No, this isn't yet another "on-screen caliper" app.
Since I bought a lathe and have started doing some metalwork I have found myself spending a lot of time measuring a piece with calipers and then trying to convert the measurement from thousandths of an inch to fractional inches - usually to find a drill bit.
I wrote CaliperTool to help.
Enter a measurement value in inches or millimeters and CaliperTool will find the closest 1/64" values above and below your measurement and print them along with their respective errors.
It will do the same thing with standard "letter" and "number" drill bits, listing the closest above and below your measurement along with errors.
It will also do a simple metric conversion and list the measurement in millimeters or inches, depending on which unit you entered.
(Press the "Measurement in ..." button to toggle inches/mm entry.)
If you like this app, you might want to check out TapDrillTool elsewhere in the App Store.