Measure a hinge by laying it flat and measuring the width and height.
Measuring Hinges Corners
Hinges can have square corners, or radius corners. If you're hinges have radius corners they're most likely 5/8" or 1/4" radius. To determine the radius hold a quarter or dime up the corner. If the quarter matches the radius you have 5/8" hinges, if the dime matches you have 1/4" hinges. Below is an example of a 5/8" radius hinge.