Fat Man on London
21 kt fission weapon · United Kingdom · Population 9,002,488 · Density 5,500/km²
About this scenario
This page calculates what would happen if the Fat Man (USA, 1945) detonated over London (United Kingdom). Nagasaki bomb.
The capital of the United Kingdom and most populous city in Western Europe, on the River Thames. With an urban-core density of about 5,500 people per km², even a relatively small detonation over the city center would affect a large population.
The Fat Man delivers 21 kt of explosive yield — comparable in yield to the Hiroshima bomb. The tables below show calculated effect radii for an air burst (optimized for blast spread) and a surface burst (which produces massive fallout).
Air-burst effects (Fat Man over London)
| Effect zone | Radius | Est. affected |
|---|---|---|
| Fireball (vaporization, 100% fatal) | 0.49 km | ~4,150 |
| Severe blast (20 PSI, ~98% fatal) | 1.32 km | ~25,193 |
| Moderate blast (5 PSI, ~50% fatal) | 2.81 km | ~53,392 |
| Light blast (1 PSI, glass injuries) | 8.00 km | ~48,484 |
| 3rd-degree thermal burns | 2.33 km | — |
| 2nd-degree thermal burns | 4.18 km | — |
Estimated total fatalities: ~82,735 · Estimated total affected (inside 1 PSI light-blast radius): ~1,106,401.
Surface-burst effects (with fallout)
A surface burst trades blast spread for radioactive fallout — much smaller blast radii but a large lethal fallout plume drifting downwind. This is what would happen if the Fat Man struck ground level rather than detonating optimally above London.
| Effect zone | Radius (surface burst) |
|---|---|
| Fireball | 0.39 km |
| Severe blast (20 PSI) | 0.72 km |
| Moderate blast (5 PSI) | 1.55 km |
| Light blast (1 PSI) | 4.40 km |
| 3rd-degree thermal burns | 1.40 km |
| Lethal fallout zone | ~40.6 km |
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See the actual blast zones overlaid on a map of London with population-density-based casualty estimates updated in real time as you move the detonation point.
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FAQ
What would happen if the Fat Man detonated over London?
In an air burst over London, the Fat Man (21 kt) would produce a fireball about 0.49 km in radius. The 5 PSI moderate-blast zone — where most residential buildings collapse — would extend to 2.81 km. Light blast damage and shattered windows would reach 8.00 km. Given London's urban density (~5,500/km²), this scenario yields an estimated 82,735 immediate fatalities and about 48,484 additional injured.
How many people would die in London from a Fat Man strike?
An air burst of the Fat Man over London could cause an estimated 82,735 immediate fatalities and 48,484 additional injuries. The fireball alone (radius 0.49 km) would kill approximately 4,150 people; the severe-blast zone (20 PSI, radius 1.32 km) would add 25,193; the moderate-blast zone (5 PSI, radius 2.81 km) would add 53,392 more. Real numbers depend heavily on time of day, sheltering, weather, and altitude of detonation.
What is the blast radius of the Fat Man on London?
For an air burst over London: fireball 0.49 km, severe blast (20 PSI) 1.32 km, moderate blast (5 PSI) 2.81 km, light blast (1 PSI) 8.00 km. Thermal radiation causes 3rd-degree burns out to 2.33 km. A surface burst would shrink the blast radii by roughly 40 percent but generate massive radioactive fallout extending ~41 km from ground zero.
Is the Fat Man bigger than the bomb that hit Hiroshima?
The Hiroshima bomb (Little Boy) had a yield of about 15 kilotons. The Fat Man at 21 kt is comparable in yield to the Hiroshima bomb.