Grams of TNT to Kilocalorie (th)

By convention, exactly 4,184 joules — the same as one thermochemical kilocalorie. Real TNT detonation yields vary by about ±2% depending on purity and confinement, but the defined value provides a fixed reference point. This makes the gram of TNT a convenient bridge between chemistry (calories) and explosive engineering.

TNT (trinitrotoluene) became the reference explosive because it is chemically stable, safe to handle, and was massively produced during both World Wars. Its consistent detonation properties made it a natural benchmark. Other explosives are rated by their "TNT equivalent" — for example, C-4 is about 1.34× TNT and ANFO is about 0.74× TNT.

A standard US consumer firecracker contains about 0.5–1 gram of TNT equivalent in flash powder. An M-80 (now illegal for consumer sale) contained roughly 3 g of TNT equivalent. Cherry bombs were about 1.5 g. Commercially sold fireworks are regulated by the CPSC to contain no more than 50 mg of flash powder per report charge.

A US M67 fragmentation grenade contains about 180 g of Composition B explosive, which has a TNT equivalence of about 1.33×, giving roughly 240 grams of TNT equivalent. The lethal radius is about 5 meters, with a casualty-producing radius of 15 meters. The fragmentation — not the blast energy alone — is the primary wounding mechanism.

One gram of TNT releases exactly 1 thermochemical kilocalorie (1 kcal = 4,184 J) by definition. This means a dietary Calorie (nutritional kcal) contains the same energy as detonating one gram of TNT. A 2,000-Calorie daily diet is energetically equivalent to 2 kg of TNT — though your body releases that energy over 24 hours, not in microseconds.

Most foundational biochemical data — ATP hydrolysis (~7.3 kcal/mol), glucose oxidation (~686 kcal/mol), amino acid combustion values — were measured and published in kcal th before SI adoption. Rewriting decades of literature, lecture notes, and exam banks to kJ would introduce conversion errors and confusion. The field maintains kcal th by convention while acknowledging SI equivalents.

The standard free energy change (ΔG°) for ATP → ADP + Pi is approximately −7.3 kcal th/mol (−30.5 kJ/mol). Under actual cellular conditions, the value is closer to −12 to −14 kcal/mol because reactant and product concentrations differ from standard state. This energy drives muscle contraction, nerve impulses, protein synthesis, and virtually every energy-requiring process in living cells.

The classic Atwater factors (4 kcal/g carb, 4 kcal/g protein, 9 kcal/g fat) are averages from 19th-century bomb calorimetry, adjusted for digestibility. They can be off by 5–25% for specific foods. Almonds deliver ~20% fewer usable calories than labels claim because cell walls trap some fat from digestion. High-fiber foods also overcount. The FDA allows ±20% tolerance on label accuracy, so a "200 kcal" bar could legally contain 160–240 kcal.

Complete aerobic oxidation of one mole of glucose (C₆H₁₂O₆) releases approximately 686 kcal th (2,870 kJ). The human body captures about 38–40% of this in ATP; the rest dissipates as body heat. This is why exercise makes you warm — over half the food energy your muscles consume is released as thermal energy rather than mechanical work.

Fat molecules are highly reduced — their carbon atoms are bonded mostly to hydrogen, with very little oxygen. Oxidising them releases maximum energy because every C-H bond is converted to C=O and O-H bonds. Carbohydrates are already partially oxidised (they contain oxygen in their structure), so less additional oxidation is possible. Gram for gram, fat stores 2.25× more energy, which is why evolution favored fat as the body's long-term energy reserve — it packs the most kcal per gram of tissue weight.