> a better answer
A cheaper answer: in the car-parts store, look in the dollar bin. (It's all $3 and $5 now.) I picked up a real cheezy "vernier" caliper. Looks like the one in Duck's post except the vernier is un-readable (badly stamped), just straight scale. But this IS good enough to tell 25/32 from 13/16, on bolts, hydraulic fittings, and drills. Where you either know it must be a Standard size (bolts), or you don't care if it runs 1/32" large (metric pots versus Imperial drills).
I think it may have metric markings too. But my 1967 USA Ford tractor has no metrics so I don't care.
The drill-gauge is also handy, and Home Depot often has plastic ones for a buck (get a couple). These may also have screw-length notches, inch/mm marks, and angles.