About the Wheel Offset and Backspacing Calculator
This wheel offset and backspacing calculator converts between the two ways wheel fitment is specified. Offset is the distance in millimetres from the wheel’s centreline to its mounting face (positive means the face is toward the outside of the car); backspacing is the distance in inches from the mounting face to the inner lip. Enter a wheel width and either value to get the other, plus the front spacing — how far the outer lip sits from the hub.
The second wheel lets you compare your current wheels with new ones before buying. The calculator shows how many millimetres closer the inner lip moves toward struts, springs and brake lines, and how much farther the outer lip moves toward the fender. Those two numbers decide whether a wider or lower-offset wheel will rub.
It assumes the common convention that a wheel’s overall width is about 1 inch more than its nominal rim width (the bead flanges add roughly half an inch per side). Tyre sidewall bulge, brake caliper shape and spoke design also affect real clearance, so treat the results as a measurement guide and test-fit when clearances are tight.
How to use the wheel offset and backspacing calculator
- 1Choose whether your wheel specs give offset (ET, mm) or backspacing (inches).
- 2Enter the width and offset or backspacing of your current wheel.
- 3Enter the same details for the wheel you are considering.
- 4Read the converted values for each wheel.
- 5Check the inner and outer change to judge suspension and fender clearance.
Formula and method
A wheel’s overall width is taken as its nominal rim width W plus 1 inch for the two bead flanges, so the centreline sits half of that from either lip. Backspacing is the distance from the mounting face to the inner lip: half the overall width plus the offset converted from millimetres to inches (÷ 25.4). Front spacing is what remains on the outside.
When comparing two wheels, the change in backspacing shows how far the inner lip moves toward the suspension, and the change in front spacing shows how far the outer lip moves toward the fender. Both are reported in millimetres.
- W
- Nominal rim width (inches)
- ET
- Offset in millimetres (positive = mounting face toward outside)
- BS
- Backspacing (inches)
- FS
- Front spacing (inches)
Worked examples
8 in +35 to 9 in +20
The 8 in wheel is 9 in overall, so its centreline is 4.5 in from the lip; adding 35 ÷ 25.4 = 1.378 in gives 5.88 in backspacing. The 9 in +20 wheel has 5.79 in backspacing, so it gains 2.3 mm inside but pokes out 27.7 mm further.
Truck wheels by backspacing: 10 in with 4.5 in vs 5 in
A 10 in wheel is 11 in overall with its centreline 5.5 in from the inner lip. 4.5 in backspacing is 1 in inboard of that, which is −25.4 mm offset; 5 in backspacing is −12.7 mm. The second wheel tucks 12.7 mm further in but uses 12.7 mm more inner clearance.
Sport compact: 7.5 in +45 to 8.5 in +35
Going an inch wider adds 12.7 mm to each side, and dropping offset by 10 mm moves the whole wheel 10 mm outward. Net: the inner lip moves 2.7 mm closer to the strut and the outer lip pokes out 22.7 mm.
Frequently asked questions
What is the difference between wheel offset and backspacing?+
Offset is measured from the wheel’s centreline to the hub mounting face, in millimetres. Backspacing is measured from the mounting face to the inner edge of the wheel, in inches. Both describe the same position; backspacing also depends on wheel width.
Does lower offset make the wheel stick out more?+
Yes. Lowering offset moves the mounting face inward relative to the wheel, so the whole wheel shifts outward. Each 1 mm lower offset pushes both lips 1 mm further out, adding poke and freeing inner clearance.
How does a wider wheel change fitment?+
At the same offset, each extra inch of width adds about half an inch (12.7 mm) on both the inner and outer side. To keep the inner lip where it was you would need to lower the offset by 12.7 mm per extra inch.
What does positive, zero and negative offset mean?+
Positive offset puts the mounting face outboard of the centreline (common on modern cars and FWD). Zero offset puts it exactly in the middle. Negative offset places it inboard, giving a deep-dish look common on lifted trucks.
How do I measure backspacing myself?+
Lay the wheel face-down, place a straightedge across the inner lip, and measure from the straightedge down to the mounting pad. That distance in inches is the backspacing.