Wheel Offset & Backspacing Calculator
Converts between offset and backspacing, the two ways of describing where a wheel sits in its arch. Enter your current offset as well and it shows how far a new wheel moves in or out.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
backspacing = (offset + overall width ÷ 2) ÷ 25.4 offset = backspacing × 25.4 − overall width ÷ 2 overall width = rim width + 1 inch
Both describe the same thing — where the mounting face sits relative to the wheel. Offset measures it from the centreline in millimetres; backspacing measures it from the inner lip in inches. The extra inch is the two flanges, which the stamped rim width excludes.
What to watch out for
The two measurements describe the same geometry from different reference points:
- Offset (ET) — millimetres from the wheel's centreline to the mounting face. Positive pulls the wheel inward, negative pushes it out.
- Backspacing — inches from the mounting face to the inner lip. More backspacing means the wheel tucks further in.
- The extra inch. A stamped 8" rim measures 8 inches bead to bead, but the flanges add about half an inch each side. Overall width is what the geometry uses.
Lower offset than standard moves the wheel outward, which can foul the arch on compression. Higher offset tucks it in, risking contact with the strut or brake caliper.
Frequently Asked Questions
What is wheel offset?
The distance in millimetres from the wheel centreline to its mounting face. ET35 means the face is 35 mm outboard of the centreline, so the wheel sits further in.
How do I convert offset to backspacing?
Add half the overall width to the offset, then convert to inches. Overall width is the stamped rim width plus one inch for the flanges.
Does lower offset push the wheel out?
Yes. Dropping from ET45 to ET35 moves the wheel 10 mm outward. That is the usual reason for a wheel fouling the arch liner after a change.
How much offset change is safe?
Within about 5 mm of standard is generally uneventful. Larger changes alter the scrub radius, which affects steering feel and loads the wheel bearing differently.