GarageCalc Offset measures the mounting face from the wheel centerline, in millimeters. Backspacing measures from the mounting face to the inner wheel lip, usually in inches. They describe related geometry, but converting between them requires the wheel's overall width.
Which measurement are you looking at?
A marking such as 18 × 8 ET35 describes an 18-inch wheel with an 8-inch nominal rim width and positive 35 mm offset. That 8-inch width is measured between the bead seats. It does not include the outer lips. Backspacing does include the inner lip, which is why using the stamped width directly in the conversion gives the wrong answer.
Offset to backspacing formula
Backspacing (in) = overall wheel width (in) ÷ 2 + offset (mm) ÷ 25.4. To reverse it: offset (mm) = [backspacing (in) − overall width (in) ÷ 2] × 25.4.
When overall width is unavailable, nominal width plus 1 inch is a common approximation. It assumes about half an inch of lip on each side. Measure the actual wheel or obtain the manufacturer's drawing before using the result for a close clearance decision.
| Nominal width | Offset | Estimated backspacing |
|---|---|---|
| 8 in | +20 mm | 5.29 in |
| 8 in | +35 mm | 5.88 in |
| 8 in | +45 mm | 6.27 in |
| 9 in | +35 mm | 6.38 in |
| 9 in | +45 mm | 6.77 in |
Notice that an 8-inch and a 9-inch wheel at the same ET35 have different backspacing. A backspacing number alone therefore cannot tell you whether the wheel will sit farther out.
Worked example: 8-inch ET35 to 9-inch ET45
The nominal width increases by 25.4 mm, placing an additional 12.7 mm on each side of the centerline. The offset increases by 10 mm, moving the wheel inward. Assuming comparable lip geometry:
- Outer edge: 12.7 − 10 = 2.7 mm farther out.
- Inner edge: 12.7 + 10 = 22.7 mm farther in. Existing inner clearance decreases by that amount.
This is wheel geometry, not a prediction of the tire sidewall's exact position. Actual tire width depends on the tire model and rim width. Use the wheel offset calculator to compare your dimensions, then check the complete assembly.
Does lower offset always mean more poke?
At the same width, yes: changing from ET35 to ET20 moves the wheel outward 15 mm. When width changes too, calculate both edges. Also check the bolt pattern, hub bore, wheel load rating, brake clearance and tire clearance through steering and suspension travel. A matching offset cannot establish those other requirements.
Measurement references
See Discount Tire's explanations of offset and backspacing and bead-seat versus overall wheel width. The examples and diagram above are GarageCalc calculations, not vehicle fitment approvals.