Black hole radius
Explore a black hole’s event horizon: set a mass and see its Schwarzschild radius and diameter. A 10-solar-mass black hole is about 59 km across.
Your experiment
How it works
The event horizon is a boundary in spacetime, not a material surface. For a non-rotating, uncharged black hole, its radius grows linearly with mass.
Schwarzschild Radius Calculator & Formula ↗About this model
The Schwarzschild solution: no rotation or electric charge. The glowing ring is an explanatory illustration, not an image prediction. It is not the size of an observed black hole shadow.
Key formulas
| Quantity | Formula | Note or example |
|---|---|---|
| Schwarzschild radius | rₛ = 2GM / c² | G = 6.674 × 10⁻¹¹ m³ kg⁻¹ s⁻², c = 299,792,458 m/s |
| Quick estimate | rₛ ≈ 2.95 km × (M / M☉) | Grows in proportion to mass |
| Event horizon diameter | D = 2rₛ | Black hole “size” |
Worked examples
Each row uses one of this lab’s presets and the same model as the interactive tool.
| Example | Horizon radius | Horizon diameter | Mass |
|---|---|---|---|
| Sun’s mass | 2.95 km | 5.91 km | 1 Sun |
| Stellar mass | 29.53 km | 59.07 km | 10 Suns |
| Sagittarius A* | 12.7 million km (0.085 AU) | 25.4 million km (0.17 AU) | 4,300,000 Suns |
| M87* | 19.2 billion km (128 AU) | 38.4 billion km (257 AU) | 6,500,000,000 Suns |
Black hole radius: common questions
Would a black hole with the Sun’s mass pull Earth in?
At Earth’s distance it would have the same gravitational pull as the Sun, neglecting other differences. Earth’s orbit would not suddenly collapse.
Is the bright ring the event horizon?
No. Glowing material and bent light can appear around a black hole; the horizon itself does not emit light that escapes.
What is the formula for the Schwarzschild radius?
rₛ = 2GM/c², where G is the gravitational constant, M the mass and c the speed of light. It is the radius at which the escape speed would equal the speed of light; for one solar mass it is about 2.95 km.
How big would a black hole with Earth’s mass be?
Its Schwarzschild radius would be about 8.87 millimeters, so the event horizon would be roughly 1.8 cm across — about the size of a marble.
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