Kilocalorie (th) to Tons of TNT

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.

One ton of TNT releases 4.184 GJ — roughly the energy of 120 liters of petrol or the electricity an average US home uses in 1.2 days. In blast terms, one ton of TNT in open air produces lethal overpressure within about 15–20 meters and can shatter windows at 100+ meters. The MOAB bomb (11 tons TNT) flattened structures across a 150-meter radius.

They differ by factors of 1,000: 1 kiloton = 1,000 tons, 1 megaton = 1,000,000 tons. Conventional bombs are rated in tons (the MOAB is 11 tons). Tactical nuclear weapons are rated in kilotons (Hiroshima was ~15 kt). Strategic thermonuclear warheads are rated in megatons (modern US warheads are 0.3–0.475 Mt). The scale spans nine orders of magnitude.

The 2020 Beirut explosion was caused by 2,750 tonnes of ammonium nitrate and was estimated at roughly 500–1,100 tons of TNT equivalent. For comparison, the Oklahoma City bombing (1995) was about 2 tons TNT equivalent, and the Halifax explosion (1917) was roughly 2,900 tons. Beirut ranked among the largest non-nuclear explosions in history.

Small meteor airbursts are rated in tons or kilotons of TNT. The 2013 Chelyabinsk meteor released about 440,000 tons (440 kt) — roughly 30 times the Hiroshima bomb. The Tunguska event (1908) is estimated at 3–15 megatons. The Chicxulub asteroid that ended the dinosaurs released roughly 100 trillion tons (100 million megatons) of TNT equivalent energy.

The largest planned non-nuclear explosion was the British demolition of Heligoland fortifications in 1947, using 6,700 tons of TNT equivalent. The largest accidental explosion was the Halifax harbour disaster (1917) at roughly 2,900 tons. The largest conventional bomb, the US GBU-43/B MOAB, yields about 11 tons — tiny compared to accidental industrial blasts.