How big is Proton? | Scale of the Universe

A proton is a particle with a positive electric charge, found in the nucleus at the center of every atom. It is about 1.7 femtometers across, measured as twice its charge radius, the average radius of its electric charge. A femtometer is a millionth of a billionth of a meter (10⁻¹⁵ meters).

Size

A proton has no hard surface. It is a cloud of three smaller particles called quarks, two up quarks and one down quark, held together by the strong force, the same force that binds protons and neutrons into a nucleus. The recommended value of its charge radius, listed by the U.S. National Institute of Standards and Technology (NIST), is 0.84075 femtometers. It is uncertain by less than 1%.

There are two main ways to measure it. One fires electrons at protons and records how they bounce off. The other measures the light given off by hydrogen atoms, whose energy levels shift very slightly because the proton is not a point.

Size comparisons

A neutron, the proton's partner in the nucleus, is about the same size. Measured the same way, by how their magnetism is spread out, their radii differ by less than 2%.
A helium nucleus, made of two protons and two neutrons, is about 3.4 femtometers across, twice the width of a single proton.
A hydrogen atom, one proton with one electron around it, is about 0.1 nanometers across, roughly 60,000 times the width of its proton. A nanometer is a billionth of a meter.
If a proton were enlarged to the size of a basketball, the hydrogen atom around it would be about 15 kilometers (9 miles) across.
About 60 billion protons lined up side by side would span the width of a human hair, about 0.1 millimeters.

The proton radius puzzle

Before 2010, both methods gave a radius of about 0.88 femtometers. That year, a team at the Paul Scherrer Institute in Switzerland measured hydrogen atoms in which the electron was replaced by a muon. A muon is like an electron but about 200 times heavier, so it orbits much closer to the proton and is far more sensitive to its size. The team found 0.84184 femtometers, about 4% smaller, a gap too large to blame on measurement error.

Physicists called the disagreement the proton radius puzzle, and some hoped it was a sign of an unknown force of nature. In 2019, the PRad experiment at Jefferson Lab in Virginia reported a new kind of electron-scattering measurement that found 0.831 femtometers, in line with the smaller value. In 2026, a precise study of ordinary hydrogen at the Max Planck Institute of Quantum Optics in Germany also agreed with it, and its authors describe the puzzle as resolved.

A building block of every element

The number of protons in a nucleus decides which element an atom is. Every atom with one proton is hydrogen, and every atom with six is carbon. A proton has 1,836 times the mass of an electron, so the protons and neutrons in the nucleus hold nearly all of an atom's mass.