Calorie (th) to Gigajoule

The thermochemical calorie (cal th) is defined as exactly 4.184 joules; the International Table calorie (cal IT) is exactly 4.1868 joules — a difference of 0.066%. The thermochemical value was fixed by the US National Bureau of Standards in 1935 for chemistry; the IT value was adopted for steam tables. In nutritional contexts, the difference is irrelevant, but in precise calorimetry it can matter.

Decades of published thermochemical data — heats of formation, bond energies, combustion enthalpies — are recorded in cal th and kcal th. Converting every reference table to joules would be error-prone and disruptive. Biochemistry textbooks still quote ATP hydrolysis at ~7.3 kcal/mol and glucose oxidation at ~686 kcal/mol. The convention persists because the existing literature is too vast to rewrite.

A dried, weighed food sample is sealed in a steel vessel filled with pure oxygen, submerged in a known mass of water. An electric spark ignites the sample, which burns completely. The temperature rise of the surrounding water — measured to 0.001°C — gives the total heat released. One degree rise per gram of water equals one calorie. Corrections for the heat capacity of the bomb itself, the ignition wire, and acid formation give results accurate to ±0.1%. Atwater then applied digestibility factors to convert bomb values to usable food energy.

Hydrogen releases about 34,000 cal th per gram; methane about 13,300 cal th/g; ethanol about 7,100 cal th/g; and glucose about 3,720 cal th/g. These values appear throughout chemistry textbooks as standard reference data. The higher the cal/g value, the more energy-dense the fuel — which is why hydrogen is attractive despite being hard to store.

Before 1935, the calorie was defined by water's heat capacity, which varies with temperature — the 15°C calorie, 20°C calorie, and mean calorie all differed slightly. The US National Bureau of Standards ended the ambiguity by defining the thermochemical calorie as exactly 4.184 J, a round value close to all the experimental variants. This gave chemists a fixed, reproducible conversion factor independent of water's quirky temperature-dependent heat capacity.

In cold-climate countries, 30–60 GJ per year is common for heating and hot water. A well-insulated modern home in Germany might use 20 GJ; a drafty older home in Canada might use 100+ GJ. Australians use about 40 GJ/year on average. Each gigajoule costs roughly $8–$15 depending on local gas prices.

One tonne of coal holds roughly 24–30 GJ depending on grade. One tonne of crude oil contains about 42–44 GJ. One tonne of LNG holds roughly 54 GJ. One tonne of dry firewood stores about 16 GJ. These figures explain why oil and gas are preferred for transport — they pack more gigajoules per kilogram than solid fuels.

One gigajoule equals 277.78 kWh. At an average electricity price of $0.15/kWh, one gigajoule of electrical energy costs about $42. The same gigajoule from natural gas costs $8–15. This price gap is the main reason gas boilers remain popular for heating in countries with cheap pipeline gas.

A single-aisle jet like the Boeing 737-800 burns about 10–12 GJ per flight hour. A six-hour transatlantic flight on a wide-body aircraft can consume 300–400 GJ of jet fuel. The entire global aviation industry uses roughly 12 billion gigajoules of fuel per year — about 3% of total world energy consumption.

At 2,000 kcal/day (8.4 MJ/day), a person consumes about 3.07 GJ of food energy per year. Over 80 years, that is roughly 245 GJ — equivalent to about 6,000 liters of petrol. Your entire lifetime food energy would fit in a medium-sized fuel tanker, which is a humbling thought.