This chart works pretty well for most folks in sizing most traditional geometry frames (level top tube) and measuring the frame size by the seat tube length from the center of the BB spindle to the top of the seat tube, which is the way Schwinn measured frame size. Inseam is leg length as explained on the chart; it is not your pants inseam.
Since the chart uses centimeters instead of inches for the frame size, multiply the seat tube length in inches (measured as shown in the photo above) by 2.54 to get centimeters. e.g., 21" = 53.34cm, 22" = 55.88cm, 23" = 58.42cm, 24" = 60.96cm, etc.