How Big Is a Black Hole? Size Comparison From Stellar-Mass to TON 618
Black Holes · Size comparison · Updated 2026-07-22
A black hole's size is one number derived from one input: its event horizon radius is about 2.95 km for every solar mass. A 10-solar-mass black hole is roughly 30 km across the radius, Sagittarius A* at the centre of our galaxy is about 12.7 million km, and TON 618 - the largest well-known estimate - is on the order of 1,300 astronomical units, wider than the Solar System's planetary region many times over.
That single relation, the Schwarzschild radius, is linear in mass, which is why black hole size comparisons look so extreme: multiply the mass by a billion and you multiply the horizon by a billion. Here is the formula, a worked example, and the full ladder with real masses, real radii and honest flags on which numbers are measured and which are inferred.
The one formula: Schwarzschild radius
For a non-rotating black hole, the event horizon sits at the Schwarzschild radius, Rs = 2GM/c². Plug in the Sun's mass and you get 2.954 km. Because G and c are constants, everything else is proportion: radius in kilometres is roughly 2.95 times the mass in solar masses.
Worked example. Take a typical stellar-mass black hole of 10 solar masses. 10 x 2.95 = 29.5 km radius, so about 59 km across - a horizon roughly the width of Greater London, which spans about 50 km east to west. Scale up to Sagittarius A* at 4.3 million solar masses and you get 4,300,000 x 2.95 = 12.7 million km, roughly 18 times the radius of the Sun itself and about a fifth of the way out to Mercury's orbit.
Black hole size comparison chart
| Object or class | Mass (solar masses) | Schwarzschild radius | Comparable to | Distance from Earth | Imaged or inferred |
|---|---|---|---|---|---|
| Earth (hypothetical) | 0.000003 | 8.9 mm | A large marble | - | Thought experiment only |
| The Sun (hypothetical) | 1 | 2.95 km | A small town | - | Thought experiment only |
| Typical stellar-mass black hole | 10 | 29.5 km | A mid-size city | Nearest candidates a few thousand ly | Inferred from orbits and X-rays |
| Cygnus X-1 | 21.2 (2021 VLBI estimate) | About 63 km | 125 km across, a large metro area | About 7,200 light-years | Inferred from the binary orbit |
| Sagittarius A* | 4.3 million | 12.7 million km | 18 solar radii; a fifth of Mercury's orbit | About 27,000 light-years | Directly imaged (EHT, 2022) |
| M87* | 6.5 billion | 19.2 billion km (about 128 AU) | Far beyond Neptune's orbit at 30 AU | About 55 million light-years | Directly imaged (EHT, 2019) |
| TON 618 | About 66 billion (contested) | About 194 billion km (about 1,300 AU) | Roughly 0.02 light-years across the radius | Light travel time about 10.8 billion years | Inferred from quasar emission lines |
Two rows deserve caveats. Cygnus X-1 was long quoted at about 15 solar masses, which is where the frequently repeated "44 km" figure comes from; the 2021 VLBI parallax work by Miller-Jones and colleagues revised the mass upward to 21.2 solar masses, and later analyses have argued for values back down near 14 to 18. TON 618 is the shakiest number on the page: the 66-billion figure comes from broad-line region scaling of the Hβ line, and a 2019 reanalysis using the C IV line gives about 40.7 billion solar masses instead, which would shrink the horizon to roughly 800 AU.
The misconception worth killing: black holes are not cosmic vacuum cleaners
The most common intuition about black hole size is that they suck things in from a distance. Gravity does not work that way. Replace the Sun, right now, with a black hole of exactly one solar mass and Earth's orbit does not change at all - same 365.25-day year, same 1 AU semi-major axis, same orbital speed. The only thing that changes is the light. Outside the original radius of a body, the gravitational field of a spherical mass depends only on the mass enclosed, not on how tightly that mass is packed.
What we have actually seen versus what we have calculated
- Directly imaged: two. The Event Horizon Telescope published M87 in April 2019 and Sagittarius A in May 2022. Both images show the bright ring around the shadow, not the horizon itself: for a non-rotating hole the shadow's radius is about 2.6 Schwarzschild radii, so the dark patch is more than five Schwarzschild radii across.
- Measured by orbits. Sagittarius A*'s mass comes from decades of tracking stars such as S2 looping around it, work that won the 2020 Nobel Prize in Physics for Genzel and Ghez.
- Measured by gravitational waves. LIGO and Virgo have weighed dozens of stellar-mass black holes from their merger signals, mostly between 5 and 80 solar masses.
- Inferred from quasar light. The ultramassive records rest on scaling relations between emission line widths and luminosity, and carry factor-of-two uncertainty. Treat any "biggest black hole in the universe" headline as a best estimate, not a measurement.
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How to read any black hole size claim in ten seconds
- Find the mass, not the size: Every size claim is downstream of a mass. If an article gives a diameter but no mass, it has hidden the only number that matters.
- Multiply by 2.95: Mass in solar masses times 2.95 gives the horizon radius in kilometres. Divide by 149,600,000 to convert to AU once you are past a few million solar masses.
- Ask how the mass was obtained: Stellar orbits and gravitational waves are tight. Single-epoch emission line scaling, which is what the record holders use, is the loosest.
- Check whether spin was included: A maximally rotating Kerr black hole has a horizon at half the Schwarzschild radius, so some sources quote smaller numbers for the same mass. The 2.95 figure is the non-rotating baseline.
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Frequently asked questions
What is the biggest black hole in the universe?
The largest widely cited estimate is TON 618, a quasar at redshift 2.219, at around 66 billion solar masses - an event horizon radius near 1,300 AU. That figure comes from emission line scaling rather than direct measurement, and a 2019 reanalysis using the C IV line puts it closer to 40.7 billion solar masses. Phoenix A and a handful of other ultramassive candidates are quoted in the same range with similar uncertainty.
What would happen if the Sun became a black hole?
Nothing gravitational. A one-solar-mass black hole has a horizon radius of 2.95 km, and outside the Sun's original surface the gravitational field is identical, so Earth would keep the same orbit and the same year length. The problem is light and heat: the sky goes dark and the surface temperature collapses within weeks.
How big is the event horizon of Sagittarius A*?
About 12.7 million km in radius, or roughly 25 million km across, from a mass near 4.3 million solar masses. That is about 18 times the radius of the Sun, and it would fit inside Mercury's orbit with room to spare. The Event Horizon Telescope published the first image of it in May 2022.
How do you calculate the size of a black hole?
Use the Schwarzschild radius, R = 2GM/c². In practice, multiply the mass in solar masses by 2.95 to get the radius in kilometres. A 30-solar-mass black hole gives 88.5 km; a billion-solar-mass one gives 2.95 billion km, about 20 AU. Rapid rotation shrinks the horizon, down to half that value in the extreme Kerr limit.