Backspacing ↔ Offset Calculator
Backspacing (in) = wheel width ÷ 2 + 0.5 + offset (mm) ÷ 25.4, and in reverse, offset (mm) = (backspacing − width ÷ 2 − 0.5) × 25.4, using the bead-seat width in inches. Example: a 9" wheel at +18 mm has 5.71" of backspacing. Offset (mm) and backspacing (inches) describe the same thing — where the mounting face sits in the wheel — in two conventions. Enter the wheel width and either number, and the backspacing calculator gives you the other.
Why two numbers
Offset is mounting face to centerline, in millimeters, positive inward. It's independent of wheel width, so street and import wheels quote it. Backspacing is mounting face to the inner lip, in inches. It tells an off-road builder directly how far the wheel reaches toward the suspension, which is why lift kits and truck wheels quote it. Same wheel, two descriptions.
Quick reference
| Width \ offset | −44 mm | −12 mm | 0 mm | +18 mm | +35 mm | +44 mm |
|---|---|---|---|---|---|---|
| 7" | 2.27" | 3.53" | 4.00" | 4.71" | 5.38" | 5.73" |
| 7.5" | 2.52" | 3.78" | 4.25" | 4.96" | 5.63" | 5.98" |
| 8" | 2.77" | 4.03" | 4.50" | 5.21" | 5.88" | 6.23" |
| 8.5" | 3.02" | 4.28" | 4.75" | 5.46" | 6.13" | 6.48" |
| 9" | 3.27" | 4.53" | 5.00" | 5.71" | 6.38" | 6.73" |
| 9.5" | 3.52" | 4.78" | 5.25" | 5.96" | 6.63" | 6.98" |
| 10" | 3.77" | 5.03" | 5.50" | 6.21" | 6.88" | 7.23" |
| 12" | 4.77" | 6.03" | 6.50" | 7.21" | 7.88" | 8.23" |
Will these wheels fit? Check the new wheel against your current one →
Wheel offset calculator (old vs new) →
Offset comparison: three wheels vs your current one →
Backspacing vs Offset →
Wheel Offset Explained →
Backspacing and offset questions
How do I convert backspacing to offset?
Offset in mm = (backspacing in inches - width in inches / 2 - 0.5) x 25.4. The width here is the bead-seat width stamped on the wheel, and the 0.5 is the half inch of flange the stamped width leaves out, a quarter inch on each side.
Why does the same backspacing give a different offset on a wider wheel?
Because backspacing is measured from the back lip and offset from the centreline, and widening a wheel moves the centreline but not the back lip. 5 inches of backspacing is ET+13 on an 8-inch wheel and ET-13 on a 10-inch wheel. Quoting backspacing without the width says nothing.
Why do shops quote backspacing instead of offset?
Because backspacing is what you can measure on a bench with a straightedge and a ruler, with the wheel face down, while offset needs the centreline you cannot see. Truck and off-road shops stayed with it; wheel makers stamp offset as ET. They describe the same geometry.
Does more backspacing push the wheel in or out?
In. More backspacing moves the mounting face toward the back of the wheel, so the wheel tucks further under the vehicle and the inner edge moves toward the suspension. Less backspacing pushes it out toward the fender.