Park Tools digital caliper
Digital calipers are the most useful tool on our work bench. They measure seat post sizes, headset crown race diameter, tire width, fender clearance, frame tube size, bolt length and diameter, BB shell width, handlebar and stem diameter.... The list goes on and on.
Park Tools digital calipers measure from 0 to 150 mm. You can switch from metric to inches to fractions of an inch with just the push of a button. And unlike cheap calipers the display doesn't die in a few months.
Made of stainless steel and an ABS plastic body. Comes with a protective case and two batteries. It has a convenient auto-off feature so your battery won't die if you forget to turn it off. There is a handy set screw, so you can lock the jaws.