Event Horizon Calculator

Calculator Inputs

Calculation mode *

Use Kerr for rotating black holes (outer horizon).

Mass value *

Mass unit

Spin a* (0 ≤ a* < 1)

Use 0 for non-rotating; try 0.9 for rapid spin.

Horizon radius value *

Horizon radius unit

Output radius unit

Decimal precision

Range: 0–12.

Extra outputs

Circumference Area Light-crossing time Surface gravity (Schwarzschild) Hawking temperature (Schwarzschild)

Surface gravity and temperature use the non-rotating approximation.

Reset

Formula Used

Schwarzschild radius (non-rotating):

R_s = 2GM / c²

Kerr outer horizon (rotating):

r₊ = r_g ( 1 + √(1 − a*²) ), r_g = GM / c²

Inverse (radius → mass, Schwarzschild):

M = R_s c² / (2G)

G is the gravitational constant, c is the speed of light, M is mass, and a* is dimensionless spin (0 to just under 1).

How to Use

Select a calculation mode for mass-based or radius-based solving.
Enter mass with units, or enter a horizon radius with units.
For rotating cases, provide a spin value between 0 and less than 1.
Pick an output unit and set decimal precision for display.
Enable extra outputs like area, circumference, or Hawking temperature.
Press Calculate to see results above the form.
Use CSV or PDF to export the latest computed result.

Example Data Table

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Object mass	Mass unit	Model	Horizon radius (km)