About this tool
The Schwarzschild Radius Calculator is a free, in-browser astrophysics tool that finds the event-horizon radius of a black hole of any given mass. It applies the classic result of general relativity, Rs = 2GM/c², where G = 6.674×10⁻¹¹ N·m²/kg² is the gravitational constant and c = 299,792,458 m/s is the speed of light. Enter a mass in kilograms, Earth masses, or solar masses (1 M☉ = 1.989×10³⁰ kg) and the radius is returned instantly.
Every value is computed locally in your browser with plain JavaScript — nothing you type is uploaded or logged, so the numbers stay on your device. As a bonus the tool also reports the Hawking temperature T = ħc³/(8πGMk_B), the tiny black-body temperature a black hole radiates at; it is inversely proportional to mass, so stellar-mass and larger black holes are colder than the cosmic microwave background.
Results are shown in metres, kilometres, and solar radii (1 R☉ = 6.9634×10⁸ m) so you can compare the horizon size to familiar scales. A one-solar-mass black hole has a Schwarzschild radius of about 2,953 metres — just under 3 km — while a supermassive black hole of millions of solar masses spans a radius comparable to the Sun-to-planet distances.