The diagram is an example of the proper method for measuring bolt patterns.
The conversion of inches to mm is 1 inch to 25.4 mm.
The bolt pattern or bolt circle is the diameter of an imaginary circle formed by the centers of the wheel lugs. Bolt patterns can be 4, 5, 6, 7, or 8 lug holes, however tuner applications are typically 4 or 5 lug. Bolt circles are measured in both inches and millimeters, therefore you may hear a bolt pattern referred to as both (i.e. 5x114.3 mm is also referred to as 5x4.5). A bolt circle of 5x114.3 mm would indicate a 5 lug pattern on a circle with a diameter of 114.3mm (or 4.5").