Biology
Spider Silk Strength Compared to Steel
Quick fact
Pound for pound, spider silk can be up to five times stronger than steel—its strength-to-weight ratio is superior—but steel still exceeds spider silk in absolute tensile strength.
Why this is interesting
You've probably heard that spider silk is stronger than steel. But is that actually true—and if so, how can a strand of spider web hold up to a material used in buildings?
Read the full explanation
Understanding Spider Silk Strength Compared to Steel
When people say spider silk is 'stronger than steel,' they are usually talking about strength-to-weight ratio, not raw pulling strength. Imagine two ropes: one made of steel and one made of spider silk. If both are the same thickness, the steel rope will resist breaking under a heavier load—that's absolute tensile strength. But spider silk is far less dense than steel (about one-sixth the density), so when you compare ropes of the same weight, the spider silk rope can be much thicker and thus support more load per weight. A good analogy is comparing a heavy steel beam to a light, yet sturdy fishing line—if you make both the same weight, the spider silk line can be made thicker and may hold more.
A deeper explanation
The core of the comparison lies in two material properties: tensile strength (the maximum stress a material can withstand before breaking) and density. Steel has a tensile strength of roughly 400–550 MPa (megapascals) for common grades, while spider silk (e.g., dragline silk from orb-weavers) ranges from 1,000 to 2,000 MPa. In absolute terms, spider silk is actually stronger than steel by about two to four times! However, the classic claim 'spider silk is stronger than steel' is often based on specific strength, which is tensile strength divided by density. Steel's density is ~7.9 g/cm³, whereas spider silk is only ~1.3 g/cm³. Thus, the specific strength of spider silk is dramatically higher. Additionally, silk is highly elastic and can stretch up to 30% before breaking, making it tougher (energy absorbed before fracture) than steel. These properties arise from the hierarchical structure of silk proteins (fibroin) forming crystalline beta-sheet regions embedded in a flexible amorphous matrix. This understanding is crucial for materials scientists: when engineering light but strong structures (e.g., in aerospace), specific strength matters more than absolute strength. Spider silk is a model for biomimetic materials aiming to combine high strength, elasticity, and low weight.