Is Silk Stronger Than Steel?

The Claim, and What It Actually Means

"Stronger than steel" attaches itself to silk constantly. It is true in one specific sense and misleading in every other, and the distinction is worth getting right.

The Numbers

Tensile strength — how much pull a material takes before it breaks:

• Steel: roughly 0.2 to 2 GPa, depending on the alloy.
• Spider silk: roughly 0.45 to 2.0 GPa.

So they overlap. Spider silk is not categorically stronger — a good steel beats a typical silk on raw tensile strength.

Where the Claim Comes From

Strength-to-density. Silk is far lighter than steel, so per unit of weight it is dramatically stronger. That is a real and useful property, and it is what people mean when they say it — usually without saying the "for its weight" part that makes it true.

Where Steel Wins Easily

Stiffness. Silk is nowhere near as stiff. A steel cable holds its shape under load; a silk thread stretches a long way first. For most engineering, stiffness matters as much as ultimate strength, and this is why nobody builds bridges from silk.

Where Silk Wins Outright

Toughness — the total energy a material absorbs before failing, which is not the same as strength. Rigid spider silk comes in around 180 megajoules per cubic metre against Kevlar's 50. Silk stretches, and stretching absorbs energy. That is why a web catches an insect instead of flinging it back.

And Silkworm Silk Is Not Spider Silk

Nearly every headline figure refers to spider silk, which is a different material from what Bombyx mori produces. Silkworm silk is not as strong. It has one enormous advantage: it can be farmed. Spiders are territorial and cannibalistic and cannot be kept at density, which is precisely why researchers put spider silk genes into silkworms rather than trying to farm spiders.

Related: the silkworm's wild ancestor · what silk is made of