On a bike with a horizontal top tube, if measured in inches, measure from center of the bottom bracket to the top of the top tube, running the tape along the seat tube. This is c to t. High end road bikes measured in centimeters are measure from the center of the bottom bracket along the seat tube to the center of the top tube, or c to c. Bikes with sloping top tubes are a whole different matter, which is why many are described as small, medium etc.
Mountain bikes are generally measured in inches. I have worked in a number of bike shops & sold various brands of bikes & this is the bike industry's standard method.