How this calculator works
The Schwarzschild horizon is a spherical causal boundary in the vacuum solution outside a nonrotating uncharged mass. The surface gravity shown is normalized relative to time at infinity. It is not the proper acceleration of a stationary observer at the horizon, which diverges. Solar-mass comparison uses IAU 2015 nominal solar GM divided by CODATA G; because G is measured, that kilogram reference is not exact.
Formula
r_s = 2GM/c^2; kappa = c^4/(4GM) = c^2/(2r_s). Solar reference mass = (GM)_sun_nom/G.
Worked example
For mass 1e30 kg, the Schwarzschild radius is about 1485.232 m. Doubling mass doubles the radius and halves the horizon surface gravity.
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Frequently asked questions
Does this work for a spinning black hole?
A rotating horizon follows the Kerr solution and depends on angular momentum. This calculator assumes zero spin.
Is the radius the size of the singularity?
No. It is the horizon radius. The classical central singularity is not a material sphere.
Is surface gravity the acceleration I would feel?
No. It is a horizon parameter normalized at infinity; local hovering acceleration grows without bound near the horizon.