Kaboomly / Blast radius
Nuclear Blast Radius
A nuclear explosion does not have one blast radius. It has several, one for each kind of effect and each level of damage. This page explains what those rings mean and how they change with the size of the weapon.
All distances here are calculated when the site is built, by the same model that runs in the Kaboomly nuke simulator map above. They are rough estimates for education, based on Glasstone and Dolan's The Effects of Nuclear Weapons (1977).
What "blast radius" means
About half of the energy of a typical nuclear explosion in the lower atmosphere goes into the blast wave, according to Glasstone and Dolan (chapter I). The wave is a shell of compressed air that moves outward faster than sound. What damages buildings is the sudden jump in air pressure as it passes, called peak overpressure, and the strong winds that follow it.
Overpressure is measured in pounds per square inch (psi) above normal air pressure. The number sounds small, but it acts on every square inch of a wall. The simulator draws three overpressure rings, because each marks a different kind of damage:
- 20 psi: heavily built concrete buildings are severely damaged or destroyed.
- 5 psi: most houses and other residential buildings collapse or are badly damaged.
- 1 psi: windows shatter and flying glass becomes a hazard; buildings take light damage.
The other effect radii
Fireball
The fireball is the glowing sphere of extremely hot gas formed in the first second. Glasstone and Dolan give its maximum size for a 1-megaton explosion as about 5,700 feet across (§2.05), and its radius grows with about the 0.4 power of the yield (§2.127). In a surface burst it touches the ground, and the simulator treats it as behaving like an explosion of twice the yield, following the same source.
Third-degree burns
The flash of thermal radiation travels at the speed of light and can burn exposed skin far beyond the worst blast damage. The simulator draws the distance at which there is a 50% chance of third-degree burns on bare skin facing the explosion. The heat needed rises with yield, because a larger explosion delivers its heat over a longer pulse (Glasstone and Dolan, Fig. 12.65). For a 15 kt explosion the model uses about 7.47 calories per square centimeter; for 1.2 Mt it uses about 9.85. The distances assume 12 miles of visibility, a typical clear day over a city.
500 rem initial radiation
Initial nuclear radiation is the burst of neutrons and gamma rays released within the first minute. The ring marks where an unshielded person would receive 500 rem. Unlike heat and blast, this radiation is absorbed quickly by air, so its range grows only slowly with yield.
Nuke blast radius by yield
The tables below list every effect radius for the seven weapon presets in the simulator, for an airburst at the fixed example height and for a surface burst. Distances are measured along the ground from ground zero.
| Weapon | Fireball | 20 psi | 5 psi | 1 psi | 3rd-degree burns | 500 rem |
|---|---|---|---|---|---|---|
| Little Boy15 kt | 0.16 km0.10 mi | 0.42 km0.26 mi | 1.6 km0.98 mi | 4.5 km2.8 mi | 2.2 km1.3 mi | 1.3 km0.84 mi |
| Fat Man21 kt | 0.18 km0.11 mi | 0.47 km0.29 mi | 1.8 km1.1 mi | 5.0 km3.1 mi | 2.5 km1.5 mi | 1.4 km0.87 mi |
| W76100 kt | 0.35 km0.21 mi | 0.79 km0.49 mi | 3.0 km1.8 mi | 8.4 km5.2 mi | 4.7 km2.9 mi | 1.6 km1.0 mi |
| W88*475 kt | 0.65 km0.40 mi | 1.3 km0.83 mi | 5.0 km3.1 mi | 14 km8.8 mi | 8.3 km5.2 mi | 1.6 km0.99 mi |
| B83*1.2 Mt | 0.93 km0.58 mi | 1.8 km1.1 mi | 6.8 km4.2 mi | 19 km12 mi | 11 km7.1 mi | 1.4 km0.89 mi |
| Castle Bravo*15 Mt | 2.6 km1.6 mi | 4.2 km2.6 mi | 16 km9.8 mi | 45 km28 mi | 24 km15 mi | noneat ground level |
| Tsar Bomba*50 Mt | 4.2 km2.6 mi | 6.3 km3.9 mi | 24 km15 mi | 67 km42 mi | 34 km21 mi | noneat ground level |
| Weapon | Fireball | 20 psi | 5 psi | 1 psi | 3rd-degree burns | 500 rem |
|---|---|---|---|---|---|---|
| Little Boy15 kt | 0.21 km0.13 mi | 0.55 km0.34 mi | 1.1 km0.71 mi | 3.5 km2.2 mi | 1.6 km1.0 mi | 1.4 km0.86 mi |
| Fat Man21 kt | 0.24 km0.15 mi | 0.61 km0.38 mi | 1.3 km0.80 mi | 3.9 km2.4 mi | 1.9 km1.2 mi | 1.5 km0.90 mi |
| W76100 kt | 0.46 km0.28 mi | 1.0 km0.64 mi | 2.2 km1.3 mi | 6.6 km4.1 mi | 3.5 km2.2 mi | 1.9 km1.2 mi |
| W88475 kt | 0.85 km0.53 mi | 1.7 km1.1 mi | 3.6 km2.3 mi | 11 km6.9 mi | 6.5 km4.0 mi | 2.4 km1.5 mi |
| B831.2 Mt | 1.2 km0.77 mi | 2.4 km1.5 mi | 4.9 km3.1 mi | 15 km9.4 mi | 9.2 km5.7 mi | 2.8 km1.8 mi |
| Castle Bravo*15 Mt | 3.4 km2.1 mi | 5.5 km3.4 mi | 11 km7.1 mi | 35 km22 mi | 22 km14 mi | 4.9 km3.0 mi |
| Tsar Bomba*50 Mt | 5.5 km3.4 mi | 8.2 km5.1 mi | 17 km11 mi | 53 km33 mi | 34 km21 mi | 6.0 km3.7 mi |
* At least one value in this row lies outside the range of the source charts (for example yields above 10 megatons, or burst heights above 5,000 feet) and is an extrapolation. "None at ground level" means the 500 rem dose does not reach the ground from that burst height. Figures come from the simulator's model and are rounded. Sources and tolerances are on the How It Works page.
How blast radius scales with yield
The most surprising thing in the table is how slowly the rings grow. A B83 (1.2 Mt) has about 80 times the yield of Little Boy (15 kt). As an airburst, its 5 psi radius is 4.3 times larger, not 80 times. That matches the cube-root scaling of blast that Glasstone and Dolan describe: the cube root of 80 is about 4.3. In everyday terms, eight times the yield is needed to double a blast distance.
Heat follows a different rule. Over the same step the burn radius grows 5.3 times, faster than blast, so over this range the burn ring sits further and further outside the 5 psi ring. At very large yields the atmosphere absorbs more of the heat and the growth slows. The initial radiation ring grows only 1.1 times over the same range, and for the largest airbursts in the table it does not reach the ground at all.
Airburst versus surface burst
Burst height changes the shape of the effects. Take the W76 preset (100 kt). As an airburst at the simulator's fixed height of about 1.0 km (0.63 mi), its 5 psi ring reaches 3.0 km (1.8 mi). As a surface burst, the same ring reaches 2.2 km (1.3 mi). The 20 psi ring behaves the other way: 0.79 km (0.49 mi) for the airburst and 1.0 km (0.64 mi) for the surface burst.
The reason is that a blast wave from above reflects off the ground and reinforces itself, spreading moderate overpressure over a wider area. A surface burst puts more of its force into the ground close by. Heat reaches farther from an airburst, while initial radiation and local fallout are greater for a surface burst. The simulator uses a single fixed airburst height, scaled from the Hiroshima explosion, and does not offer any height choice. The How It Works page explains why.
What these radii leave out
The rings assume flat, open ground and clear air. Hills and dense buildings shield areas behind them, and haze, rain, or snow change burn distances. Real effects also fade gradually rather than stopping at a line. Read the numbers as a sense of scale, not as a prediction. The full list is under limitations.
Try it on the map
Scroll back up to the map, pick a city you know, and compare an airburst with a surface burst for the same yield. The nuke simulator draws every ring from this page to scale, and the nuclear bomb map guide explains how to read the building colors and share what you find.
Frequently asked questions
What is the blast radius of a nuclear bomb?
There is no single number. Each effect reaches a different distance, depending on the yield and on whether the bomb explodes in the air or at the surface. Blast is usually described by overpressure rings: 20 psi for heavy damage to strong buildings, 5 psi for the collapse of most houses, and 1 psi for broken windows. The tables on this page list them for seven historical yields.
What does psi mean in a nuclear blast?
Psi stands for pounds per square inch. In blast charts it means peak overpressure: how far the air pressure in the blast wave rises above normal atmospheric pressure, which is about 14.7 psi at sea level. Even a few psi of overpressure pushes on a wall or window with a very large total force.
Does a bigger bomb have a proportionally bigger blast radius?
No. Blast distances grow roughly with the cube root of the yield, so a bomb with eight times the yield reaches only about twice as far. Burn distances grow faster than that at small and medium yields but are limited by the atmosphere at very large yields, and the initial radiation ring grows only a little.
Is an airburst or a surface burst more damaging?
They spread effects differently. An airburst reaches farther at the moderate 5 psi and 1 psi levels and for heat, because the blast reflects off the ground and the flash is not blocked by terrain. A surface burst gives stronger blast close to ground zero, a larger crater and fireball contact with the ground, and heavy local fallout. The simulator shows both options so you can compare them, and it does not recommend one.
Which ring is the most important?
It depends on the question. The 5 psi ring is the one most often quoted for widespread building damage, the 1 psi ring shows how far broken glass can reach, and the burn ring reaches well beyond the 5 psi ring at every preset yield. Looking at all of them together gives a better sense of scale than any single number.
Why is the 500 rem ring missing for the largest bombs?
In the simulator an airburst happens at a fixed scaled height that grows with yield. For the largest yields the burst is several kilometers up, and at that distance the initial radiation reaching the ground falls below 500 rem, so the ring has no size at ground level. Blast and heat still reach the ground over a very wide area.
How accurate are these blast radius numbers?
They are idealized estimates for flat ground, a standard atmosphere, and clear weather. The blast model agrees with worked examples in Glasstone and Dolan (1977) to within about 10 to 15 percent in most tested cases, but real terrain, buildings, and weather would change the outcome. See the How It Works page for the tested cases and limitations.