.31 gauge.

D'Addario Folk Nylon guitar strings are high quality nylon strings with ball ends on each string to facilitate easy installation on classical guitars, as well as steel string acoustic guitars if desired. BEBW single strings are 80/20 bronze wound on nylon.
D'Addario is the world's largest manufacturer of strings for musical instruments. Many top guitarists won't play any other brand. Regardless of your playing style, D'Addario has a set that's right for you.