A frequent mistake made in the mainstream media is to confuse strength and toughness, when comparing silk to other materials. As shown below in detail, weight for weight, silk is stronger than steel, but not as strong as Kevlar. Silk is, however, tougher than either.
It is important to note that the variability of mechanical properties of spider silk fibers may be important and it is related to their degree of molecular alignment. Besides, mechanical properties depend strongly on the ambient conditions, i.e. humidity and temperature.
In detail a dragline silk's tensile strength is comparable to that of high-grade alloy steel (450 - 2000 MPa), and about half as strong as aramid filaments, such as Twaron or Kevlar (3000 MPa).
Consisting of mainly protein, silks are about a sixth of the density of steel (1.3 g/cm3). As a result, a strand long enough to circle the Earth would weigh less than 500 grams (18 oz). (Spider dragline silk has a tensile strength of roughly 1.3 GPa. The tensile strength listed for steel might be slightly higher—e.g. 1.65 GPa, but spider silk is a much less dense material, so that a given weight of spider silk is five times as strong as the same weight of steel.)