Spring Thickness

So I have been thinking about ordering some spring stock and cutting my own springs again. I am a little confused by the terminology I guess. I always remember sprigs being categorized by their gauge, namely 18 and 20 gauge with the lower number meaning thicker. So I was looking at the Eikon website and all their springs are listed by their thickness in 1000ths of an inch. Anybody know what the conversion is? Tried looking at some engineering materials sites but the numbers don't make any sense to me.

try measurement conversions on google? that might help i hope. good luck.