Metric screws are measured differently than American screws. In addition to using metric units, instead of measuring threads per inch, the metric screws list the distance between the threads. If the threads are measured improperly, you can end up with a screw that will not fit.
Measure a known distance in millimetres on the screw using the calipers, such as 4 millimetres.
Count the number of threads in the area you have measured.
Divide the distance found in Step 1 by the number of threads counted in Step 2 to determine the screw thread distance. For example, if you counted 10 threads in 4 millimetres, the thread count would be 2.5 threads per millimetre.