What is TNT equivalent?

A standard measure of explosive energy. 1 kg TNT = 4.184 MJ. Using TNT equivalent makes different explosive scenarios easier to compare on the same energy scale.

When the blast wave reflects off the ground and merges with the incident wave, it forms an amplified shock front called a Mach stem. That is why surface bursts can produce stronger ground-level overpressure than free-air bursts of the same yield.

How accurate are these estimates?

They are useful for open-terrain screening and education, but actual blast effects vary with confinement, elevation, obstacles, and atmospheric conditions. Treat the numbers as approximate standoff guidance, not a detailed site-specific prediction.

What overpressure causes injuries?

Human injury thresholds are approximate and depend on exposure and shielding. Eardrum rupture is often discussed around 5 psi, serious lung injury around 15 psi, and life-threatening effects at still higher pressures.

What is the cube-root scaling law?

Hopkinson-Cranz scaling: similar blast effects occur at distances proportional to the cube root of the yield. Doubling the yield only extends the damage radius by 26%.

Does this account for fragments?

No. Fragment range can far exceed the blast damage radius for cased explosives. If fragmentation is important, it needs to be assessed separately from overpressure.

Blast Radius Calculator

Calculate blast overpressure zones, damage radii, and fireball size for explosions using Hopkinson-Cranz scaling. Educational physics tool.

This calculator is for educational physics/engineering purposes only. Results are approximate using simplified Kingery-Bulmash scaling.

Presets

Yield (kg TNT equivalent)

Burst Type

Target Distance (m)

Atmospheric Pressure (kPa)

Heavy Damage Radius

18.0 m

~10 psi / 69 kPa — destruction of reinforced structures

Moderate Damage

30.6 m

~5 psi / 34 kPa — collapse of residential buildings

Light Damage

82.8 m

~1 psi / 6.9 kPa — minor structural damage, injuries

Glass Breakage

126.0 m

~0.5 psi / 3.4 kPa — windows shatter

Fireball Radius

20.0 m

Approximate radius of the fireball

Overpressure at Target

0.18 kPa

0.03 psi at 100 m

Blast Energy

4.184e+9 J

Total energy release (TNT × 4.184 MJ/kg)

Cube-Root Scaling

10.00 m/unit

W^(1/3) — Hopkinson-Cranz scaling factor

Damage Zones

Heavy: 18.0 m

Moderate: 30.6 m

Light: 82.8 m

Glass: 126.0 m

Overpressure vs Distance

Distance (m)	Scaled Z	Overpressure (kPa)	Overpressure (psi)
10	1.00	17.92	2.60
25	2.50	2.20	0.32
50	5.00	0.52	0.08
100	10.00	0.18	0.03
200	20.00	0.08	0.01
500	50.00	0.03	0.00
1000	100.00	0.01	0.00

Planning notes, formulas, and examples

About the Blast Radius Calculator

The **Blast Radius Calculator** estimates the overpressure damage zones from an explosion using Hopkinson-Cranz cube-root scaling and simplified Kingery-Bulmash blast parameters. This is a standard engineering tool used in safety analysis, building design near explosive hazards, and munitions effects evaluation.

Given the TNT-equivalent yield, burst type (surface or free-air), and atmospheric conditions, the calculator returns the radii for heavy structural damage, moderate damage, light damage, and glass breakage. It also computes the overpressure at any specified distance, the fireball radius, and the total blast energy.

This calculator is intended for **educational and engineering safety** purposes — understanding safe standoff distances, designing blast-resistant structures, and teaching the physics of shock-wave propagation. All results are approximate; real-world blast effects depend on terrain, structures, confinement, reflections, and fragment distribution. Use it to compare scenarios and understand order-of-magnitude effects rather than as a substitute for a formal blast assessment. It is best treated as a first-pass screening tool that helps frame the right standoff question before a specialist model is used.

When This Page Helps

Blast effects do not scale intuitively, and rough mental estimates can be badly misleading when comparing different charge sizes or standoff distances. This calculator gives a structured way to compare overpressure zones, point overpressure, and fireball size using standard scaling laws so engineers and students can discuss the same assumptions from the start.

How to Use the Inputs

Select a preset or enter the explosive yield in kg of TNT equivalent.
Choose surface burst (ground reflection doubles effective yield) or free-air burst.
Enter the target distance for point-specific overpressure.
Adjust atmospheric pressure if not at sea level.
Read damage-zone radii, overpressure at target, fireball size, and blast energy.
Use the distance table for a full overpressure profile.

Formula used

Cube-Root Scaling: Z = R / W^(1/3)
Damage radii scale as R ∝ W^(1/3) × K (constant for each overpressure level)
Total Energy: E = W × 4.184 MJ/kg
Surface Factor: ×1.8 (ground reflection / Mach stem amplification)

Simplified Kingery-Bulmash overpressure used for the distance profile.

Example Calculation

Result: Heavy damage ≈ 18 m, moderate ≈ 31 m, light ≈ 83 m, glass ≈ 126 m

A 1-tonne TNT surface burst produces heavy structural damage (~10 psi) within about 18 m, and glass breakage out to ~126 m. At 100 m, overpressure is approximately 2.5 kPa (0.36 psi).

Tips & Best Practices

Damage radius scales as the cube root of yield — a 1 000× larger bomb only extends the radius ~10×.
Surface bursts are more damaging at ground level due to Mach stem amplification.
For safety analysis, always use conservative (higher-yield) estimates.
Fragments, thermal radiation, and cratering are separate hazards not captured by overpressure alone.
Compare the overpressure at your standoff to building resistance ratings.

How To Read The Damage Zones

The zone labels are shorthand for overpressure ranges, not guarantees of a specific outcome. Heavy, moderate, and light damage bands help compare relative severity, but real structures respond differently depending on materials, orientation, openings, and reflected pressure. Use the zones as screening thresholds and then move to a dedicated structural review if the stakes are high.

Limits Of Cube-Root Scaling

Hopkinson-Cranz scaling is a good first-order model for open-air blasts, but it does not capture every real-world effect. Confinement, nearby walls, urban reflections, partial shielding, and unusual charge geometry can all shift the pressure field. That is why the calculator is most useful for education, standoff comparisons, and early safety checks rather than final design approval.

Hazards Not Included

Overpressure is only one part of the picture. Fragments, thermal radiation, crater effects, and secondary debris can extend the dangerous area well beyond the pressure zone shown here. If you are reviewing a real hazard scenario, treat this output as one layer in a broader safety analysis.

Sources & Methodology

Last updated: January 15, 2025

Frequently Asked Questions

A standard measure of explosive energy. 1 kg TNT = 4.184 MJ. Using TNT equivalent makes different explosive scenarios easier to compare on the same energy scale.