You can get the right measurement and avoid future problems with this handy tutorial -
Take the guess work out of the equation by following this short tutorial on how to measure your brake disc rotor
Please note: All measurements are in mm
1. ID = Inside Diameter: Measure from inside edge to inside edge.
2. OD = Outside Diameter: Measure from outside edge to outside edge.
3. Measurement from inside edge of mounting hole to inside edge of bolt hole.
4. Bolt hole diameter: measure from inside edge of bolt hole to opposite inside edge.
5. PCD = Pitch Circle Diameter: To measure this, use the following equation;
ID + (2 x Measurement 3) + Measurement 4
Eg: 90 + ( 2 x 1.5 ) + 17 = 110mm
6. Thickness: Measure disc thickness by either standing disc vertically or lay on a flat surface. 
7. Offset: Place your disc on a flat surface and measure from the surface to the top of the disc (including the disc thickness)
8. Types of bolt holes
I really hope this can help you with your rotor jobs!