Small Calipers are a must when working on 3D projects (among other things). I have larger (6″+) metal ones, but wanted something that would be easier to toss on my desk for quick measurements. This General digital is small (3″) and non-metallic, so I can fiddle around electronics. MM, Inches, and Inches Fractions. Though, if you are doing 3D printing work, you only care about the mm. When working on custom projects, I use this dozens of times a day. About $14. https://amzn.to/2Ce0604

I also keep this little brass caliper, designed for jewelry work, in my shop coat pocket at all times. It’s handy for doing rough measurements. $9 https://amzn.to/2SP6wd0