How big can a black hole get?

In one line

The rule

A black hole's size (radius) increases as its mass increases; there is no known maximum size for a black hole.

The formula for the Schwarzschild radius (the size of a simple, non-rotating black hole): Rₛ = 2GM/c² where:

• Rₛ = Schwarzschild radius (the "size" of the black hole)
• G = gravitational constant
• M = mass of the black hole
• c = speed of light

Let's understand it simply

Imagine a dustbin in your home. The more garbage you put in, the bigger the pile gets. Now, think of a black hole as a cosmic dustbin, but it can swallow not just dust, but stars, gas, planets—anything nearby. The more it eats, the bigger it gets. There is no lid on this dustbin! So, if a black hole keeps finding things to swallow, it keeps growing. In space, some black holes are small (like the size of a city), and some are huge (bigger than our whole solar system).

How it works

1. Black holes form when a massive star collapses under its own gravity.
2. Initial size depends on the mass of the star—bigger stars make bigger black holes.
3. Black holes can merge: When two black holes come close, they can combine into one larger black hole.
4. They can eat matter: If gas, dust, or stars fall in, the black hole grows.
5. No upper limit: As long as there is more matter to swallow, the black hole can keep growing.
6. Supermassive black holes: In the centers of galaxies, black holes can reach millions or even billions of times the Sun’s mass.

The story behind it

The idea of a black hole's size comes from Einstein’s theory of gravity (General Relativity, 1915). In 1916, Karl Schwarzschild found the formula for the radius of a black hole. At first, scientists thought black holes were just a strange math idea. But in the late 20th century, astronomers found real evidence of black holes in space. They also found that some black holes in galaxy centers are millions or billions of times heavier than the Sun. The biggest ones are called supermassive black holes.

Where you see it in real life

• Milky Way’s center: Our galaxy has a black hole about 4 million times the Sun’s mass—its size is about as big as Mercury’s orbit.
• M87 galaxy: The black hole in the center is about 6.5 billion times the Sun’s mass—its shadow was photographed in 2019.
• Merging black holes: When two black holes merge, the new one is bigger; this was detected by LIGO in 2015.
• Quasars: These are super-bright centers of galaxies powered by supermassive black holes eating lots of gas.
• Stellar-mass black holes: These are smaller, formed from single stars, and are about 3–20 times the Sun’s mass.
• Intermediate black holes: Rare, but found in some star clusters, with hundreds to thousands of Sun’s mass.

Why it matters

• Explains galaxy growth: Supermassive black holes shape how galaxies form and grow.
• Helps us understand gravity: Studying big black holes tests Einstein’s theory in extreme conditions.
• Gravitational waves: When big black holes merge, they send ripples through space we can detect on Earth.
• Space science and technology: Learning about black holes pushes the limits of telescopes and computers.

Worked example

Let’s find the Schwarzschild radius of a supermassive black hole with mass 1 billion times the Sun (M = 1,000,000,000 × mass of Sun).

Given: M = 1,000,000,000 × M_sun (M_sun = 2 × 10³⁰ kg), G = 6.67 × 10⁻¹¹ N·m²/kg², c = 3 × 10⁸ m/s
Step 1: Write the rule        Rₛ = 2GM/c²
Step 2: Put in the numbers    Rₛ = 2 × 6.67 × 10⁻¹¹ × (1 × 10⁹ × 2 × 10³⁰) / (3 × 10⁸)²
Step 3: Calculate numerator   = 2 × 6.67 × 10⁻¹¹ × 2 × 10³⁹ = 26.68 × 10²⁸
Step 4: Calculate denominator = 9 × 10¹⁶
Step 5: Divide                Rₛ = (26.68 × 10²⁸) / (9 × 10¹⁶) = 2.96 × 10¹² m
Step 6: Convert to km         Rₛ ≈ 2.96 trillion meters = 2.96 billion km

So, this black hole’s size is about 2.96 billion kilometers—about 20 times the distance from the Sun to Earth!

Common mistakes

• Thinking black holes are always small: Some are huge, bigger than the whole solar system.
• Believing there is a maximum size: There is no known upper limit; they grow as long as they find matter.
• Confusing mass and size: The black hole’s size is not the same as its mass; size grows with mass, but not in a simple straight line.
• Forgetting about merging: Black holes can combine to make even bigger ones.

Remember it

A black hole can grow endlessly—its size depends on how much it eats, with no fixed maximum.