Blast Radius Calculator
Calculate blast radius and safe stand-off distance from explosive detonations using the Hopkinson-Cranz scaling law. Free online blast radius calculator for safety planning.
About This Calculator
The Blast Radius Calculator estimates the safety distance from explosive detonations using the Hopkinson-Cranz cube-root scaling law — the internationally recognized method for blast radius estimation based on decades of empirical data from military and industrial testing. Whether you are an engineer, safety professional, or student studying blast effects, this calculator provides instant safety distance estimates for bare explosives and fragmenting munitions.
The calculator uses three standard formulas from the International Ammunition Technical Guidelines (IATG 01.80): for bare exposed explosives R = 130 × W⅓, for fragmenting munitions with public access R = 634 × W⅙, and for restricted access R = 444 × W⅙. The scaled distance Z = R / W⅓ is also calculated, which is the universal parameter used to estimate peak overpressure from any explosive.
The chart shows how blast radius scales with explosive mass — the radius grows as the cube root (for bare explosives) or sixth root (for fragmenting) of the mass, meaning doubling the mass does not double the danger radius. The damage level table shows at what distances different overpressure thresholds (1 psi to 20 psi) would be expected based on the calculated scaled distance.
Regional Notes
India: The Petroleum and Explosives Safety Organisation (PESO) governs explosive safety. The Hopkinson-Cranz law is the standard method for estimating safety distances in mining, construction, and demolition applications across India.
US: The US Army Technical Manual TM 5-1300 (now UFC 3-340-02) uses the same Hopkinson-Cranz scaling for blast-resistant design. The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) references these formulas for safe storage distance calculations.
UK: The Health and Safety Executive (HSE) and the Ministry of Defence use the IATG standards for ammunition safety. UK regulations for explosives storage and handling follow similar cube-root scaling principles for quantity-distance relationships.
Warning: This calculator provides indicative distances only. Actual safety planning must account for terrain, atmospheric conditions, structural shielding, and other factors. Always consult qualified explosives safety engineers.
Frequently Asked Questions
What is blast radius?
Blast radius is the distance from an explosion center beyond which no fragments would be expected. It is calculated using the Hopkinson-Cranz scaling law based on explosive mass and munition type. The blast radius is an indicative safety distance — it is still unsafe to be anywhere close to this boundary in a real-world scenario.
How is blast radius calculated?
The blast radius is calculated using the Hopkinson-Cranz cube-root scaling law. For bare exposed explosives: R = 130 × W^⅓. For fragmenting munitions with public access: R = 634 × W^⅙. For fragmenting munitions with restricted access: R = 444 × W^⅙, where W is the TNT equivalent mass in kilograms and R is the radius in meters.
What is the Hopkinson-Cranz scaling law?
The Hopkinson-Cranz scaling law states that two identical explosives with the same geometry but different sizes produce self-similar blast waves when the scaled distance Z = R / W^⅓ is the same. This allows estimating large explosion effects from smaller test data. The law is also known as the cube-root scaling law.
What damage levels correspond to different overpressures?
At 1 psi (scaled distance Z ≈ 40): broken windows. At 5 psi (Z ≈ 20): moderate damage to buildings. At 10 psi (Z ≈ 10): severe damage, near total destruction. At 20 psi (Z ≈ 5): reinforced concrete buildings heavily damaged. These thresholds are based on empirical blast data from military testing.
What is scaled distance Z?
Scaled distance Z = R / W^⅓ is the blast parameter used to compare explosions of different sizes. It represents the distance normalized by the cube root of explosive mass. The peak overpressure and other blast effects are functions of this scaled distance, making it a universal parameter for blast analysis.
What is the difference between bare exposed and fragmenting munitions?
Bare exposed explosives are unconfined explosives that produce a spherical blast wave. Fragmenting munitions are enclosed in a casing that breaks into fragments upon detonation, producing additional fragmentation hazards. Fragmenting munitions have larger safety distances due to the fragmentation hazard in addition to the blast overpressure.
Can this calculator be used for safety planning?
This calculator provides an indicative blast radius based on standard empirical formulas from IATG (International Ammunition Technical Guidelines). However, real-world safety planning requires professional assessment considering terrain, structures, atmospheric conditions, and other factors. Always consult qualified explosives safety engineers for actual safety planning.
How does explosive type affect blast radius?
Different explosives have different energy densities relative to TNT. The calculator uses TNT equivalent mass to standardize results. For example, C4 has approximately 1.13 times the energy density of TNT, while ANFO has about 0.88 times. The TNT equivalent factor normalizes all explosives to a common reference for consistent blast radius estimation.