Pocket Caliper

This yellow plastic caliper is lightweight, reasonably durable (I lose it before it wears out) and pocket-friendly (only 4 inches long). I use it frequently during house or auto repairs to ensure the right size replacement part (such as nuts and bolts, or o-rings and sealing washers) comes home with me from the store or junkyard. I find this easier, quicker, and more accurate in many cases than using a small rule. It is not a precision machinist’s instrument. However, in most of the work I need to get done, measurement to the closest 1/32 of an inch or 1 mm will get the right part or a fit which is good enough to work.

-- Ken Johnson  

General Tools 141ME Pocket Caliper
$8

Available from Amazon

Also from Art Stuff
Similar tool from Harbor Freight for $3