Little Boy vs Fat Man
Nuclear weapon side-by-side comparison
Little Boy
USA · 1945 · pure fission
15 kt
Hiroshima bomb.
Fat Man
USA · 1945 · pure fission
21 kt
Nagasaki bomb.
Verdict
The Fat Man is 1.4× more powerful than the Little Boy by yield. Both weapons are described in detail below, with side-by-side blast and thermal radii, casualty estimates at a reference urban density, and historical context.
Blast & thermal effects (air burst)
| Effect zone | Little Boy | Fat Man |
|---|---|---|
| Yield | 15 kt | 21 kt |
| Fireball radius | 0.43 km | 0.49 km |
| Severe blast (20 PSI) | 1.18 km | 1.32 km |
| Moderate blast (5 PSI) | 2.52 km | 2.81 km |
| Light blast (1 PSI) | 7.16 km | 8.00 km |
| 3rd-degree thermal burns | 2.03 km | 2.33 km |
| 2nd-degree thermal burns | 3.64 km | 4.18 km |
Casualty estimate (dense urban target)
Estimated immediate fatalities if each weapon air-bursts over a city center with a population density of about 8,000 per km² (representative of dense urban cores like Manhattan or central Paris):
Real casualties depend strongly on time of day, sheltering, weather, and altitude of detonation. See the methodology page for the underlying formulas.
Hiroshima reference
The Hiroshima bomb (Little Boy) had a yield of approximately 15 kilotons. The Little Boy is 1× more powerful than the Hiroshima bomb. The Fat Man is 1× more powerful.
Run them on the simulator
Related comparisons
FAQ
Is the Little Boy bigger than the Fat Man?
No. The Fat Man (21 kt) is 1.4× more powerful than the Little Boy (15 kt).
What is the blast radius of the Little Boy compared to the Fat Man?
For an air burst, the Little Boy produces a 5 PSI moderate-blast radius of 2.52 km, versus 2.81 km for the Fat Man. Severe blast (20 PSI): 1.18 km vs 1.32 km. 3rd-degree thermal burns: 2.03 km vs 2.33 km.
How many people would die from a Little Boy compared to a Fat Man?
Detonated over a typical dense city center (8,000 people/km²), the Little Boy would cause an estimated 96,377 immediate fatalities. The Fat Man would cause an estimated 120,341. Real numbers depend on time of day, sheltering, weather, and altitude of detonation.
How do the Little Boy and Fat Man compare to the Hiroshima bomb?
The Little Boy is 1× more powerful than the Hiroshima bomb (Little Boy, 15 kt). The Fat Man is 1× more powerful than the Hiroshima bomb.