This calculator is designed to allow you to determine the size of a black holes event horizon based upon its mass.

The smallest natural black hole is thought to be at least 1.4 solar masses because this number represents the Chandrasehkar limit (the most massive a neutron star can be before collapsing into a black hole).

Vesc = square root(2*G*m)/r   ->   r = (2*G*m) / Vesc²  

a = acceleration (meters/sec/sec)
Vesc = Escape velocity (299792458 meters/sec)
G = Gravitational constant (6.6732E-11)
m = mass (Kg)
r = radius (meters)