Offset decides where the wheel sits in the fender: tucked in, flush, or poking out. Two wheels with the same diameter and width can look and fit completely different because of offset.
Offset in plain words
Offset (written ET or +/- mm) is the distance from the wheel's mounting face to the center line of the wheel.
Backspacing
Backspacing is the distance from the mounting face to the inner edge of the wheel, in inches. It tells you how close the wheel comes to the strut, spring and brake caliper.
Formula: backspacing = (wheel width + 1) รท 2 + offset in mm รท 25.4
Example: 18x9.5 +40 โ (9.5 + 1) รท 2 = 5.25 in, plus 40 รท 25.4 = 1.57 in โ 6.82 in of backspacing.
How a new size moves the wheel
When you change width and offset together, both edges of the wheel move:
- Outer edge moves out by: half the extra width (in mm) + the drop in offset.
- Inner edge moves in by: half the extra width (in mm) โ the drop in offset.
Example: going from 18x8 +45 to 18x9 +35. One inch wider is 25.4 mm, so each side gets 12.7 mm. Offset dropped 10 mm.
| Change | What it means | |
|---|---|---|
| Outer lip | 12.7 + 10 = 22.7 mm further out | Closer to the fender, more aggressive look |
| Inner barrel | 12.7 โ 10 = 2.7 mm further in | Barely closer to the strut, almost no change inside |
Easy Fit vs Aggressive Fit
Our Fitment Finder sorts every wheel into two groups for your exact car and suspension:
What else limits offset
- Brake calipers. Big brake kits and some performance trims need more spoke clearance. A low offset with a flat face can hit the caliper even when the size looks right.
- Suspension. Lowering brings the fender down onto the tire. Coilovers and camber plates open up more room, which is why our finder asks for your suspension.
- Tire size. A wider or stretched tire changes clearance as much as the wheel does. See the tire size guide.
- Spacers. A 10 mm spacer has the same effect as 10 mm less offset. See spacers and adapters.