Blast Shock Wave Pressure Calculator

Blast overpressure — shock wave

When a high explosive detonates it throws off a shock wave, and the peak overpressure P (how far above atmospheric the pressure spikes, in kPa) falls off with distance while also tracking the TNT-equivalent mass W of the charge. The engineering correlation most people reach for feeds the scaled distance Z = R/W^(1/3) (m/kg^⅓), where R is in metres and W in kilograms of TNT, into empirical curves such as Kingery–Bulmash or Sadovsky. As a rough guide to damage: ~1 kPa is a light noise, ~7 kPa is enough to break ordinary window glass, ~35 kPa means serious structural damage, and ~70 kPa+ can be lethal. For scale, the Hiroshima Little Boy bomb came in around 15 kt TNT equivalent.

Applications

Civil defence agencies lean on it to set evacuation and exclusion radii. In controlled demolition with RDX, dynamite or emulsions, it helps size the charges and the safety distances. Forensic investigators of bomb attacks and industrial accidents work it backwards, estimating the charge mass from the damage they find on site (broken windows, collapsed walls). It also turns up in military engineering for fortification and standoff design, and in safety analysis of fuel depots and pyrotechnics warehouses.

FAQ

What is "TNT equivalent"? It's the mass of TNT that would release the same energy as the explosive you actually have. RDX runs about ~1.6× TNT, ANFO ~0.8×, gunpowder ~0.5×, so the conversion gives you a common yardstick across very different materials.

Why use scaled distance? Cube-root scaling lets one curve cover charges of wildly different sizes. Double W and you have to push the distance out by 2^(1/3) ≈ 1.26× just to hold the same overpressure.

Does this account for reflections? No, these are free-air values. A surface burst roughly doubles the effective charge, and a confined space pushes the pressure higher still.

At what overpressure do windows break? Ordinary glass starts cracking around 3–4 kPa, and most residential windows are gone by 7 kPa. Hardened or laminated glass holds out longer.