The Pitch Circle Diameter (PCD) is the diameter of the circle that passes through the centre of all the flange mounting holes.

Measure the distance between the centre or two adjacent holes. This applies whether you have four, five, six or eight holes to measure between. Once you have the measurement between two studs, you can then work out the measurements.
You use the PCD value to check the "M" dimension on the motor dimension sheet.

In the below calculator, the measurement between two holes is value “S.”