A similar concept to an offset, a backspace is simply the space between the wheel's mounting surface (the mounting pad) and the inboard flange (lip) of a wheel.
Therefore the backspace depends on both the overall width of the wheel and the offset of the wheel.
It shows where exactly the mounting pad is in relation to wheel's overall width. Terms: the offset and the backspace are often used interchangeably. Both terms determine where the wheel sits within the wheel well, how much of the wheel will protrude inboard, towards the suspension components or jut out, towards the fender. A wheel with the positive offset has more backspace, conversely a wheel with the negative offset has less backspace.
A little heads-up: the backspace is not measured from the inboard bead seat but from the very edge of the rim including the metal thickness (the back side rim lip or the rim flange) to the mounting pad.
e.g., A 6 inches wheel with the mounting pad positioned right in the center of the wheel would measure a 3½ inch backspace if it has a ½ inch wall (lip width). The overall width of the wheel would be: 6in + 2 x ½in = 7in.
How to measure the wheel backspace?
For this measurement you will need a straight edge and a metric ruler. Lay the wheel flat on the ground with the back facing upward. Next, lay the straight edge across the wheel and measure the distance from the back of the mounting surface (pad) to the straight edge.
Another, simpler way, if you have the offset, is to measure the wheel's overall width. Input that value and the value for the offset, from the offset designation on the wheel, into our wheel offset/backspace calculator to get the backspace. Also, the overall width can be calculated from the declared wheel width and the lip width. This can also be done by our calculator.