The heaviest black holes known are so massive that astronomers have started calling them ultramassive: tens of billions of times the mass of the Sun. Which one is the largest of all is surprisingly hard to say, because each is weighed in a different way, and the methods often disagree.
The contenders
TON 618 is a quasar nearly 11 billion light-years away. Its black hole has been estimated at about 40 billion solar masses from the motion of gas glowing near it; an earlier estimate from the same data gave 66 billion.
The Cosmic Horseshoe, a galaxy that bends the light of a more distant one into an almost complete ring, has a black hole of about 36 billion solar masses. In 2025 Melo-Carneiro and colleagues measured it by combining the lensing with the motions of stars, one of the more direct methods.
Abell 1201, the central galaxy of a cluster, was found in 2023 by James Nightingale and colleagues to bend background light in a way that requires a black hole of about 33 billion solar masses, though the uncertainty is large.
Holm 15A was announced in 2019 at about 40 billion solar masses. More detailed modelling in 2025 brought it down to about 22 billion.
Phoenix A, at the heart of the Phoenix Cluster, has a model-based estimate of up to 100 billion solar masses, but this has not been measured directly, and another estimate is about eight times lower.
Why the numbers disagree
The most reliable way to weigh a black hole is to watch stars or gas orbit it. For the most distant ones that is impossible, so astronomers rely on indirect methods: the width of spectral lines in a quasar’s light, the way a galaxy’s core is shaped, or how its gravity bends light. Each depends on assumptions. As the list of most massive black holes itself notes, the masses derived by different methods often contradict each other.
Is there a limit?
There may be. In 2016 Andrew King argued that a black hole feeding from a disc of gas cannot grow much beyond about 50 billion solar masses: beyond that, the disc would form stars instead of feeding the black hole. A rapidly spinning black hole might reach about 270 billion. Black holes could still grow larger by merging with each other.