Megatons of TNT to Calorie (nutritional)

One megaton equals 4.184 × 10¹⁵ joules — the energy of burning about 120 million liters of petrol or the total electricity output of a large power plant running for 50 days. A 1-megaton airburst would flatten reinforced concrete buildings within 2 km, cause third-degree burns at 10 km, and break windows at 40+ km. It is roughly 67 times the Hiroshima bomb.

The Soviet AN602 "Tsar Bomba," detonated on 30 October 1961, yielded approximately 50 megatons — the largest human-made explosion in history. It was a three-stage thermonuclear device originally designed for 100 Mt but scaled down by replacing the uranium tamper with lead to reduce fallout. The fireball was 8 km wide, and the mushroom cloud rose 67 km. It was a propaganda weapon with no practical military use.

Modern strategic warheads are smaller than Cold War designs because accuracy improved. The US W88 yields about 0.475 Mt; the W76-1 about 0.1 Mt. Russian RS-28 Sarmat MIRVs carry warheads estimated at 0.5–0.8 Mt each. Military planners found that several smaller warheads (MIRVs) destroy more area than one large one due to the cube-root scaling of blast radius with yield.

The Chicxulub impact that ended the dinosaurs released roughly 100 million megatons (10²³ J). The Tunguska event (1908) was 3–15 megatons. NASA's planetary defense threshold is objects capable of 1+ megatons of damage. A 50-meter iron asteroid striking Earth at 20 km/s would release about 10 megatons — enough to obliterate a major city.

Accuracy replaced raw yield. A 0.5 Mt warhead landing within 100 meters of a target destroys it just as effectively as a 10 Mt warhead landing 1 km away. MIRVed missiles carrying 6–10 smaller warheads also cover more total area than one massive bomb. The US retired its last megaton-class warhead (the B83) in 2022, relying entirely on sub-megaton weapons.

In the late 19th century, nutritionists adopted the kilocalorie as the practical unit for food energy but dropped the "kilo" prefix in everyday speech. A banana labelled "90 calories" actually contains 90 kilocalories (90,000 small calories). Some labels use a capital "C" (Calorie) to distinguish it from the small calorie, but this convention is inconsistently applied and remains a source of confusion worldwide.

One kcal (kilocalorie) equals 1,000 cal (calories). European and Australian labels typically show energy in both kJ and kcal explicitly. US labels use "Calories" (capital C), which actually means kcal. If a label says 200 Calories, it means 200 kcal = 200,000 small calories = 836.8 kJ. The small calorie (4.1868 J) is rarely seen outside laboratory contexts.

Adults typically need 1,600–2,500 kcal per day depending on sex, age, weight, and activity level. Sedentary women average about 1,800 kcal; active men about 2,500 kcal. Endurance athletes during competition can burn 4,000–8,000 kcal/day. These figures are based on the International Table calorie (4.1868 J), though the thermochemical calorie gives near-identical results in practice.

Australia, New Zealand, and EU member states mandate SI-based labeling, so they use kilojoules (kJ) as the primary energy unit. The US and Canada use kilocalories (labelled as "Calories"). To convert, multiply kcal by 4.1868 to get kJ, or divide kJ by 4.1868 for kcal. A 2,000 kcal daily diet equals 8,374 kJ.

Wilbur Atwater and colleagues in the 1890s used bomb calorimeters to burn food samples and measure heat released. They established that carbohydrates yield ~4 kcal/g, protein ~4 kcal/g, and fat ~9 kcal/g — the Atwater factors still printed on food labels today. Modern methods use chemical analysis and Atwater factors rather than direct calorimetry for every product.