Fuel Efficiency (Volume) Converter: J/m³, Btu/ft³ & More
Convert volumetric energy density and fuel efficiency by volume—energy per cubic meter or cubic foot, plus volume-per-energy for automotive and HVAC.
Convert energy per volume (J/m³, Btu/ft³, therm/ft³) and volume per energy (m³/J, L/J, gal/(hp·h)) for fuels, gases, and heating value. SI, imperial, therm, and CHU. Free, runs in your browser—no sign-up.
What is fuel efficiency by volume?
Volumetric energy density is energy per unit volume—how many joules (or Btu) you get per cubic meter or cubic foot. It tells you how much energy is packed into a tank or pipeline. The same idea is sometimes expressed as volume per energy (e.g. m³/J or gallon per horsepower-hour), common in engine specs and HVAC. This converter handles both: pick J/m³, Btu/ft³, therm/ft³, or inverse units like L/J and gal/(hp·h).
Engineers use it for natural gas heating value, battery volumetric density, and fuel comparison. Contractors and HVAC specs often use Btu/ft³ or therm; automotive may use gallon per hp·h. All conversions use the SI base J/m³ under the hood.
How conversion works
Every energy-per-volume unit is converted via J/m³. Multiply your value by the source unit’s factor (how many J/m³ one unit equals), then divide by the target unit’s factor:
For volume-per-energy units (m³/J, L/J, gal/(hp·h)), the tool uses the reciprocal: energy density in J/m³ = factor ÷ value. Example: 1 Btu/ft³ ≈ 37,259 J/m³; 1 m³/J = 1 J/m³. The converter above does both directions for all 15 units.
Key units explained
Joule per cubic meter (J/m³)
Factor: 1 (SI base)
Standard unit for volumetric energy density. Used in physics and engineering.
Btu (IT) per cubic foot [Btu/ft³]
Factor: 1 Btu/ft³ ≈ 37,259 J/m³
Common in US gas heating value and HVAC. 1 ft³ ≈ 0.0283 m³.
Therm per cubic foot [therm/ft³]
Factor: 1 therm/ft³ ≈ 3.73×10⁹ J/m³
Large unit for gas billing and pipeline energy density. 1 therm = 100,000 Btu.
Gallon (US) per horsepower-hour
Volume per energy (reciprocal)
Fuel consumption: volume of fuel per mechanical hp·h. Lower is more efficient.
Who uses this converter?
HVAC and gas engineers move between Btu/ft³, therm/ft³, and J/m³ for heating value and pipeline specs. Battery and energy storage work in J/m³ or Wh/L for volumetric density. Automotive and marine use gal/(hp·h) or L/J for fuel consumption. Students and educators need to match textbook units (often SI) to real-world Btu or therm. Conversions run locally—nothing is sent to a server.
Common conversions at a glance
| From | To | Formula |
|---|---|---|
| Btu/ft³ | J/m³ | × 37,259 |
| J/L | J/m³ | × 1000 |
| cal/cm³ | J/m³ | × 4,186,800 |
Avoiding common mistakes
Don’t mix energy per volume (J/m³, Btu/ft³) with volume per energy (m³/J, L/J)—they are reciprocals. Check whether your datasheet uses “J per m³” or “m³ per joule.” Therm (IT) and Btu (th) differ slightly from IT; this converter supports both. For gal/(hp·h), we use mechanical horsepower (745.7 W). When in doubt, convert to J/m³ first, then to your target unit.
Fuel Efficiency (Volume) FAQ
? How do I convert Btu/ft³ to J/m³?
Multiply Btu (IT)/ft³ by about 37,259. So 1 Btu/ft³ ≈ 37,259 J/m³. The factor comes from 1 Btu ≈ 1055 J and 1 ft³ ≈ 0.0283 m³. Use the tool above for therm, CHU, or volume-per-energy units.
? What is the difference between energy per volume and volume per energy?
Energy per volume (e.g. J/m³) answers “how much energy in one cubic meter?” Volume per energy (e.g. m³/J) answers “how much volume for one joule?” They are reciprocals. The converter supports both so you can match datasheets and regional units.
? What is a therm?
One therm equals 100,000 Btu (IT)—used for natural gas billing in the US. Therm per cubic foot is a large unit for pipeline or storage energy density. This converter includes therm/ft³ and therm/gallon (UK).
? How do I convert J/m³ to Btu/ft³?
Divide J/m³ by about 37,259. So 1 J/m³ ≈ 0.0000268 Btu (IT)/ft³. Use the converter for therm/ft³, CHU/ft³, and volume-per-energy units (m³/J, ft³/Btu, L/MJ, etc.) so you can match HVAC, fuels, or gas datasheets.
? What is volumetric energy density used for?
Energy per volume (J/m³, Btu/ft³) is used for heating value of gases, fuel energy density, and HVAC (e.g. energy in a volume of air or gas). Volume per energy (m³/J, ft³/Btu) appears in ventilation or flow per unit energy. This tool supports both so you can convert between SI, imperial, therm, and CHU.
? How do I convert natural gas energy density from Btu/ft³ to kJ/m³?
Multiply Btu (IT)/ft³ by about 37.26. So 1 Btu/ft³ ≈ 37.26 kJ/m³ (or 37,259 J/m³). Use the converter for therm/ft³, CHU/ft³, and other energy-per-volume units common in gas billing and pipeline specs.