Since this thread is about converting measurements to and from inferial fractions, let me present a clever approximation for converting fractional inches to millimeters. The arithmetic is simple enough to do in your head.

Keep doubling numerator and denominator until the denominator is 256. Then the numerator divided by ten is the equivalent in mm with an error of only 0.79%.

Some examples...

13/64 = 26/128 = 52/256 -> 52/10 -> 5.2 mm (correct value = 5.159375 mm)

1&13/16 = 29/16 = 464/256 -> 46.4 mm (correct value = 46.0375 mm)