Wheel offset decides how far a wheel sits in or out of the wheel well. A few millimetres can mean the difference between a wheel that clears the brake caliper and the fender and one that rubs on both. Offset, wheel width and backspacing are all tied together, and when you change wheels the question is always the same: how much closer will the wheel be to the suspension on the inside, and how much closer to the fender on the outside? This calculator compares your current wheel with a new one and answers exactly that.
How to use the wheel offset calculator
- Enter your current wheel: width in inches and offset (ET) in millimetres. Both are stamped on the back of the wheel, for example 8J ET35.
- Enter the new wheel's width and offset.
- Read the backspacing of both wheels and the shift of the inner and outer edges. Positive inner means the inside edge moves toward the suspension; positive outer means the outside edge moves toward the fender.
Fitment also depends on the stud layout: use the bolt pattern calculator to check it, and the tire size comparison calculator to compare the tires that go on the wheel.
Wheel offset formula
Outboard distance (frontspacing) = width − backspacing
Inner edge shift = new backspacing − old backspacing
Outer edge shift = new outboard distance − old outboard distance
- Width in millimetres (inches × 25.4)
- Offset (ET) positive when the mounting face is toward the outside of the centerline
- Backspacing is in the same unit as the wheel; many shops quote it in inches (divide by 25.4)
Worked example: 8J ET35 → 9J ET25
- Current: width 8 × 25.4 = 203.2 mm; backspacing 101.6 + 35 = 136.6 mm (5.38 in).
- New: width 9 × 25.4 = 228.6 mm; backspacing 114.3 + 25 = 139.3 mm (5.48 in).
- Inner edge: 139.3 − 136.6 = +2.7 mm toward the suspension.
- Outer edge: (228.6 − 139.3) − (203.2 − 136.6) = 89.3 − 66.6 = +22.7 mm toward the fender.
The wider wheel with lower offset grows mostly outward: it clears inside by almost the same amount as before, but pushes 23 mm closer to the fender.
Offset and backspacing of common wheels
Each row is calculated with the formulas above.
| Wheel width | Offset (ET) | Backspacing | Distance outboard of the hub face |
|---|---|---|---|
| 7 in | +40 mm | 128.9 mm (5.07 in) | 48.9 mm |
| 7.5 in | +45 mm | 140.2 mm (5.52 in) | 50.2 mm |
| 8 in | +35 mm | 136.6 mm (5.38 in) | 66.6 mm |
| 8 in | +0 mm | 101.6 mm (4.00 in) | 101.6 mm |
| 9 in | +30 mm | 144.3 mm (5.68 in) | 84.3 mm |
| 10 in | -25 mm | 102.0 mm (4.02 in) | 152.0 mm |
Reading the result
The outer edge shift is what you check against the fender and body. Small increases are usually fine with a tire that has not been stretched or flared fenders; larger ones can rub on bumps or at full lock. The inner edge shift matters for the suspension, brake calipers and, on front wheels, the steering components. A positive inner number means less clearance on the inside, so check how much room you have there before you commit.
Lowering the offset is the quickest way to give a wheel a more aggressive stance, but it also changes the scrub radius of the steering and loads the wheel bearings differently. Wheels offered for your car are normally within a few millimetres of the factory offset for that reason. Moving more than about 10 mm usually calls for checking clearances and, sometimes, using spacers or fender work.
The tire's width and sidewall add to the picture, since the tire is wider than the wheel. Use the tire size calculator to get the tire's width in inches and the tire size comparison calculator to compare the old and new tires; this calculator deals only with the wheel itself.
Sources
The definitions of offset (ET) and backspacing, and the relationship backspacing = width ÷ 2 + offset, follow the standard wheel-geometry definitions described in the Wikipedia article “Offset (vehicle)” and wheel-fitment guides. Table values were computed from the formulas shown. The result is a geometric comparison only; brake and suspension clearances must be measured on the vehicle.
Frequently asked questions
What is wheel offset?
Offset (ET) is the distance in millimetres from the wheel’s mounting face to its centerline. Positive offset moves the wheel inward toward the car, negative offset pushes it outward.
What is backspacing?
Backspacing is the distance from the mounting face to the wheel’s inner edge. It equals half the wheel width plus the offset, and is often measured in inches.
What does a lower offset do?
It moves the whole wheel outward. Reducing offset by 10 mm moves the outer edge 10 mm toward the fender and the inner edge 10 mm away from the suspension.
What happens when I fit a wider wheel?
A wider wheel grows in both directions around its centerline: each extra inch of width adds 12.7 mm to the inside and 12.7 mm to the outside, before the offset change is applied.
Does this prove the wheel will fit?
No. Brake calipers, suspension parts, fender lips and tire width all set the limits. Use the change in clearance as a guide, then check with measurements or fitment data.