Wheel Offset Calculator

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Compare Two Wheel Offsets

Enter your current and new wheel to compare any two offsets.

Current wheel

in
The second number in 18×8.
mm
Stamped as ET or +/−.

New wheel

in
mm
mm
Leave at 0 without a spacer.

Enter your numbers and press Calculate.

Same-width offset differences

FromToChange
+35+40−5 mm in
+35+45−10 mm in
+40+45−5 mm in
+30+35−5 mm in
+25+35−10 mm in
+18+35−17 mm in
+20+35−15 mm in
+38+45−7 mm in
+45+50−5 mm in
+30+40−10 mm in
0−12+12 mm out
0+18−18 mm in
−12−24+12 mm out
−12+18−30 mm in
−18−44+26 mm out

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.

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

Sources

  • https://mickeythompsontires.com/tech-bulletins/backspacing-and-offset