Little Boy on San Francisco
15 kt fission weapon · USA · Population 873,000 · Density 7,300/km²
About this scenario
This page calculates what would happen if the Little Boy (USA, 1945) detonated over San Francisco (USA). Hiroshima bomb.
The financial and technology capital of the western United States, on a dense 121 km² peninsula. With an urban-core density of about 7,300 people per km², even a relatively small detonation over the city center would affect a large population.
The Little Boy delivers 15 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 (Little Boy over San Francisco)
| Effect zone | Radius | Est. affected |
|---|---|---|
| Fireball (vaporization, 100% fatal) | 0.43 km | ~4,208 |
| Severe blast (20 PSI, ~98% fatal) | 1.18 km | ~26,982 |
| Moderate blast (5 PSI, ~50% fatal) | 2.52 km | ~56,754 |
| Light blast (1 PSI, glass injuries) | 7.16 km | ~51,536 |
| 3rd-degree thermal burns | 2.03 km | — |
| 2nd-degree thermal burns | 3.64 km | — |
Estimated total fatalities: ~87,944 · Estimated total affected (inside 1 PSI light-blast radius): ~1,176,057.
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 Little Boy struck ground level rather than detonating optimally above San Francisco.
| Effect zone | Radius (surface burst) |
|---|---|
| Fireball | 0.34 km |
| Severe blast (20 PSI) | 0.65 km |
| Moderate blast (5 PSI) | 1.38 km |
| Light blast (1 PSI) | 3.94 km |
| 3rd-degree thermal burns | 1.22 km |
| Lethal fallout zone | ~35.5 km |
Run this scenario in the simulator
See the actual blast zones overlaid on a map of San Francisco 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 Little Boy detonated over San Francisco?
In an air burst over San Francisco, the Little Boy (15 kt) would produce a fireball about 0.43 km in radius. The 5 PSI moderate-blast zone — where most residential buildings collapse — would extend to 2.52 km. Light blast damage and shattered windows would reach 7.16 km. Given San Francisco's urban density (~7,300/km²), this scenario yields an estimated 87,944 immediate fatalities and about 51,536 additional injured.
How many people would die in San Francisco from a Little Boy strike?
An air burst of the Little Boy over San Francisco could cause an estimated 87,944 immediate fatalities and 51,536 additional injuries. The fireball alone (radius 0.43 km) would kill approximately 4,208 people; the severe-blast zone (20 PSI, radius 1.18 km) would add 26,982; the moderate-blast zone (5 PSI, radius 2.52 km) would add 56,754 more. Real numbers depend heavily on time of day, sheltering, weather, and altitude of detonation.
What is the blast radius of the Little Boy on San Francisco?
For an air burst over San Francisco: fireball 0.43 km, severe blast (20 PSI) 1.18 km, moderate blast (5 PSI) 2.52 km, light blast (1 PSI) 7.16 km. Thermal radiation causes 3rd-degree burns out to 2.03 km. A surface burst would shrink the blast radii by roughly 40 percent but generate massive radioactive fallout extending ~35 km from ground zero.
Is the Little Boy bigger than the bomb that hit Hiroshima?
The Hiroshima bomb (Little Boy) had a yield of about 15 kilotons. The Little Boy at 15 kt is comparable in yield to the Hiroshima bomb.