The strange quark is the third-lightest of the six kinds of quark. It is found in short-lived particles such as kaons, while ordinary matter is built from the lighter up and down quarks. No experiment has measured a size for it, so the figure shown is not a measurement. Experiments have found only that quarks, if they have any size at all, are no more than about a two-thousandth of the width of a proton.

Size

The size line reads 4 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. No experiment has measured a size for any kind of quark.

Experiments have set an upper limit instead. At the HERA accelerator, the ZEUS experiment collided electrons with protons and found no sign that the quarks inside had any size. In 2016 it reported that a quark's radius must be less than 4.3 × 10⁻¹⁹ meters, so a quark is less than about 8.6 × 10⁻¹⁹ meters across.

That limit comes from quarks inside protons, not from strange quarks in particular. It is about 200 times the explorer's figure, so no experiment contradicts that figure, and none supports it.

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 proton were enlarged to the size of a basketball, about 24 centimeters (9.4 inches) across, a quark at that limit would be about 0.12 millimeters across, a little wider than a typical human hair.
At the same scale, a hydrogen atom would be about 15 kilometers (9.4 miles) across.

The first strange particles

In 1947, Clifford Butler and George Rochester of the University of Manchester reported tracks of particles unlike any seen before. They had photographed them in a cloud chamber, a device in which electrically charged particles leave trails of tiny clouds. The particles came to be known as kaons.

Kaons did not fit any picture physicists had at the time. They were heavy, about half the mass of a proton, yet they lasted a relatively long time before decaying. In the 1950s, Murray Gell-Mann, Kazuhiko Nishijima, Abraham Pais and others explained this with a new property called strangeness. The name reflected how odd the particles had always seemed.

Today strangeness is traced to the strange quark. A positive kaon, for example, is an up quark paired with a strange antiquark, the strange quark's antimatter partner.

A heavier quark

The Particle Data Group, which collects the world's particle measurements, gives the strange quark's mass as 93.5 MeV. Particle physicists give masses in units of energy, and an MeV is a million electronvolts. The strange quark's mass is about 20 times the mass of a down quark and 43 times the mass of an up quark.

Heavier quarks are unstable, and the weak force turns them into lighter ones. That is why particles containing strange quarks eventually decay. A positive kaon survives about 12 billionths of a second on average. The lambda particle, made of an up, a down and a strange quark, lasts about a quarter of a billionth of a second.

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.
Meet the kaon. Nathan Collins, Symmetry, 2020.
Charge Independence Theory of V Particles. Kazuhiko Nishijima, Progress of Theoretical Physics, 1955.
SI prefixes. International Bureau of Weights and Measures.
Proton rms charge radius. NIST CODATA, 2022.
Bohr radius. NIST CODATA, 2022.
Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.
Just How Small Is "Nano"?. National Nanotechnology Initiative.

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