Measuring a C Clamp means measuring the opening of the jaws of the clamp. When you have the C Clamp open, you will measure the space that fits between the top part of the clamp and the bottom part. Most of the time, a C Clamp is going to range from 2 to 10 inches and will usually have a measurement that is rather exact. If your measurement is something like 4 3/4, you may have measured it wrong; try to measure from the inside of the clamp and not the outside bars.