Fuel Efficiency by Volume converter

This category describes volumetric energy density — how much energy is packed into a given volume of fuel — using joule per cubic meter (J/m³) as the reference. Metric multiples scale up directly, with 1 megajoule per cubic meter equal to 1,000,000 J/m³ and 1 joule per liter equal to 1,000 J/m³. Energy-industry units carry their own historical factors: 1 BTU per cubic foot equals 37,258.9458078313 J/m³, 1 BTU per US gallon equals 278,716.2699391016 J/m³, and 1 therm per US gallon — a large unit used for high-density liquid or compressed fuels — equals 27,871,626,993.910156 J/m³.

To convert megajoule per cubic meter to BTU per cubic foot, multiply by BTU/ft³ = MJ/m³ × 26.83919199.

Pipeline-quality natural gas typically carries about 37 MJ/m³ of heating value, which converts to 37 × 26.83919199 ≈ 993 BTU/ft³ — very close to the widely quoted "roughly 1,000 BTU per cubic foot" rule of thumb that utility bills and furnace ratings are built around. Using the page's other quick pair, a biogas or renewable gas blend rated at 10 kWh/m³ converts to 10 × 3,600,000 ÷ 1,000,000 = 36 MJ/m³, almost identical to natural gas — one reason some blends can be substituted into existing gas infrastructure with little modification. Gasoline, stored as a liquid rather than a gas, carries a far larger figure around 34,200 MJ/m³ (equivalently about 34.2 MJ per liter), which is precisely why liquid fuels pack so much more energy into a given tank size than any practical gas storage option.

Volumetric energy density is the figure that actually matters at the pump or the gas meter, since vehicles and appliances are filled and billed by volume, not mass: utility companies report natural gas heating value in BTU per cubic foot to calculate customer bills from metered gas volume, fuel engineers compare gasoline, diesel and alternative fuels by volumetric energy density because tank size is a fixed physical constraint on a vehicle, and LNG and CNG infrastructure planning depends heavily on this figure since compressing or liquefying gas is specifically about packing more energy into a smaller volume for transport and storage.

Unlike a physical constant such as water's density, a fuel gas's BTU-per-cubic-foot rating is not perfectly fixed — it depends on the exact mixture of methane, ethane and other hydrocarbons in that particular gas supply, as well as the temperature and pressure at which the volume was measured, so utilities periodically re-certify their heating-value figures rather than treating them as a universal constant. It's also easy to confuse this volumetric category with fuel-efficiency-by-mass: a gas can have a high BTU-per-cubic-foot rating as a compressed or liquefied fuel while having an unremarkable BTU-per-pound rating, since volumetric figures are heavily affected by how tightly the fuel is compressed or liquefied, not just its chemistry.

The therm-per-gallon unit on this page is worth a second look precisely because its factor looks so extreme: a therm is already a large quantity of energy (roughly 100,000 BTU), so expressing it per gallon of a dense liquid fuel produces one of the largest numbers on this entire site. It is included mainly for specialized energy-trading and liquid-fuel accounting contexts rather than everyday use, and most practical conversions on this page will instead run through the more familiar BTU, joule or kilowatt-hour based units.