Blast Radius Calculator
When an explosive detonates it drives a shock wave outward, and the damage it does depends on the peak overpressure, the sudden spike in air pressure above normal, that reaches a given point. This calculator estimates the blast radius: the distance from the charge at which a chosen overpressure level is reached. You enter the charge size as a TNT equivalent in kilograms and the overpressure you care about in kilopascals, and it returns the radius, the scaled distance, the pressure in pounds per square inch, and a plain description of the damage expected at that level. It works using the Kinney-Graham equation, a well-established fit to measured blast data that relates peak overpressure to scaled distance, where scaled distance is the true distance divided by the cube root of the charge weight. Because of that cube-root scaling, making a charge eight times larger only doubles the radius for a given effect, which is why blast effects grow more slowly than people expect. Common reference levels help set the target: around 3.5 kilopascals shatters windows, around 20 kilopascals collapses ordinary houses, and around 35 kilopascals destroys most buildings. The tool is intended for education, emergency planning and understanding how blast effects scale, not as a precise safety boundary, since real explosions depend on the explosive type, confinement and surroundings. Change the charge or the target overpressure and the radius updates immediately.
Kinney-Graham scaling for a bare TNT charge in open air. Scaled distance Z is in metres per kilogram to the one-third power. Real events vary widely, so treat this as an estimate.
How it works
The calculator relies on the Kinney-Graham equation, which expresses peak overpressure as a function of the scaled distance Z. Scaled distance is the true distance R divided by the cube root of the charge weight W in kilograms of TNT, so R equals Z times the cube root of W. Because the overpressure depends only on Z for a given explosive, the whole problem scales with the cube root of the charge: an eightfold increase in charge doubles every radius. To find the blast radius for your chosen overpressure the calculator works backwards, searching for the scaled distance that produces that pressure in the Kinney-Graham equation, then multiplying by the cube root of the charge weight to get the distance in metres. The overpressure is also shown in pounds per square inch, since blast damage thresholds are often quoted in those units, and the likely damage is read from standard overpressure bands.
Worked example
Take a 10 kilogram TNT-equivalent charge and ask for the distance to 20 kilopascals of overpressure, roughly the level that collapses an ordinary house. The cube root of 10 is about 2.154. Solving the Kinney-Graham equation for 20 kilopascals gives a scaled distance of about 6.23 metres per kilogram to the one-third, so the radius is about 6.23 times 2.154, which is roughly 13.4 metres. That 20 kilopascals equals about 2.90 pounds per square inch, and the expected effect at that level is: Houses collapse; serious to fatal injuries. These match the defaults shown in the calculator above.
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Frequently asked questions
What does a blast radius calculator estimate?
It estimates the distance from an explosion at which the air blast reaches a chosen peak overpressure, the pressure spike above normal atmospheric pressure that causes injury and structural damage. You enter the charge weight as a TNT equivalent and a target overpressure, and it returns the radius at which that pressure level is reached, along with the scaled distance and the likely level of damage.
How is the blast distance calculated?
It uses the Kinney-Graham equation, a widely cited fit to measured blast data, which gives peak overpressure as a function of the scaled distance. Scaled distance is the true distance divided by the cube root of the charge weight, so distances scale with the cube root of the charge. The calculator solves the equation for the distance that produces your chosen overpressure.
How accurate is the estimate?
The scaling law is a good general model for a bare TNT charge in open air, but real events depend on the explosive type, confinement, height of burst, terrain and reflections. TNT equivalence itself is approximate. Treat the result as an order-of-magnitude estimate for education and planning, not a precise safety boundary.
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