How Do I Determine the Size of a Canvas Boat Cover Snap?

Button and Screw-Stud Fasteners

1). Place the jaws of a digital micrometer -- the anvil and the spindle -- around the post of the eyelet (the bottom part of the button -- the only part of the snap that's visible when the snap is in use). Turn the micrometer’s ratchet until the spindle -- the moving part of the micrometer’s jaw -- meets the post.
2). Visually inspect the micrometer's position to ensure the post is centered between the ratchet and spindle. Turn the micrometers ratchet until you hear three quiet “click” sounds; inspect the micrometer to verify that the eyelet post is centered in the micrometer’s jaws.
3). Move the lever of the thimble lock to lock the spindle in place. Remove the eyelet. Read the size of the post on the digital micrometer’s display.
4). Repeat the process for the shank of the stud -- the bottom part of the snap into which you press the button.

1). Lay a machinists' rule over the surface of fastener, so the "0" mark of the rule is the edge of the snap.
2). Move the machinists rule so that the rule crosses the widest part of the snap, while the "0" mark remains on the edge of the snap.
3). Read the width of the widest part of the snap from the machinists' rule.