Little Boy vs Fat Man
Nuclear weapon side-by-side comparison
Verdict
The Fat Man is 1.4× more powerful than the Little Boy by yield. Both weapons are detailed below, with side-by-side blast and thermal radii for an air burst.
Blast & thermal radii (air burst)
| Effect zone | Little Boy | Fat Man |
|---|---|---|
| Yield | 15 kilotons | 21 kilotons |
| Fireball radius | 0.18 km | 0.21 km |
| Severe blast (20 psi) | 0.54 km | 0.61 km |
| Moderate blast (5 psi) | 1.54 km | 1.73 km |
| Light blast (1 psi) | 4.26 km | 4.78 km |
| 3rd-degree thermal burns | 2.18 km | 2.55 km |
| 2nd-degree thermal burns | 2.80 km | 3.26 km |
Air burst at optimum height, computed by the site's effects engine. [2, 3] Casualty and fallout estimates are withdrawn pending a verified model.
Hiroshima reference
The Hiroshima bomb (Little Boy) had a yield of approximately 15 kilotons. [1] The Little Boy is the Hiroshima bomb. The Fat Man is 1.4× more powerful than the Hiroshima bomb.
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FAQ
Is the Little Boy bigger than the Fat Man?
No. The Fat Man (21 kilotons) is 1.4× more powerful than the Little Boy (15 kilotons) by yield.
What is the blast radius of the Little Boy compared to the Fat Man?
For an air burst at optimum height, the Little Boy produces a 5 psi moderate-blast radius of 1.54 km, versus 1.73 km for the Fat Man. Severe blast (20 psi): 0.54 km vs 0.61 km. 3rd-degree thermal burns: 2.18 km vs 2.55 km.
How do the Little Boy and Fat Man compare to the Hiroshima bomb?
The Hiroshima bomb (Little Boy) was about 15 kt. The Little Boy is the Hiroshima bomb. The Fat Man is 1.4× more powerful than the Hiroshima bomb.
Sources
- 1.Source: US DOE / NNSA Nevada Field Office, United States Nuclear Tests, July 1945 through September 1992 (DOE/NV-209 Rev. 16, Sep 2015), NNSS publication library — retrieved Oct 2026
- 2.Source: U.S. DoD / DOE (Glasstone & Dolan, via Atomic Archive), The Effects of Nuclear Weapons, 3rd ed. (1977) §2.127 — retrieved Oct 2026
- 3.Source: Wikipedia, Effects of nuclear explosions — scaling-law table — retrieved Oct 2026