Calorie (th) to Kilograms of TNT

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.

One kilogram of TNT releases 4.184 MJ — enough to shatter windows within several meters and cause serious injury at close range. In open air, 1 kg of TNT produces a blast overpressure lethal to humans within about 2–3 meters. The effect depends heavily on confinement: the same charge inside a vehicle or building is far more destructive than in open ground.

One kilogram of TNT (4.184 MJ) is roughly the kinetic energy of a 1,500 kg car traveling at 75 km/h, or the energy stored in about 120 mL (half a cup) of petrol. It is also the chemical energy in roughly one large meal (1,000 kcal). The difference is that TNT releases its energy in microseconds rather than hours.

Mining engineers express blast charge sizes in kg of TNT equivalent to standardize across different commercial explosives. A typical quarry blast hole uses 5–50 kg of ANFO (ammonium nitrate/fuel oil), equivalent to roughly 4–37 kg TNT. Building demolition charges range from 10 to several hundred kg TNT equivalent, carefully placed at structural weak points.

A standard 155 mm artillery shell contains about 7–11 kg of TNT equivalent. A 500 lb (Mk 82) air-dropped bomb holds roughly 87 kg of TNT equivalent. An RPG-7 warhead is about 1–2 kg TNT equivalent. Anti-tank mines range from 5–10 kg TNT equivalent. These figures represent explosive fill, not total weapon weight.

A standard stick of commercial dynamite (about 200 g, 20 cm long) has a TNT equivalence of roughly 0.25–0.30 kg, since dynamite is about 1.25–1.5× as powerful as TNT by weight. Eight sticks of dynamite are roughly equivalent to one kilogram of TNT. Modern mining rarely uses traditional dynamite, preferring cheaper ANFO or emulsion explosives.