TM has mentioned before — but I’ve had the General 141ME for at least a couple of years, and have found it especially handy lately while doing some volunteer work setting up a search-and-rescue (SAR) van. It’s only 4″ long, made of fiberglass-reinforced plastic, can measure up to 3″ inner or outer diameters with its caliper, and has both metric and English readings.
I have used it to check what size u-bolts I need to clamp a bracket to existing tubing, confirm hole diameters so I can get cables through, and determine screw lengths for a locking clasp on an exterior door. It fits easily in my pocket, so it travels with me on the multiple trips I’ve been making to various hardware stores to pick up all the things we don’t seem to have in the barn where the SAR van is stored.
The $6–$7 price and small size have made it a worthwhile addition to my traveling tool kit. What’s your favorite — and handy — method/tool for diameter measurements?