When measuring a flange, the most important dimension is the PCD (Pitch Circle Diameter). The PCD is the diameter of a circle through the centre of each hole.
When you are measuring a valve or flange that is already installed, if there are no stampings on the flanges or valves here is a handy tip to calculate.
Assuming you can get to the bolts measure the centres of two adjacent bolts then use the following multipliers:
4 holes on flange multiply by 1.414
8 holes on flange multiply by 2.613
12 holes on flange multiply by 3.864
16 holes on flange multiply by 5.126
If you know the size or DN (Diameter Nominale) then check out our Flange Tables
The flange tables include dimensions from 1/2" (DN15) to 24" (DN600) for flange specifications BS4504 PN6/10/16/25/40 ANSI 150/300 & BS10 Table D and E
Please ask about other specifications.