Fuel Efficiency by Mass converter

This category measures gravimetric energy density — how much energy is packed into a given mass of fuel or battery material — using joule per kilogram (J/kg) as the reference. Metric multiples scale up cleanly: 1 kilojoule per kilogram equals 1,000 J/kg and 1 megajoule per kilogram equals 1,000,000 J/kg. Calorie- and BTU-based units bring in their own historical conversion factors: 1 calorie per gram (equivalently 1 kilocalorie per kilogram) equals 4,184 J/kg, and 1 BTU per pound equals 2,326 J/kg, while 1 kilowatt-hour per kilogram equals 3,600,000 J/kg.

To convert megajoule per kilogram to BTU per pound, multiply by BTU/lb = MJ/kg × 429.9226139.

Gasoline carries roughly 46 MJ/kg of chemical energy, which converts to 46 × 429.9226139 ≈ 19,776 BTU/lb — a figure that explains why liquid fuels remain hard to beat for energy per kilogram. A modern lithium-ion battery cell, by contrast, stores around 0.9 MJ/kg, which converts to kilowatt-hours per kilogram as 0.9 × 1,000,000 ÷ 3,600,000 = 0.25 kWh/kg, roughly fifty times less energy-dense than gasoline by mass — the central challenge in electric-vehicle range engineering. Coal, depending on grade, sits somewhere in between at around 24 MJ/kg, converting to 24 × 429.9226139 ≈ 10,318 BTU/lb.

Gravimetric energy density is the number that decides how heavy a fuel tank or battery pack needs to be for a given range: aviation and spacecraft designers care about it intensely because every kilogram of fuel mass reduces the payload a vehicle can carry, battery manufacturers quote it (often in Wh/kg) to compare chemistries like lithium-ion, lithium-iron-phosphate and emerging solid-state cells, and food science uses the same underlying physical quantity — calories per gram — to describe how much chemical energy a food contains per unit mass, even though nobody calls it "fuel efficiency" there. Rocket designers watch it especially closely, since propellant mass dominates a launch vehicle's total weight budget far more than it does for cars or aircraft.

A frequent point of confusion is mixing up this mass-based ("gravimetric") energy density with the volume-based ("volumetric") energy density covered on the fuel-efficiency-by-volume page: a fuel can rank very differently on the two scales. Hydrogen, for example, has an outstanding energy-per-kilogram figure but a poor energy-per-liter figure because it is so light and diffuse, which is why hydrogen vehicles need heavily compressed or cryogenic storage to carry a useful amount of energy in a reasonably sized tank. Always check which of the two a spec sheet is actually quoting before comparing fuels, since the same word "efficiency" gets used loosely for both.

It's also worth noting that the real-world energy a fuel delivers (its useful output after engine or battery losses) is always lower than its raw gravimetric energy density, because no real engine or chemical reaction converts 100% of stored energy into useful work. The figures on this page describe the fuel's stored chemical potential, not the delivered mileage or runtime, which depends heavily on the efficiency of whatever converts that energy into motion.