The up quark is the lightest of the six kinds of quark, the particles that make up protons and neutrons. No experiment has measured a size for it, so the figure shown is not a measurement. Experiments have found only that a quark, if it has any size at all, is no more than about a two-thousandth of the width of a proton.

Size

The size line reads 10 zeptometers. A zeptometer is 10⁻²¹ meters, a thousandth of a billionth of a billionth of a meter. The notation 10⁻²¹ means 1 divided by a 1 followed by 21 zeros. The explorer gives each kind of quark a different figure, but no experiment has measured a size for any of them.

Physicists look for a particle's size by firing other particles at it and watching how they scatter. The ZEUS experiment studied collisions between electrons and protons at the HERA accelerator. It found no sign that the quarks inside the protons had any size. In 2016 it reported that a quark's radius, if it has one, must be less than 4.3 × 10⁻¹⁹ meters.

A quark is therefore less than about 8.6 × 10⁻¹⁹ meters across. That is an upper limit, not a size. It is about 86 times the explorer's figure, so experiments do not rule that figure out, but they do not support it either.

Size comparisons

A proton, about 1.7 femtometers (1.7 × 10⁻¹⁵ meters) across, is about 2,000 times as wide as the largest size experiments allow for a quark.
If a quark at that limit were enlarged to the size of a tennis ball, about 6.7 centimeters (2.6 inches) across, a proton would be about 130 meters (430 feet) across.
At the same scale, a hydrogen atom would be about 8,200 kilometers (5,100 miles) across, about two-thirds the width of Earth.
About 100 trillion quarks at that limit, lined up side by side, would span a human hair about 0.1 millimeters wide.

Inside protons and neutrons

Ordinary matter is built from two kinds of quark, the up quark and the down quark. A proton is made of two up quarks and one down quark. A neutron is made of one up quark and two down quarks.

Each quark carries a fraction of the electric charge of a proton: +2/3 for an up quark and −1/3 for a down quark. The charges of a proton's three quarks add up to +1. The charges of a neutron's three quarks add up to 0.

Quarks are held together by gluons, the particles that carry the strong force. No one has ever found a quark on its own. Physicists have searched for single quarks in sea water, meteorites, cosmic rays and particle accelerators, without success.

Where a proton's mass comes from

The Particle Data Group, which collects the world's particle measurements, gives the up quark's mass as 2.16 MeV. Particle physicists give masses in units of energy, and an MeV is a million electronvolts. The up quark has less than half the mass of the down quark, which is 4.70 MeV.

A proton's mass is about 938 MeV. Its two up quarks and one down quark add up to only about 9 MeV, or about 1 percent of that. Almost all of the rest comes from the interactions between the quarks and the gluons that bind them.

Sources

Summary Tables: Quarks. Particle Data Group, 2025.
Limits on the effective quark radius from inclusive ep scattering at HERA. H. Abramowicz et al. (ZEUS Collaboration), Physics Letters B, 2016.
Introduction to Particle Physics. In University Physics Volume 3. OpenStax, 2016.
Quarks. In University Physics Volume 3. OpenStax, 2016.
DOE Explains: Quarks and Gluons. U.S. Department of Energy, Office of Science.
The Nobel Prize in Physics 1990: Press release. Royal Swedish Academy of Sciences, 1990.
SI prefixes. International Bureau of Weights and Measures.
Proton rms charge radius. NIST CODATA, 2022.
Proton mass energy equivalent in MeV. NIST CODATA, 2022.
Bohr radius. NIST CODATA, 2022.
Rules of Tennis 2026. International Tennis Federation, 2026.
Earth Fact Sheet. NASA, 2024.
Just How Small Is "Nano"?. National Nanotechnology Initiative.

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