Wheel offset calculator
Put in the wheels you have and the wheels you want. You get backspacing, how far the wheel moves toward the fender, and how much closer it sits to the strut.
Offset and backspacing
Current wheel
17x8 ET+35
Backspacing 5.88" · Overall width 9.0"
New wheel
17x9 ET+18
Backspacing 5.71" · Overall width 10.0"
Outer edge (fender side)
+1.17"
further out, toward the fender lip
Inner edge (suspension side)
−0.17"
further from the suspension
| Measurement | Current | New | Change |
|---|---|---|---|
| Backspacing | 5.88″ | 5.71″ | −0.17″ |
| Frontspacing | 3.12″ | 4.29″ | +1.17″ |
| Overall width | 9.0″ | 10.0″ | +1.0″ |
| Offset | 35 mm | 18 mm | −17 mm |
How offset and backspacing relate
Offset and backspacing measure the same thing from two different places. Offset (ET) is millimeters from the wheel centerline to the mounting face. Backspacing is inches from the mounting face to the inner flange. Converting between them only needs the wheel width:
backspacing = (width + 1) \u00f7 2 + offset \u00f7 25.4
Width in inches, offset in millimeters, result in inches. The +1 accounts for the flanges, which add about half an inch per side beyond the stamped bead seat width.
The number that matters for rubbing is not offset on its own, it is how far each edge of the wheel moves. Going wider at the same offset splits the change evenly: one inch of extra width moves the outer edge out half an inch and the inner edge in half an inch. Dropping offset moves the whole wheel outward without changing its width.
Outward movement is a fender problem: the lip and the liner are what you catch, usually at full lock or full compression. Inward movement is a suspension problem: the strut body, the control arm and sometimes the brake caliper. The calculator reports both directions separately because they fail in different places.
Limits
This is geometry, not a fitment guarantee. It does not know your ride height, your tire width, your suspension travel or your brake package, all of which decide whether a given setup actually clears. Treat the numbers as the first check and measure the car before you buy. The thresholds used for the verdict are common rules of thumb, not a standard.
Last updated September 2026.
Common questions
- What does wheel offset actually mean?
- Offset, stamped as ET, is the distance in millimeters from the wheel’s mounting face to its centerline. Positive offset pushes the mounting face outboard, which tucks the wheel further under the car. Negative offset does the opposite and makes the wheel stick out. A change of 25 mm is roughly one inch of movement.
- What is the difference between offset and backspacing?
- They describe the same geometry from different reference points. Offset is measured in millimeters from the centerline; backspacing is measured in inches from the mounting face to the inner flange. Backspacing = (wheel width + 1) ÷ 2 + offset ÷ 25.4. A 17x9 with +18 mm offset has 5.71 inches of backspacing.
- Why add one inch to the wheel width?
- The width stamped on a wheel is the bead seat width, measured inside the flanges. The flanges add about half an inch per side, so a 9-inch wheel is 10 inches flange to flange. Backspacing is measured to the flange, which is why the extra inch belongs in the formula.
- How much offset change is too much?
- As a rule of thumb, under a quarter inch of movement is hard to notice, a quarter to half an inch is worth measuring before you buy, and more than half an inch commonly rubs. Which end rubs depends on direction: outward movement hits the fender lip and liner, inward movement hits the strut and control arm.
- Does a wider wheel stick out more?
- It splits the difference. Keeping the same offset and going one inch wider moves the outer edge out half an inch and the inner edge in half an inch. That is why a wider wheel often needs more positive offset to sit in the same place.