Belts usually have their size marked on the outer- non wearing side. If it is a V belt and broken, just stretch it out and measure the length- that will be your size. IE if it is 35 inches long then it will be a 350 belt- then the width will usually be 3/8 (3L) or 1/2 (4L) so a 3/8 wide belt 35 inches long will be a 3L350