A 50-meter stony asteroid hitting at 20 km/s releases energy equivalent to roughly 60 million tons of TNT, about four times the estimated force of the 1908 Tunguska event that flattened 2,000 square kilometers of Siberian forest. This calculator runs the same kinetic-energy math astronomers use for a first-pass estimate, from your own diameter, speed, and composition.
The Physics: Mass, Velocity, Energy
Every number on this page comes from one equation from introductory physics: kinetic energy equals one-half times mass times velocity squared. Mass is calculated by treating your impactor as a sphere, using its volume (four-thirds pi times radius cubed) multiplied by a density figure for its composition. Velocity gets converted from kilometers per second to meters per second before it's squared, since squaring a value that's already off by a factor of 1,000 would overstate the result by a million times. The result, in joules, then gets converted to tons of TNT equivalent by dividing by 4.184×10^9, the standard energy-per-ton-of-TNT conversion used across nuclear weapons yield and impact physics literature alike.
It's worth sitting with how much that squared term matters. Doubling an impactor's diameter increases its volume, and therefore its mass, eightfold (radius cubed). But doubling its velocity quadruples its kinetic energy on its own, independent of mass entirely. A small, fast object can carry more destructive energy than a much larger, slower one, which is exactly why velocity gets its own section below.
Why Composition Changes Everything
This calculator offers three composition presets: icy comet material at roughly 1,000 kg/m³ (close to water ice), a stony asteroid at roughly 3,000 kg/m³ (typical of the rocky, silicate-rich asteroids that make up the majority of near-Earth objects), and iron asteroid material at roughly 8,000 kg/m³ (rarer, but the kind that survives atmospheric entry best and is responsible for craters like Arizona's Barringer Crater). Since mass scales directly with density, an iron object of a given size carries eight times the mass, and therefore eight times the kinetic energy, of an icy comet the exact same size at the exact same speed. Real asteroids and comets vary well outside these three midpoints, this is a representative estimate, not a measurement of any specific known object.
Why Velocity Matters More Than Size
Near-Earth objects typically strike the atmosphere somewhere between about 11 km/s (the slowest possible, roughly Earth's escape velocity, for an object that was barely moving relative to Earth before gravity accelerated it) and up to 72 km/s for a long-period comet on a retrograde orbit meeting Earth head-on. Most asteroid impacts cluster in the 12 to 20 km/s range. Comets, especially ones on highly eccentric orbits from the outer solar system, routinely arrive faster than typical asteroids, which is part of why cometary impacts are treated as a distinct, often higher-energy hazard case in planetary defense research despite comets usually being less dense than rocky asteroids.
Putting the Number in Context
Raw joules or even tons of TNT don't mean much without a reference point, which is why the result panel compares your input against four well-documented events. The Hiroshima bomb released energy equivalent to roughly 15,000 tons of TNT. The 2013 Chelyabinsk meteor, a roughly 20-meter object that exploded over Russia and injured over a thousand people mostly from shattered glass, is estimated at around 440 to 500 kilotons, or nearly 500,000 tons, more than 30 times Hiroshima despite never reaching the ground intact. The 1908 Tunguska event, still the largest impact event in recorded history, is estimated at 10 to 15 megatons, flattening an area of Siberian forest larger than metropolitan Tokyo. And the Chicxulub impactor, the roughly 10 to 15 kilometer object linked to the end-Cretaceous mass extinction 66 million years ago, is estimated at around 100 million megatons, a scale so far beyond the others that the comparison bars on this page use a logarithmic scale just to fit all four on the same screen.
What This Calculator Doesn't Try to Do
Deliberately left out: a precise crater diameter. Real crater scaling depends on far more than mass and velocity alone, including impact angle, whether the target is rock or ocean, local gravity, and whether the impactor even reaches the ground intact rather than airbursting in the atmosphere the way the Chelyabinsk and Tunguska objects both did. Publishing a single crater-size number from just diameter and velocity would be more precise-looking than the underlying physics actually supports. If you want a full crater and blast-radius model with all of those variables included, Purdue University and Imperial College London jointly maintain the Earth Impact Effects Program, an actual research-grade impact calculator, linked in the sources below. This page sticks to the part of the physics, kinetic energy, that a diameter, velocity, and density genuinely can calculate on their own.
A Worked Example
Set diameter to 50 meters, velocity to 20 km/s, and composition to stony asteroid. Radius is 25 meters, so volume works out to roughly 65,450 cubic meters. At 3,000 kg/m³, that's about 196.3 million kilograms, or roughly 196,300 tonnes. Velocity in meters per second is 20,000. Kinetic energy is 0.5 × 196,300,000 kg × 20,000², which comes out to roughly 3.93×10^16 joules. Divide by 4.184×10^9 joules per ton of TNT, and that's about 9.4 million tons, roughly 9.4 megatons, comparable to Tunguska and well over 600 times the Hiroshima bomb, from a rock smaller than a football field.
How to Use This Calculator
Enter a diameter in meters, a velocity in kilometers per second, and pick the composition that best matches what you're modeling. The result updates instantly with estimated mass, kinetic energy in joules, and a TNT-equivalent figure, plotted against four real, well-documented reference events on a logarithmic scale so you can see roughly where your scenario lands.
Frequently Asked Questions
Does a bigger object always release more energy than a faster one?+
Not necessarily. Doubling diameter increases mass eightfold (volume scales with the cube of radius), but doubling velocity quadruples energy on its own. A small, very fast object can outrelease a much larger, slower one.
Why doesn't this calculator show a crater size?+
Real crater diameter depends on impact angle, target material, local gravity, and whether the object even survives atmospheric entry, variables this simple tool doesn't collect. Purdue and Imperial College London's Earth Impact Effects Program, linked in the sources below, models all of that properly.
Was Chelyabinsk really more powerful than Hiroshima?+
Yes, by most published estimates, roughly 30 times more energetic, despite the object itself exploding in the atmosphere roughly 30 km up rather than reaching the ground. Most injuries came from shattered window glass, not a direct blast at ground level.
What velocity should I actually use?+
For a typical near-Earth asteroid, 12 to 20 km/s is a realistic range. Comets can arrive considerably faster, sometimes 50 km/s or more, due to their more eccentric orbits.
Small numbers in, genuinely enormous numbers out.
Kinetic energy scales brutally with velocity, and that's the whole reason a relatively modest asteroid can outrelease a nuclear weapon by orders of magnitude. If solar-system scale is the part that got you here, the Weight on Other Planets Calculator covers the other end of that physics, standing on a world instead of falling toward one.
Try 20 meters at 19 km/s. That's roughly Chelyabinsk.
Sources and References
- Collins, G. S., Melosh, H. J., and Marcus, R. A. Earth Impact Effects Program. impact.ese.ic.ac.uk. Accessed August 2026.
- NASA Jet Propulsion Laboratory, Center for Near-Earth Object Studies. Chelyabinsk and Tunguska event data. cneos.jpl.nasa.gov. Accessed August 2026.