To calculate offset you'll need the following measurements:
|Wheel Center line (outboard flange to inboard flange measurement / 2)|
|Wheel center line from Wheel backspace to get offset. |
|To convert from inches to mm multiply by 25.4|
|To convert from mm to inches divide by 25.4|
Backspace to Offset Conversion Chart
The table on the right is q quick reference for finding offset, pick the rim width and follow the row over to the backspace of your wheel.