much fuel volume is consumed per unit of energy or work.
The two forms are reciprocals. If a fuel gives 37.3 MJ/m³, one megajoule needs 1 ÷ 37.3 ≈ 0.0268 m³ of it. This converter therefore inverts the value when you cross between the two families, instead of multiplying by a constant.
Gallons need care. A US gallon is 3.785411784 litres and an Imperial (UK) gallon is 4.54609 litres, about 20 % larger. The unit list therefore labels the gallon wherever one appears. The therm per UK gallon is an energy-per-volume unit; the gallon-per-horsepower-hour units are US gallons, shown once with mechanical horsepower (745.7 W) and once with metric horsepower (735.5 W).
This is not distance-based fuel economy. Miles per gallon, kilometres per litre and litres per 100 km relate distance to fuel volume and depend on the vehicle, so they are outside this tool. Mass-based units are in the Fuel Efficiency - Mass Converter.
Supported Units
This converter supports the following 15 units. “Direct” units are energy per volume; “Reciprocal” units are volume per energy. The last column is the size of each unit in the base unit J/m³.
| Unit | Symbol | Type | Reference value in J/m³ |
|---|---|---|---|
| Joule per cubic meter | J/m³ | Direct | 1 |
| Joule per liter | J/L | Direct | 1,000 |
| Megajoule per cubic meter | MJ/m³ | Direct | 1,000,000 |
| Kilojoule per cubic meter | kJ/m³ | Direct | 1,000 |
| Kilocalorie (IT) per cubic meter | kcal/m³ | Direct | 4,186.8 |
| Calorie (IT) per cubic centimeter | cal/cm³ | Direct | 4,186,800 |
| Therm per cubic foot | therm/ft³ | Direct | 3,725,894,581 |
| Therm per gallon (UK) | therm/gal (UK) | Direct | 23,207,984,280 |
| Btu (IT) per cubic foot | Btu/ft³ | Direct | 37,258.94581 |
| Btu (th) per cubic foot | Btu (th)/ft³ | Direct | 37,234.0282 |
| CHU per cubic foot | CHU/ft³ | Direct | 67,066.10245 |
| Cubic meter per joule | m³/J | Reciprocal | 1 |
| Liter per joule | L/J | Reciprocal | 1,000 |
| Gallon (US) per horsepower-hour (mechanical hp) | gal (US)/(hp·h) | Reciprocal | 709,175,035.9 |
| Gallon (US) per horsepower-hour (metric hp) | gal (US)/(hp(metric)·h) | Reciprocal | 699,473,571.5 |
How to Use the Fuel Efficiency - Volume Converter
- Type the value in the value 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
- Find the size of each unit in the common base unit (J/m³): 1 Btu/ft³ = 37,258.94581 J/m³ and 1 MJ/m³ = 1,000,000 J/m³.
- Divide the source size by the target size to get the conversion factor: 37,258.94581 ÷ 1,000,000 = 0.03725894581, so 1 Btu/ft³ = 0.03725894581 MJ/m³.
- Multiply by your value: 1,000 Btu/ft³ × 0.03725894581 = 37.25894581 MJ/m³.
Values must be positive. A reciprocal unit cannot be zero, because it would mean infinite energy per volume.
Common Fuel Efficiency (Volume) Conversions
- Natural gas heating value: 1,000 Btu/ft³ equals 37.25894581 MJ/m³.
- Metric to imperial: 38 MJ/m³ equals 1,019.889296 Btu/ft³.
- Therm to Btu: 0.01 therm/ft³ equals 1,000 Btu/ft³.
- Per litre: 35,000 J/L equals 35 MJ/m³.
- Reciprocal: 0.05 gal (US)/(hp·h) equals 7.050445584e-8 L/J.
- Mechanical vs metric hp: 0.05 gal (US)/(hp·h) is 0.04931600353 gal (US)/(hp(metric)·h).
Btu per Cubic Foot to MJ per Cubic Metre
Quick reference for frequently used values
| Btu/ft³ | MJ/m³ | kJ/m³ | therm/ft³ |
|---|---|---|---|
| 500 | 18.6294729 | 18,629.4729 | 0.005 |
| 800 | 29.80715665 | 29,807.15665 | 0.008 |
| 1,000 | 37.25894581 | 37,258.94581 | 0.01 |
| 1,030 | 38.37671418 | 38,376.71418 | 0.0103 |
| 1,100 | 40.98484039 | 40,984.84039 | 0.011 |
| 2,500 | 93.14736452 | 93,147.36452 | 0.025 |
Where and Why This Conversion Is Used
- Gas supply and billing: natural gas energy content is quoted in MJ/m³, Btu/ft³ or therm per cubic foot.
- Fuel data sheets: liquid fuel energy density is given per litre or per cubic metre.
- Engine specifications: consumption per horsepower-hour is quoted in gallons in US documents.
- Engineering comparison: bringing mixed sources to one unit before calculations.
Limitations
- Gas volumes depend on temperature and pressure; the unit values here are fixed definitions, not measured at a reference condition.
- Reciprocal units describe the same energy only; they do not account for engine or burner efficiency.
- Distance-based economy (mpg US, mpg Imperial, km/L, L/100 km) is not converted here, and US and Imperial gallons must never be treated as equal.
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.