Determine the mass of the celestial object, expressed in kilograms. In astronomy study problems, the mass will usually be provided.
Enter the mass of the object into the expression "2MG/c^2." G is the gravitational constant, or 6.67*10^-11. C is the speed of light in a vacuum: 300,000 meters per second.
Express the initial result in meters. Convert the meters measurement to kilometers or millimeters as necessary for simplicity. For example, a radius of 3,900 meters would be expressed as 3.9 kilometers.