Due to the manufacturing processes the actual dimension of a cutter can be slightly different from the nominal size.
If you want to use the actual tool size in toolpath generation in a CAD/CAM system, you need to measure the diameter of the cutter.
New cutters normally measure under the specified nominal dimension. Used cutters will measure under the nominal dimension because of wear.
The method shown here only works for 2 or 4 flute cylindrical cutters.
You will need to use a micrometer.
Take a measurement across 2 points on opposing flutes. Make sure you align the cutter between measurement faces of the anvil and the spindle of the micrometer.