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Recall that every massive object has an escape velocity:

v_{esc}={\sqrt{\frac{2GM}{r}}}

Where M and r are the mass and the radius of an object respectively. The escape velocity from any body depends on its mass, and on the starting distance. The escape velocity is larger for larger masses and smaller distances.

The radius at which the escape speed from the black hole equals the speed of light is called the Schwarzschild radius:

G = Newton’s gravitational constant (~6.67x10^-11 m³ kg^-1 s^-2 )

c = speed of light (~300,000,000 m/s)

M = Mass of the object

This equation can also be expressed in units of masses of the Sun and it is then easer to use:

Earth’s Schwarzschild radius is about a centimeter; the Sun’s is about 3 km.