Impact Energy Calculator (asteroid)

Understanding the asteroid impact energy calculator — concept and principle

This calculator estimates the kinetic energy an asteroid or meteoroid carries when it reaches Earth, from its diameter, density and speed, and converts it to megatons of TNT, the unit used to compare impacts with explosions. It is useful for understanding news about near-Earth objects, planetary defence and historical events such as Tunguska and Chelyabinsk.

Speed is squared, so it matters far more than size alone: 17 km/s turns a 50 m rock into 6.8 Mt.

How it is calculated

Assume a sphere: mass m = ρ × (π/6) × D³, with density ρ in kg/m³ and diameter D in metres.
Kinetic energy E = ½ × m × v², with v converted from km/s to m/s.
Megatons of TNT = E ÷ 4.184 × 10¹⁵ J (1 kiloton is defined as 4.184 × 10¹² J).

The principle

Energy grows with the cube of size and the square of speed. Doubling the diameter multiplies the volume, mass and energy by eight; doubling the speed multiplies the energy by four. That is why a body only tens of metres across can release more energy than the largest nuclear tests. Speed matters enormously: nothing falling from space can hit Earth slower than about 11 km/s, Earth's escape velocity, and typical asteroid encounters are in the high teens of km/s. The formula gives the energy available on arrival; how it is released (high-altitude airburst, ground impact, crater) depends on strength, angle and atmosphere.

Worked example

Inputs: D = 50 m, ρ = 3,000 kg/m³ (a stony asteroid), v = 17 km/s.
Volume = (π/6) × 50³ = 65,449.8 m³, so m = 196,349,541 kg = 196,349.5408 t.
v = 17,000 m/s, v² = 2.89 × 10⁸ m²/s².
E = 0.5 × 196,349,541 × 2.89 × 10⁸ = 2.837251 × 10¹⁶ J.
E ÷ 4.184 × 10¹⁵ = 6.781192317 Mt TNT, roughly 450 times the Hiroshima bomb (about 15 kt).

Each point is the calculator re-run with only “Diameter (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Diameter (m)	Energy (megatons TNT)
12.5	0.1059561299
25	0.8476490396
37.5	2.860815509
50	6.781192317
62.5	13.24451624
75	22.88652407
100	54.24953853

How to read the result

For scale: the Chelyabinsk meteor of 2013, about 20 m across, released roughly 0.4–0.5 Mt in an airburst; published estimates for the 1908 Tunguska event, caused by an object of a few tens of metres, range from a few to over ten megatons. Bodies of around a kilometre and larger could have global effects. Typical densities: comets around 500–1,000 kg/m³, stony asteroids about 2,000–3,500, iron about 7,800.

Common mistakes

Entering the radius instead of the diameter, which underestimates energy eightfold.
Leaving the speed in km/s inside your own calculation; it must be squared in m/s.
Assuming all energy reaches the ground; small stony bodies usually break up and explode in the air.

Keep in mind: This is a simple kinetic-energy estimate for a solid sphere and ignores atmospheric deceleration, fragmentation, entry angle and shape. Crater size, blast radius and damage require full impact models.

Frequently asked questions

How is a TNT equivalent defined?

By convention 1 ton of TNT equals 4.184 × 10⁹ J, so 1 megaton is 4.184 × 10¹⁵ J, regardless of the chemical's real energy content.

What is the fastest possible impact speed?

For objects bound to the Sun, about 72 km/s, for a head-on collision with Earth's orbital motion.

How often do Tunguska-sized events happen?

Estimates are of the order of once every few centuries, but uncertainty is large.

Sources: G. S. Collins, H. J. Melosh and R. A. Marcus, Earth Impact Effects Program, Meteoritics & Planetary Science 40(6), 817–840 (2005) · NASA Center for Near Earth Object Studies (CNEOS), Jet Propulsion Laboratory

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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