Wheel offset calculator
Offset and backspacing describe the same thing from different ends of the wheel. Enter a width with either one and this calculator gives you the other, then draws both wheels on the same hub face so you can see exactly how much room you gain on the outside and lose on the inside.
| Measurement | Current | New | Change | Formula |
|---|---|---|---|---|
| Nominal width | 7.50 in | 8.50 in | +1.00 in | bead seat to bead seat |
| Overall width | 8.50 in | 9.50 in | +1.00 in | nominal + flange allowance |
| Offset (ET) | +45 mm | +35 mm | -10 mm | hub face to centreline |
| Backspacing | 6.02 in | 6.13 in | +0.11 in | overall ÷ 2 + offset |
| Frontspacing | 2.48 in | 3.37 in | +0.89 in | overall ÷ 2 − offset |
| Inner clearance | reference | +2.7 mm | +2.7 mm | new backspacing − current |
| Outer position | reference | +22.7 mm | +22.7 mm | new frontspacing − current |
How the wheel offset calculator works
Three numbers describe where a wheel sits. The nominal width is measured bead seat to bead seat, not across the rim edges. The offset, stamped as ET followed by a number in millimetres, is the distance from the hub mounting face to the wheel's centreline, positive when the mounting face sits toward the street side. Backspacing is the distance from the mounting face to the inboard rim edge, usually quoted in inches. Only two of the three are independent:
overall width = nominal width + flange allowance backspacing = overall ÷ 2 + offset offset = (backspacing − overall ÷ 2) × 25.4
Both equations need the overall width, which is why a width must be given with every offset or backspacing figure. A bare "+35 offset" says nothing about clearance until you know how wide the wheel is: a 7-inch wheel at ET35 and a 9-inch wheel at ET35 put their inner edges an inch apart. The flange allowance converts nominal to overall. Modern Tire Dealer's wheel-dimensions reference states that rim width is the distance from the outboard bead seat to the inboard bead seat, and that the distance across the outside rim edges is not the figure quoted on the wheel, which is the gap this allowance covers.
Reading the two changes separately
Changing a wheel usually changes both edges at once, and they have different consequences. The inboard edge moving toward the car eats the clearance to the strut, spring perch, control arm and inner liner, and that gap is often the smallest one on the vehicle. The outboard edge moving away from the car changes how far the wheel sits out relative to the fender, and it also changes the scrub radius, which is what the steering feels. This calculator reports the two movements as separate lines because a wheel can be safe on one and not the other.
The one number this tool cannot produce is the clearance you actually have. That is a measurement on your own car, with the suspension at full compression and the steering at full lock, and it differs between the left and right side of the same axle. The reasoning behind each term is set out in wheel offset vs backspacing, and the bolt circle that has to match before any of this matters is covered in the bolt pattern guide. If the tires are changing too, run them through the tire size calculator first: a taller tire and a lower offset together can close a gap that neither would close alone.
What this calculator cannot tell you. It has no vehicle data, so it cannot say whether a wheel fits your car, whether the bolt pattern or centre bore match, or whether the load rating is adequate. It converts the numbers you type. Check the fit on the vehicle, confirm the wheel's load rating against the axle load on the Tire and Loading Information label, and have the fit checked by a fitter before driving.