How to use the backspacing calculator
- Choose whether you are converting an offset to backspacing, or a backspacing figure to offset.
- Enter the marked wheel width in inches, then the offset in millimetres or the backspacing in inches.
- Leave the flange allowance at 0.5 in per side unless you know the wheel’s measured overall width.
- Press Calculate for backspacing or offset, front spacing and a cross-section.
Offset and backspacing describe the same thing from different reference points, so converting between them is exact once the overall width is known. The flange allowance is the only assumption, and the result tells you which value was used.
Worked example
An 8-inch wheel with +35 mm offset. With half an inch of flange per side, the overall width is 9 in and the centreline is 4.5 in from the inboard edge. A +35 mm offset puts the mounting face 35 ÷ 25.4 = 1.378 in outboard of the centreline, so backspacing is 4.5 + 1.378 = 5.88 in and front spacing is 3.12 in.
Going the other way, a 10-inch wheel with 4.5 in backspacing has its mounting face one inch inboard of its 5.5 in centreline, which is an offset of -25.4 mm.
Formulas used
overall width (in) = marked width + 2 × flange allowance
backspacing (in) = overall width ÷ 2 + offset (mm) ÷ 25.4
offset (mm) = (backspacing − overall width ÷ 2) × 25.4
front spacing (in) = overall width − backspacing The convention of adding about one inch to the marked width for the flanges is described in Wheel-Size.com’s offset and backspacing guide. Offset sign follows Tire Rack’s definition: positive when the mounting face is outboard of the centreline.
Limitations
- Flange thickness varies between wheels; for exact backspacing, measure the wheel or use the maker’s published figure.
- The result describes the wheel only. It is not a clearance, bolt-pattern, centre-bore or load check.
- To see how a new wheel moves relative to your current one, use the wheel offset calculator.
Frequently asked questions
What is the difference between offset and backspacing?
Both locate the hub mounting face within the wheel. Offset is measured from the wheel’s centreline, in millimetres, and can be positive, zero or negative. Backspacing is measured from the inboard edge of the wheel, in inches, and is always positive. Offset is the usual measure for passenger cars; backspacing is common for trucks and off-road wheels in North America.
Why does the calculator add width for the flanges?
The marked width of a wheel (the 8 in 8J or 8x17) is measured between the bead seats where the tyre sits. Backspacing is measured to the outer lip of the inboard flange, so the overall width is wider. A common convention adds about half an inch per side, which is the default here. If you have measured the overall width of your wheel, set the flange allowance so the overall width matches your measurement.
Does more backspacing move the wheel in or out?
For the same overall width, more backspacing puts more of the wheel behind the mounting face, so the wheel sits further inward toward the suspension, and the outer face sits further in under the fender. Less backspacing pushes the wheel outward.
Is backspacing the same as front spacing?
No. Front spacing (sometimes called front space or outer spacing) is the distance from the mounting face to the outboard edge. Backspacing plus front spacing equals the overall width.
Can this tell me whether a wheel will clear my brakes or suspension?
No. It converts between two ways of describing the same wheel. Clearance also depends on spoke shape, brake caliper size, tyre width, suspension travel and steering lock, and bolt pattern and centre bore are separate checks.
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