How Many Atoms Are in the Observable Universe?

The usual answer is about 10⁸⁰ atoms: a 1 followed by 80 zeros. It appears in textbooks, quizzes and documentaries, but it’s rarely explained. Where does such a number come from? Nobody counted. Instead, scientists combine a few measured quantities with simple arithmetic. It’s a wonderful example of estimation, and you can follow every step.

First, what “observable universe” means

The universe may be infinite, or at least far larger than what we can see. Light travels at a finite speed, and the universe has a finite age of about 13.8 billion years, so there’s a limit to how far away we can see. That region is the observable universe. Because space has been expanding while the light travelled, its radius today is about 46 billion light-years, not 13.8 billion.

Our estimate counts atoms only inside this sphere.

Method 1: count the stars

Almost all the ordinary matter we can see is in stars and the gas between them. Let’s start with stars.

Step 1: How many galaxies? Deep images from the Hubble and James Webb telescopes suggest the observable universe holds somewhere between a hundred billion and a couple of trillion galaxies. Take 10¹² galaxies (a trillion) as a round number.

Step 2: How many stars per galaxy? Galaxies vary enormously. Our Milky Way has perhaps 100 to 400 billion stars; small galaxies have far fewer. Take an average of 10¹¹ stars per galaxy.

So the number of stars is roughly:

10¹² × 10¹¹ = 10²³ stars

Step 3: How many atoms in a typical star? Use the Sun as a typical star. Its mass is about 2 × 10³⁰ kg. The Sun is mostly hydrogen, and a hydrogen atom has a mass of about 1.67 × 10⁻²⁷ kg. (The helium and heavier atoms in the Sun are heavier, but for a rough count, hydrogen is good enough.)

Atoms in the Sun ≈ (2 × 10³⁰ kg) ÷ (1.67 × 10⁻²⁷ kg) ≈ 1.2 × 10⁵⁷ atoms

Step 4: Multiply.

10²³ stars × 10⁵⁷ atoms per star = 10⁸⁰ atoms

That’s where the famous number comes from.

Method 2: use the density of the universe

A completely different route gives a similar answer, which is reassuring.

Measurements of the cosmic microwave background and the abundance of light elements tell cosmologists the average density of ordinary matter in the universe. It’s astonishingly low: about 0.25 hydrogen atoms per cubic metre on average, spread over all of space, including the vast empty voids between galaxies. (A room on Earth contains around 10²⁷ molecules of air per cubic metre.)

Now the volume. A sphere of radius 46 billion light-years is about 4.4 × 10²⁶ m in radius, so:

Volume = (4⁄3)πr³ ≈ (4⁄3) × 3.14 × (4.4 × 10²⁶ m)³ ≈ 3.6 × 10⁸⁰ m³

Atoms ≈ 0.25 atoms per m³ × 3.6 × 10⁸⁰ m³ ≈ 9 × 10⁷⁹ atoms

Again, about 10⁸⁰.

How precise is this?

Not very, and that’s fine. Each input is uncertain by a factor of several: the number of galaxies, the average number of stars, the average star mass. Scientists usually quote the answer as somewhere between 10⁷⁸ and 10⁸². What matters is the order of magnitude, the power of ten. When the quantities are this large, getting the exponent right is the real achievement.

A few other things to note:

• Dark matter isn’t included. About 85% of the matter in the universe is dark matter, which isn’t made of atoms as far as we know.
• Most atoms are hydrogen. Roughly 92% of atoms in the universe are hydrogen and most of the rest are helium. Everything else, including the carbon and oxygen in you, is a small minority. See the most abundant elements in the universe.
• Some matter is ionised. Much of the gas between galaxies is so hot that its atoms have lost their electrons. Strictly, these are ions and electrons rather than neutral atoms, but they’re usually included in the count.

Putting 10⁸⁰ in perspective

Large powers of ten are hard to feel. Here are some comparisons, from small to vast:

Thing	Approximate number of atoms (or molecules)
A single drop of water	1.7 × 10²¹ molecules
A human body	7 × 10²⁷ atoms
Planet Earth	1 × 10⁵⁰ atoms
The Sun	1 × 10⁵⁷ atoms
The Milky Way’s stars	~10⁶⁸ atoms
The observable universe	~10⁸⁰ atoms

Some striking consequences:

• One drop of water holds about 10²¹ molecules, roughly a hundredth of the number of stars in the observable universe (about 10²³). About a hundred drops, a small spoonful, contain as many molecules as there are stars.
• The universe contains about 10²³ times as many atoms as the Sun, and the Sun contains about 10⁷ times as many atoms as the Earth.
• 10⁸⁰ is still far smaller than numbers mathematicians use casually. A googol, 10¹⁰⁰, is 10²⁰ times larger than the number of atoms in the observable universe.

Where the atoms came from

Almost all the hydrogen and helium atoms were made in the first few minutes after the Big Bang. Heavier elements, including the carbon in your cells, the oxygen you breathe and the iron in your blood, were forged later inside stars and in stellar explosions and collisions. Many of the atoms in your body have passed through at least one star. Our guide to how new elements are made covers the laboratory side of element-making.

Why this estimate matters

Estimating the atoms in the universe is a classic example of a Fermi problem: a question that seems impossible but can be answered roughly by breaking it into steps you can estimate. Scientists and engineers use the same technique constantly, whether sizing a power station or checking whether a lab result is reasonable. The skills involved, powers of ten and scientific notation, sensible rounding, and checking one method against another, are useful far beyond astronomy.

Common misconceptions

• “Scientists counted the atoms.” It’s an estimate built from measured quantities, not a count.
• “10⁸⁰ is the number of atoms in the whole universe.” It’s for the observable universe. The whole universe may be much larger, possibly infinite.
• “The number includes everything.” Dark matter, and the energy in radiation, aren’t atoms and aren’t included.
• “10⁸⁰ is twice as big as 10⁴⁰.” It’s 10⁴⁰ times bigger. Each extra power of ten multiplies by ten.

Key takeaways

• The observable universe contains roughly 10⁸⁰ atoms, give or take a factor of a hundred either way.
• One estimate multiplies galaxies × stars per galaxy × atoms per star: 10¹² × 10¹¹ × 10⁵⁷ ≈ 10⁸⁰.
• A second estimate multiplies the average density (about 0.25 atoms per m³) by the volume of the observable universe, giving a similar answer.
• Most atoms are hydrogen, made shortly after the Big Bang; heavier elements were made in stars.
• For the other end of the scale, see how small is an atom? or convert between atoms and grams with the grams to moles converter.