Spanx founder Sara Blakely is now a billionaire.

Forbes Magazine says the 41-year-old is the youngest self-made woman to make their billionaire list.

Ms Blakely says she came up with the idea for her shapewear line after cutting off the feet of her control-top pantyhose and putting them on under her white pants.

"All the shapewear that was out there was so thick it felt almost like wearing workout clothes underneath your beautiful clothes, so it was the first time that anyone sort of put the concept of hosiery material to make shapewear."