Backspacing ↔ Offset Calculator
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 to get the other.
Backspacing uses the wheel's overall width, which
is the bead-seat width plus about 1 inch of flanges. Formula: backspacing =
width ÷ 2 + 0.5 + offset ÷ 25.4. A few makers quote ±¼" differently — if a
listing gives both numbers, trust them over this conversion.
Why two numbers
Offset is mounting face to centerline, in millimetres, 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 | −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" |