Centre caps
This is how we measured our caps.

In the table under 'Accessories' you can see both these measures of the caps with gripping claws.

4x4 caps
This is how we measured our 4x4 caps.

Boltcircle
Measuring the boltcircle diameter on a five hole rim.

Measure the entire diameter of the bolthole=A

Measure from the edge of the bolthole to the edge of the hubhole=B

Multiply the B-measure with 2.

Measure the hubhole diameter=C

Boltcircle diameter= A+Bx2+C

On a four bolt rim you measure from the boltholes outer edge to the boltholes inner edge across the hubhole.