A black hole’s “size” is usually measured by the radius of its event horizon —the boundary from which nothing, not even light, can escape. That radius depends on the black hole’s mass , so black holes can range from smaller than a marble to wider than our entire Solar System.

Quick size scale

  • A stellar‑mass black hole (about 10 times the Sun’s mass) has an event‑horizon radius of roughly 30 km , so its “diameter” is about 60 km.
  • A black hole with the same mass as Earth would be squeezed into a sphere only about 9 mm across—smaller than a golf ball.
  • The supermassive black hole at the center of our Milky Way, Sagittarius A*, is about 4.3 million times the Sun’s mass and has an event‑horizon radius of roughly 12 million km , or about 0.08 AU (less than 10% of Earth’s orbital radius).
  • The most extreme known supermassive black holes (around 40 billion solar masses) can have event‑horizon diameters roughly the size of the Solar System.

Typical black‑hole size classes

Class| Approx. mass (Sun = 1)| Approx. event‑horizon radius| Size comparison
---|---|---|---
Micro / primordial| up to Moon mass| up to ~0.1 mm| smaller than a grain of sand 9
Stellar‑mass| 2–150 M☉| ~3–150 km| city‑ to country‑scale 9
Intermediate‑mass| 10²–10⁵ M☉| ~1,000 km| roughly Earth‑size 9
Supermassive| 10⁶–10¹⁰ M☉| millions to billions of km| up to Solar‑System‑scale 59

Why “how big” is tricky

  • The singularity at the center is thought to be a point of infinite density with effectively zero volume , so the “object” itself isn’t spread out in space.
  • What we usually mean by “size” is the event‑horizon radius , given by the Schwarzschild formula R=2GM/c2R=2GM/c^2R=2GM/c2, which scales linearly with mass.

In short: black holes can be tiny or gigantic , but their visible “size” is defined by how much mass is packed inside their event horizon.

Information gathered from public forums or data available on the internet and portrayed here.