For a flat button, the buttonhole length should be the diameter of the button, plus 1/8".
For fat buttons, add the button's height and width, plus 1/8".
Measure a flat, irregular-shaped button at it's widest and add 1/8".
To measure rounded, ball or odd-shaped buttons, wrap a tape measure or narrow strip of paper around the button; fold the paper in half and measure for the correct buttonhole length.