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Automotive

Wheel Offset Calculator

Convert between wheel offset and backspacing.

Your measurementsEvery assumption editable01

Positive moves the mounting face outward toward the street side.

Wheel geometryCalculated live02

Result

5.88in backspacing

35 mm offset = 5.88 in backspacing

Nominal wheel width
8 in
Overall widthnominal plus 1 in for the flanges
9 in
Offset
35 mm
Backspacinghub face to the inner lip
5.88 in
Frontspacinghub face to the outer lip
3.12 in

Offset and backspacing describe the same thing from different reference points. Offset is measured in millimetres from the wheel's centreline; backspacing in inches from the inner lip. Neither is more correct — suppliers simply use different conventions.

The flange allowance matters. An 8 inch wheel measures 9 inches across the lips, and forgetting that inch is the commonest error in converting between the two.

Lower offset pushes the wheel outward, which risks fouling the arch and adds scrub radius that the steering will feel. Higher offset tucks it in toward the strut and the inner liner.

Offset and backspacing describe the same geometry from different reference points. Converting between them requires adding an inch to the nominal width for the flanges — the step most conversions miss.

Why use this tool?What it does differently03

Why use this tool?

Both conventions

Offset in millimetres and backspacing in inches, since suppliers use whichever they prefer.

The flange inch

An 8 inch wheel measures 9 across the lips. Forgetting that is the commonest conversion error.

Frontspacing too

Hub face to outer lip, which is what determines whether the wheel sits proud of the arch.

Explains the consequences

Lower offset pushes the wheel out and adds scrub radius the steering will feel.

How this worksThe method04

How this wheel offset calculator works

Overall wheel width is the nominal width plus one inch, because the nominal figure measures between the bead seats and excludes the flanges on either side.

Backspacing is half the overall width plus the offset, converted from millimetres to inches. Frontspacing is whatever remains of the overall width.

Lower or negative offset pushes the wheel outward, which risks fouling the arch and adds scrub radius that the steering will feel as increased kickback. Higher offset tucks the wheel inward toward the strut and inner liner.

How to use itStep by step05

How to use it

  1. Step 1: Enter the nominal width

    From the wheel — an 8J wheel is 8 inches nominal.

  2. Step 2: Enter the offset

    Usually stamped on the back of the spokes as ET followed by a number in millimetres.

  3. Step 3: Read backspacing

    Hub mounting face to the inner lip, in inches.

  4. Step 4: Check clearance both ways

    Inner clearance to the strut, outer clearance to the arch.

Example usageWorked figures06

Example usage

A typical factory wheel
8 inches wide with 35 mm offset gives 5.88 in backspacing and 3.12 in frontspacing.
An aggressive fitment
The same 8 inch wheel at 0 mm offset gives 4.5 in backspacing — an inch and a half further out, which is where arch clearance problems begin.
A wider wheel, same offset
A 10 inch wheel at 35 mm offset gives 6.88 in backspacing. The extra width goes almost entirely inboard toward the suspension.
Frequently asked questionsCommon questions07

Frequently asked questions

What is the difference between offset and backspacing?

The same geometry from different references. Offset is measured in millimetres from the wheel's centreline; backspacing in inches from the inner lip. Neither is more correct.

How do I convert offset to backspacing?

Add one inch to the nominal width for the flanges, halve it, then add the offset converted to inches. The flange inch is the step most people miss.

Does lower offset stick the wheel out further?

Yes. Lower or negative offset moves the wheel outward toward the arch, and adds scrub radius the steering will feel.

Why is a wheel wider than its stated size?

The nominal figure measures between the bead seats where the tyre sits. The flanges add about half an inch on each side.

Can I run a different offset safely?

Within limits. Substantial changes alter scrub radius and bearing loading as well as clearance, so large deviations from factory offset are not purely cosmetic.

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