How the wheel offset calculator works
A wheel's overall width is about 1 inch more than its nominal (bead seat) width because of the rim flanges. Half of that sits each side of the centre line, and the offset shifts the mounting face from that centre:
- Backspacing = (width + 1″) ÷ 2 + offset
- Front spacing = (width + 1″) ÷ 2 − offset
- Inner edge change = (new width − old width) ÷ 2 + (new ET − old ET)
- Outer face change = (new width − old width) ÷ 2 − (new ET − old ET)
Worked example: 8x17 ET35 → 9x17 ET30
The new wheel is 1 inch (25.4 mm) wider, so each side grows by 12.7 mm. The offset drops by 5 mm, which moves the whole wheel 5 mm outward. Result: the inner edge comes 7.7 mm closer to the strut (12.7 − 5) and the outer face sticks out 17.7 mm more (12.7 + 5). Backspacing goes from 5.88″ to 6.18″.
Backspacing chart
| Width | ET −12 | ET 0 | ET 20 | ET 35 | ET 45 |
|---|---|---|---|---|---|
| 6" wide | 3.03" | 3.50" | 4.29" | 4.88" | 5.27" |
| 7" wide | 3.53" | 4.00" | 4.79" | 5.38" | 5.77" |
| 8" wide | 4.03" | 4.50" | 5.29" | 5.88" | 6.27" |
| 9" wide | 4.53" | 5.00" | 5.79" | 6.38" | 6.77" |
| 10" wide | 5.03" | 5.50" | 6.29" | 6.88" | 7.27" |
The factory widths and offsets for many European models are listed on our vehicle pages, taken from official Swiss type approvals.
Frequently asked questions
What is wheel offset (ET)?
Offset is the distance in millimetres from the wheel's centre line to the face that bolts to the hub. ET is the German abbreviation (Einpresstiefe) stamped on most wheels. Positive offset puts the mounting face towards the outside of the wheel, tucking the wheel in.
Does a lower offset push the wheel out?
Yes. Lowering the offset by 10 mm moves the whole wheel 10 mm outward: the outer face sticks out 10 mm more and the inner edge moves 10 mm away from the suspension.
What is backspacing?
Backspacing is the distance from the hub mounting face to the inner edge of the wheel, usually given in inches in the US. It combines width and offset, so it tells you directly how much room the wheel needs on the inside.
How much offset change is safe?
It depends on the car. A few millimetres is usually fine; larger changes alter the scrub radius, can increase load on wheel bearings and may cause rubbing. Check the wheel and tire combinations approved for your vehicle.
Are wheel spacers the same as lowering the offset?
Geometrically yes — a 10 mm spacer has the same effect as 10 mm less offset. Spacers must be good quality, hub-centric and, in many countries, approved for road use.