Black Hole Calculator — Schwarzschild Radius & Properties
Enter a black hole's mass in solar masses to compute its Schwarzschild radius (event horizon), event horizon surface area, photon sphere, innermost stable circular orbit, mean density and surface gravity.
solar masses
Radius of the event horizon — nothing escapes from inside
- 1
Mass in kilograms
10 M☉ × 1.989e30 kg/M☉ = 1.989e+31 kg - 2
Schwarzschild radius (m)
2 × 6.674e-11 × 1.989e+31 ÷ (2.998e8)² = 2.954e+4 mr_s = 2GM/c² — the radius at which escape velocity equals c. - 3
Convert to kilometres
2.954e+4 ÷ 1 000 = 29.54
How does this calculator work?
A black hole's event horizon (Schwarzschild radius) is r_s = 2GM/c². For 1 solar mass this is ≈ 2.95 km, scaling linearly with mass. The photon sphere sits at 1.5 r_s (closest light orbit) and the ISCO at 3 r_s (closest stable matter orbit). Density inside r_s falls as 1/M².
Formula
How this is calculated
A black hole's defining boundary is the event horizon — the surface from inside which not even light can escape. For a non-spinning (Schwarzschild) black hole this is a sphere of radius r_s = 2GM/c², where G is Newton's gravitational constant (6.674×10⁻¹¹ N·m²/kg²), M is the mass, and c is the speed of light (2.998×10⁸ m/s). For one solar mass, r_s ≈ 2.95 km; scale linearly with mass.
The photon sphere at 1.5 r_s is the closest orbit where photons (light) can travel in circles — any closer and they spiral inward. The ISCO (innermost stable circular orbit) at 3 r_s is the closest orbit where matter can orbit without inevitably falling in; accretion discs around black holes reach their inner edge here. The mean density inside r_s falls rapidly with mass — a stellar-mass black hole is extraordinarily dense, but a billion-solar-mass supermassive black hole would have a mean density lower than water.
All results assume a non-rotating Schwarzschild black hole. Rotating (Kerr) black holes have a smaller ISCO and an ergosphere outside the event horizon — this calculator does not model those.
Frequently asked questions
About 2.95 km. The Sun is far too large to be a black hole at its current size (its actual radius is ~696,000 km). It would have to be compressed to under 3 km for its own gravity to prevent light from escaping.
Schwarzschild radius scales linearly with mass (r_s ∝ M) but volume scales as the cube of radius (V ∝ M³). So density = M/V ∝ 1/M². A billion-solar-mass black hole has a mean density inside its event horizon lower than Earth's atmosphere — your body would feel almost nothing crossing the horizon.
Yes — enter a tiny mass (e.g. 0.000001 solar masses ≈ 2×10²⁴ kg, about Earth's mass). Very small black holes evaporate quickly via Hawking radiation; use the Hawking Temperature calculator to see how hot they are.
TG we-Calculate Editorial Team. (2026). Black Hole Calculator — Schwarzschild Radius & Properties [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/black-hole-calculator
TG we-Calculate Editorial Team. "Black Hole Calculator — Schwarzschild Radius & Properties." TG we-Calculate. 2026. https://we-calculate.com/calculator/black-hole-calculator.
TG we-Calculate Editorial Team, "Black Hole Calculator — Schwarzschild Radius & Properties," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/black-hole-calculator
@misc{wecalculate_black_hole_calculator, title = {Black Hole Calculator — Schwarzschild Radius & Properties}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/black-hole-calculator}}, year = {2026}, note = {TG we-Calculate} }
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