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Blast Radius Calculator

Use our blast radius calculator to estimate non-fragment flying distance outside which no less than one fragment is expected to fly.

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Radius for severe structural damage46 mZ = 4.6 — indicative only, and highly dependent on confinement and surroundings
Energy released4.184 GJ1,162.222 kWh, 1 tonnes of TNT
TNT equivalent1,000 kgat the conventional 4.184 MJ per kilogram — a defined figure, since real TNT varies
In tonnes of TNT1 t
Equivalent to burning petrol120.9249 litrespetrol holds about eight times more energy per kilogram than TNT — an explosive is not a particularly energy-dense fuel, it simply releases its energy in microseconds
Scaled distance Z5 m/kg^⅓the Hopkinson–Cranz parameter — blast effects depend on this rather than on distance alone
Cube root of the charge10
Radius for window glass shatters, minor injury95 mZ = 9.5 — indicative only, and highly dependent on confinement and surroundings
Radius for glass cracks, little else200 mZ = 20 — indicative only, and highly dependent on confinement and surroundings
Radius for noise and vibration only400 mZ = 40 — indicative only, and highly dependent on confinement and surroundings
You are at50 min the glass and injury range
Charge needed to double this radius8,000 kgeight times the charge for twice the radius — the cube-root law cuts both ways

The formula

E = 4.184 MJ per kg of TNT; R = Z × W^⅓

Cube roots, and why an explosive is a poor fuel

A kilogram of TNT is defined as releasing 4.184 megajoules — the same number as a thermochemical kilocalorie times a million, which is not a coincidence but a convention. Real TNT varies somewhat, so the figure is a definition used for comparison rather than a measurement.

Petrol holds roughly eight times more energy per kilogram than TNT. What makes an explosive dangerous is not energy density but power density: it carries its own oxidiser and releases everything in microseconds, producing a shock front. Petrol needs air and burns comparatively slowly.

Blast effects scale with the cube root of charge mass — the Hopkinson–Cranz law. Doubling the radius of a given effect requires eight times the charge, which is why blast damage is so localised and why scaled distance, not distance, is the meaningful parameter.

The radius figures here are indicative reference values for free-air bursts. Real blast depends enormously on confinement, reflections, ground coupling and structure type, and a charge inside a building behaves nothing like the same charge in the open. Treat them as orders of magnitude.