Free wheel and tyre tools
Offset and backspacing converter
Convert wheel offset in millimetres to backspacing in inches, in both directions, with an adjustable allowance for rim flanges.
How it works
Offset is measured from the wheel centreline to the mounting face. Backspacing is measured from the inboard outer rim edge to that face. Nominal wheel width is measured between the bead seats, so we add a total flange allowance, normally estimated at 1 inch. Backspacing = (nominal width + flange allowance) ÷ 2 + ET ÷ 25.4. Measure the actual wheel for an exact result.
A worked example
An 8 inch wide ET0 wheel has an estimated backspacing of 4.5 inches when the total flange allowance is 1 inch. The same wheel at ET25.4 has 5.5 inches of backspacing. Converting 5.5 inches back with the same width and allowance gives ET25.4.
Quick reference
Backspacing in inches by width and offset
| Offset | 8 inch wheel | 9 inch wheel | 10 inch wheel |
|---|---|---|---|
| ET-12 | 4.03 in | 4.53 in | 5.03 in |
| ET0 | 4.5 in | 5 in | 5.5 in |
| ET18 | 5.21 in | 5.71 in | 6.21 in |
| ET35 | 5.88 in | 6.38 in | 6.88 in |
| ET45 | 6.27 in | 6.77 in | 7.27 in |
Assumes a 1 inch total flange allowance. Measure the actual wheel for an exact figure.
Explained in our guides
- ET vs backspacing: measure and convert wheel positionConvert offset to backspacing with worked examples, understand the rim flange allowance, and measure a loose wheel without confusing nominal width.
- Wheel offset explained: choosing ET without guessingUnderstand positive and negative wheel offset, calculate inner clearance and outer position, and choose an ET using measurements from your own car.
- Lifted truck wheel fitment: backspacing, 33s, 35s and trimmingPlan larger truck tyres around lift-kit requirements, backspacing, steering and articulation. Compare 33 and 35 inch tyres without assuming a lift guarantees clearance.
Questions
Why add an inch to the wheel width?
An 8 inch wheel is measured between its bead seats, but backspacing starts at its outer rim edge. The default assumes both flanges together add 1 inch. That is an estimate, so use the measured allowance if you have it.
Does more positive offset mean more backspacing?
For the same wheel width and flange allowance, yes. More positive offset moves the mounting face towards the outside of the wheel and increases the distance from the inner edge to that face.
Can backspacing confirm brake clearance?
No. It describes one position of the rim, not the shape of its spokes or barrel. Ask the wheel supplier for a brake template and check the specific wheel design.
Keep checking
Use these results to plan a setup. Confirm fitment, tyre approval and clearances with a qualified professional before buying or fitting parts.