Wheel Offset Calculator
Convert wheel offset to backspacing and front spacing (and back, from an entered backspacing) for a rim width.
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What this tool does
Convert wheel offset to backspacing and front spacing (and back, from an entered backspacing) for a rim width. Geometry conversion only — the +1 inch lip allowance is the standard approximation. It does not check vehicle fitment or clearance.
How to use the Wheel Offset Calculator
- Enter or select rim width in in.
- Enter or select wheel offset (mm).
- Enter or select backspacing for reverse conversion in in.
- Read the calculated result; change any measurement to compare alternatives.
Formula
backspacing = (width + 1)/2 + offset/25.4; offset from backspacing = (backspacing − (width + 1)/2) × 25.4
- width
- Rim width (in, before metric conversion)
- offset
- Wheel offset (mm)
- backspacing
- Backspacing for reverse conversion (in, before metric conversion)
Geometry conversion only — the +1 inch lip allowance is the standard approximation. It does not check vehicle fitment or clearance.
Worked example
For wheel offset calculator, the following measurements illustrate the exact method: Rim width: 8; Wheel offset (mm): 35; Backspacing for reverse conversion: 5.88.
Inputs
- Rim width8 in
- Wheel offset (mm)35
- Backspacing for reverse conversion5.88 in
Result
- Backspacing from offset5.88 in
- Front spacing3.12 in
- Offset from entered backspacing (mm)35.05
Results explained
- Backspacing from offset
- Backspacing from offset from the formula above, expressed in in before metric conversion. Geometry conversion only — the +1 inch lip allowance is the standard approximation. It does not check vehicle fitment or clearance.
- Front spacing
- Front spacing from the formula above, expressed in in before metric conversion. Geometry conversion only — the +1 inch lip allowance is the standard approximation. It does not check vehicle fitment or clearance.
- Offset from entered backspacing (mm)
- Offset from entered backspacing (mm) from the formula above. Geometry conversion only — the +1 inch lip allowance is the standard approximation. It does not check vehicle fitment or clearance.
Frequently asked questions
Backspacing (in) = (rim width + 1) ÷ 2 + offset (mm) ÷ 25.4. The +1 accounts for the rim's outer lip: overall width is about an inch more than the nominal bead-seat width.
The hub mounting face sits 35 mm toward the outside of the wheel from its centreline — typical of front-wheel-drive and modern cars. Negative offset pushes the wheel outward, 'deep dish' style.
(8 + 1) ÷ 2 + 35 ÷ 25.4 = 4.5 + 1.378 ≈ 5.88 inches, leaving about 3.12 inches of front spacing.
This tool only converts the geometry. Fitment also depends on brake and suspension clearance, fender room, tire size and scrub radius — verify with the wheel and vehicle manufacturers before buying.
American wheels are often sold by backspacing while imports are specified by offset (ET). The conversion lets you compare like with like at the same rim width — offset alone does not transfer between different widths.