[ tool ]
Black hole size comparison, from Gaia BH1 to TON 618
A black hole with Earth's mass would fit in a marble; TON 618's would swallow the whole Solar System. Compare them with familiar objects, at the same scale.
Updated 8 October 2026
- Mass
- 4.3 × 10⁶ solar masses
- Horizon diameter
- 25.4 million km
- The horizon is 12 times smaller than Earth's orbit. The supermassive black hole at the centre of the Milky Way.
- The horizon is drawn in black with its ring of light; the comparison object is dashed.
A simple rule
The horizon size is proportional to the mass: double the mass, and the horizon doubles. That is why horizon radii span more than fifteen orders of magnitude, from 9 mm for a hypothetical Earth-mass black hole to more than a thousand astronomical units for TON 618.
For precise values for each black hole, use the calculator.
Frequently asked questions
What is the largest known black hole?
Among direct measurements, the one in the galaxy Holm 15A reaches about 40 billion solar masses. Indirect estimates, such as for the quasar TON 618, give between 40 and 70 billion, uncertain by a factor of 2 to 3.
Is a supermassive black hole very dense?
No, counter-intuitively: since the radius grows with mass, the mean density drops as 1/M². M87*'s, computed over the volume of its horizon, is lower than that of air.
Sources
- El-Badry et al. (2023), A Sun-like star orbiting a black hole, MNRAS 518, 1057
- Gaia Collaboration, Panuzzo et al. (2024), Discovery of a dormant 33 solar-mass black hole in pre-release Gaia astrometry, A&A 686, L2
- Miller-Jones et al. (2021), Cygnus X-1 contains a 21-solar mass black hole, Science 371, 1046
- GRAVITY Collaboration (2022), mass and distance of Sgr A* from stellar orbits, A&A 657, L12
- EHT Collaboration (2019), First M87 Event Horizon Telescope results. I. The shadow of the supermassive black hole, ApJL 875, L1
- Mehrgan et al. (2019), A 40 billion solar mass black hole in the extreme core of Holm 15A, ApJ 887, 195
- Shemmer et al. (2004), The hard X-ray spectral properties of high-redshift quasars, ApJ 614, 547 (TON 618 mass estimate)
Keep exploring
- Calculator →Pick a mass and get the horizon radius, Hawking temperature, evaporation time and entropy of the black hole.
- Black holes →How black holes form, which types exist, from Gaia BH1 to M87*, and how we photographed and heard them.
- Falling in →Spaghettification, time dilation, crossing the horizon: falling into a black hole step by step, with a gravitational lensing render.
- Quiz →Ten questions about black holes, horizons, Hawking and gravitational waves, with an explanation for every answer.