Offset to Backspacing Calculator
Convert wheel offset in millimeters to backspacing in inches for any width.
Enter your numbers and press Calculate.
Formula
Backspacing = (width + 1) ÷ 2 + offset ÷ 25.4
Mickey Thompson's tech bulletin uses the same conversion, multiplying offset by 0.03937 (1 ÷ 25.4).
Overall wheel width is the stated width plus about 1 inch, because each bead flange adds about half an inch. Mickey Thompson, Speedway Motors and Discount Tire all use that convention, and so does every calculator on this site.
How to use it
- Read the size off your current wheel: it is often cast on the back of a spoke, as something like 18×8 ET35.
- Enter the new wheel's width and offset from the listing.
- Press Calculate. The drawing shows both wheels on the same hub face.
- Compare the outer and inner numbers with the clearance you measured on the car.
Examples
| Swap | Outer lip | Inner lip |
|---|---|---|
| 8×+35 → 8.5×+35 | +6.3 mm | +6.3 mm |
| 8×+35 → 9×+20 | +27.7 mm | −2.3 mm |
| 8.5×+45 → 9.5×+35 | +22.7 mm | +2.7 mm |
| 9×+18 → 10×0 | +30.7 mm | −5.3 mm |
| 9×0 → 10×−24 | +36.7 mm | −11.3 mm |
| 10×−24 → 12×−44 | +45.4 mm | +5.4 mm |
Positive outer = more poke. Positive inner = closer to the suspension.
Check clearance before you buy
Offset and width tell you where the wheel's lips end up; they do not tell you what is in the way. Before ordering, measure your current setup:
- Inner clearance. With the wheels straight, measure from the inner lip of the current wheel to the nearest suspension part: strut body, spring perch, upper control arm or sway bar link. If the new wheel's inner lip moves in by more than that gap, it will hit.
- Brake clearance. Big calipers sit inside the barrel. The spoke face, not just the lip, has to clear them, which is why a wheel's spoke design matters as much as its offset.
- Outer clearance. Hang a plumb line or hold a level against the fender lip and measure in to the tire's sidewall. A new wheel that moves out more than that gap will stick past the fender.
- Steering and suspension travel. Turn to full lock and check the fender liner, then remember the suspension compresses over bumps. Static clearance is the best case.
Enter both wheels in the calculator above: the outer-lip number is the extra space you need at the fender, and the inner-lip number is the extra space you need at the suspension.
Common mistakes
- Using the stated width as the overall width. A 9-inch wheel measures about 10 inches lip to lip because each bead flange adds roughly half an inch. Skip that inch and every backspacing figure is half an inch off.
- Dropping the minus sign. −12 and +12 are 24 mm apart. A wheel stamped ET-12 or −12 is negative.
- Comparing offsets across different widths. A wider wheel adds half its extra width on each side, so a 10-inch wheel at +25 can still stick out further than a 9-inch wheel at +18.
- Forgetting the tire. A wider tire grows half its extra width on each side of the wheel, and sidewall bulge varies between brands of the same size.
Sources
- https://mickeythompsontires.com/tech-bulletins/backspacing-and-offset
- https://speedwaymotors.com/the-toolbox/wheel-offset-vs-backspacing-explained/30059
- https://www.tirerack.com/wheels/tech/techpage.jsp?techid=101