A single strand of silk from a silkworm's cocoon is about 10–25 micrometers wide, and the 15 micrometers shown here falls within that range. A micrometer is a millionth of a meter. At 15 micrometers, five to seven strands side by side would match the width of a human hair.

Size

The width is the diameter of one strand as it is unwound from a cocoon, measured under a microscope. Each strand is made of two filaments of a protein called fibroin, held together by a gluey protein called sericin. Each fibroin filament is about 10 micrometers across. The sericin makes up 25–30% of the strand's weight.

Width varies from one cocoon to another and along a single strand. Studies of silkworm silk have measured strands from about 8 to 26 micrometers wide, and several report averages of about 20 micrometers. The strand is not perfectly round, so its measured width also depends on the direction it is viewed from.

Size comparisons

• A human hair, 80–100 micrometers wide, is five to seven times as wide as a 15-micrometer strand.
• A red blood cell, 7–8 micrometers across, is about half the width of a 15-micrometer strand.
• The widest strands, about 25 micrometers, are about as wide as a thou, one thousandth of an inch.
• The single strand in one cocoon can be up to 1,500 meters (4,900 feet) long. That is about 14 American football fields placed end to end, counting the end zones.
• If a 15-micrometer strand were enlarged to 1 centimeter (0.4 inches) wide, a 1,500-meter strand would stretch 1,000 kilometers (620 miles).

How a silkworm spins

Silkworms are the caterpillars of the silk moth Bombyx mori, and they are raised in large numbers for the textile industry. Before it changes into a moth, the caterpillar spins a cocoon from one unbroken strand. It works from the outside in, building the cocoon up in layers.

A silkworm has two silk glands, and the silk protein moves through them as a liquid. In the long, narrow front section of each gland, the protein turns solid, forming very thin fibrils that line up along the strand. The two glands join into a single tube at the spinneret, the spinning organ at their end, and the finished strand leaves with its two filaments side by side.

Silk in medicine

To make silk into materials for medicine and research, the sericin glue is first removed, usually by boiling the silk in a solution of sodium carbonate. This is called degumming. What remains are the fibroin filaments, each about 10 micrometers wide.

Silk is also used as surgical thread, often coated in wax to stop it fraying. Silk was once thought to cause allergies in some patients, but later research found that the sericin was responsible.

Sources

• Cocoon microstructures through the lens of topological persistence. Raichenko et al., Journal of the Royal Society Interface, 2024.
• Fine Structural Analysis of Degummed Fibroin Fibers Reveals Its Superior Mechanical Capabilities. Eliaz et al., ChemSusChem, 2024.
• The stage-wise macromolecular assembly and structure evolution of silk along the silk gland. Brookstein et al., Nature Communications, 2026.
• Ultrafine and High-Strength Silk Fibers Secreted by Bimolter Silkworms. Guo et al., Polymers, 2020.
• Materials Fabrication from Bombyx mori Silk Fibroin. Rockwood et al., Nature Protocols, 2011.
• Just How Small Is "Nano"?. National Nanotechnology Initiative.
• Erythrocytes. In Anatomy and Physiology, 2nd edition. OpenStax, 2022.
• Handbook 44 – 2026, Appendix C: General Tables of Units of Measurement. NIST, 2026.
• 2026 Official Playing Rules of the National Football League. NFL, 2026.