Wheel offset calculator
Enter the width and offset of your current wheel and the new one. Each extra inch of width adds 12.7 mm per side, and each millimeter less offset (or each millimeter of spacer) moves both edges 1 mm outward. If neither edge moves more than about 5 mm toward the car, it usually fits like stock. Past 15 mm it usually rubs.
Result
Fix the highlighted box to update the results. Widths run 4 to 14 in, offsets −100 to +100 mm, spacers 0 to 50 mm.
| Current | New | |
|---|---|---|
| Backspacing (in) | – | – |
| Backspacing (mm) | – | – |
| Effective offset (with spacer) | – | – |
Verdicts use TireWheelGuide's rule of thumb for how far each edge moves toward the car: within about 5 mm fits like stock, 5 to 15 mm means check clearance, over 15 mm usually rubs. An edge that moves away gains room.
How to use the wheel offset calculator
Type in the width and offset of the wheel you have now, then the wheel you want. The results update as you type. You'll find both numbers stamped on the wheel, often on the back of a spoke. "7.5J x 17 ET51" means 7.5 inches wide, 17-inch diameter and +51 mm offset.
If you plan to run spacers, enter the thickness in the spacer box. A spacer works exactly like lower offset (KSP). A 10 mm spacer on an ET45 wheel puts it where an ET35 wheel would sit.
You get four things back. The outer lip change is the poke. The inner lip change is how much closer the wheel gets to the strut, spring and brake lines. Backspacing is shown for both wheels in inches and millimeters. The drawing shows the two wheels over each other around the same hub face.
How the math works
Two rules do all the work (TireWheelGuide). Extra width splits evenly, so one more inch adds 12.7 mm on each side. Offset moves the whole wheel, so each millimeter less offset moves both edges 1 mm out.
Inner lip move = (new width − old width) × 12.7 − (old offset − new offset)
A spacer is subtracted from the new offset first. So the effective offset is the new offset minus the spacer thickness.
Backspacing uses the real edge-to-edge width. A wheel sold as 8 inches is measured at the bead seats, and each rim flange adds about half an inch (TireWheelGuide). That's why the Wheel-Size.com formula adds 1 inch:
Here's the default example, a VW GTI going from 17x7.5 ET51 to 18x8.5 ET45. The width grows 1 inch, so each side gets 12.7 mm. The offset drops 6 mm, so the whole wheel also moves 6 mm out.
| Step | Outer lip | Inner lip |
|---|---|---|
| Extra width, 1 in × 12.7 | +12.7 mm | +12.7 mm |
| Offset 51 → 45 | +6 mm | −6 mm |
| Total | 18.7 mm farther out | 6.7 mm closer to the strut |
Backspacing goes from (7.5 + 1) ÷ 2 + 51 ÷ 25.4 = 6.26 inches to (8.5 + 1) ÷ 2 + 45 ÷ 25.4 = 6.52 inches. That's 159.0 mm to 165.7 mm. The backspacing grew by 6.7 mm, which matches the inner lip move. It should, because backspacing is measured to the inner lip.
What the numbers mean
Judge each edge on its own, and let the worse one decide. TireWheelGuide's rule of thumb works like a traffic light (TireWheelGuide):
| Edge moves toward the car by | Verdict | What to do |
|---|---|---|
| Up to about 5 mm | ✓ Fits like stock | Usually nothing |
| 5 to 15 mm | ! Check clearance | Measure to the strut, spring and fender lip, at full lock and full compression |
| Over 15 mm | ✕ Usually rubs | Expect to roll fenders, add camber or pick another offset |
A negative inner number means the inner lip moves away from the suspension. You gain room there. The calculator labels that "More room" rather than giving it a rub warning. The same goes for a negative outer number, which tucks the wheel in.
The GTI example gets amber on the inside and red on the outside. That doesn't mean it won't fit. It means you should measure the gap to the fender lip before you buy. A Mazda MX-5 going from 17x7 ET45 to 17x8 ET35 shows the same pattern: 22.7 mm out, 2.7 mm in.
If your strut is the tight spot, drop the offset by 12.7 mm for every extra inch of width. The inner lip then stays where it was, and all the extra width goes outward.
What this calculator can't tell you
It only knows about the wheel. The tire adds its own width on top, about 10 mm per side going from a 225 to a 245 tire. Our tire size calculator works that out.
It also can't see your brakes. Spoke shape and barrel profile decide caliper clearance, so use the wheel maker's brake template if you run a big brake kit (Konig). Lowering, camber and suspension travel all change the real gap too. The 5 and 15 mm thresholds are a rule of thumb, not a guarantee.
Bolt pattern and center bore still have to match. Our wheel offset guide covers those, plus how offset changes scrub radius and bearing load. If you're using spacers, read whether wheel spacers are safe first.
Sources are linked in the text. Main references: TireWheelGuide, Wheel-Size.com and KSP.