Go to your local fastener store (maybe Fastenal), or try your local farm/implement store if you are in a rural area. Sometimes they have a plastic, giveaway type gauge for measuring nuts and bolts. I would also get a low priced dial caliper (under $50) so that I could measure directly.
http://www.use-enco.com/CGI/INPDFF?PMPAGE=297&PARTPG=INLMK32