Offset vs backspacing?

[Rhino]

Offset is the width in mm (positive or negative) that the wheel mounting surface is inboard or outboard from the wheel center line.
To calculate it you need the true rim width (usually 1" wider than advertised) and the backspace. The equation would be as follows

Offset = (backspace - ((wheelwidth + 1) /2)) * 25.4



Essentially you're taking the backspace and subtracting half of the true wheel width. This gives you your offset in inches. Simply multiply that by 25.4 and you have your offset in mm.