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Wheel Offset Calculator

Compare a new wheel with your current one to see how the width, offset (ET) and any spacer change the inner clearance and outward poke.

Current wheel
inches
millimetres offset (ET)

Can be positive, zero or negative.

New wheel
inches
millimetres offset (ET)
millimetres

Thickness of any spacer behind the new wheel. 0 for none.

Results

Outward poke

17.7 mm more

Compared with the current wheel, the new wheel has 17.7 mm more outward poke and 7.7 mm less inner clearance.

Inner clearance
7.7 mm less
New wheel effective offset
ET35
Wheel position, measured from the hub mounting face
Current8" ET40New9" ET35
Width (mm)203.2228.6
OffsetET40ET35
Offset with spacerET40ET35
Inner edge (mm)141.6149.3
Outer edge (mm)61.679.3

Compared with the current wheel, the new wheel has 17.7 mm more outward poke and 7.7 mm less inner clearance.

Formula

Width (mm)
= width (in) × 25.4
Effective offset (mm)
= offset (ET) − spacer thickness
Inner edge (mm)
= width (mm) ÷ 2 + effective offset
Outer edge (mm)
= width (mm) ÷ 2 − effective offset
Inner clearance change
= current inner edge − new inner edge (positive = more clearance)
Outward poke change
= new outer edge − current outer edge (positive = further out)

Both edges are measured from the hub mounting face, the flat surface that bolts to the car. Offset is the distance from the centre of the wheel to that face, so half the width sits either side of the centre line and the offset shifts both edges the same way.

Worked example

You are replacing 8" ET40 wheels with 9" ET35 wheels and no spacer.

  1. Step 1: 8" ET40 inner edge

    8 × 25.4 ÷ 2 + 40 = 141.6 mm

  2. Step 2: 8" ET40 outer edge

    8 × 25.4 ÷ 2 − 40 = 61.6 mm

  3. Step 3: 9" ET35 inner edge

    9 × 25.4 ÷ 2 + 35 = 149.3 mm

  4. Step 4: 9" ET35 outer edge

    9 × 25.4 ÷ 2 − 35 = 79.3 mm

  5. Step 5: Inner clearance

    141.6 − 149.3 = 7.7 mm less

  6. Step 6: Outward poke

    79.3 − 61.6 = 17.7 mm more

Compared with the current wheel, the new wheel has 17.7 mm more outward poke and 7.7 mm less inner clearance. The extra width is split between both sides, and the lower offset moves the whole wheel further out.

What wheel offset (ET) means

Offset is the distance in millimetres between the centre line of the wheel and the hub mounting face. It is often stamped on the wheel as ET, from the German Einpresstiefe, for example ET40.

  • Positive offset: the mounting face is outboard of the centre line, so more of the wheel sits inside, towards the suspension. Most modern cars use positive offset.
  • Zero offset: the mounting face is exactly on the centre line.
  • Negative offset: the mounting face is inboard of the centre line, so more of the wheel sits outside, towards the wheel arch.

How width and offset move the wheel

A wider wheel at the same offset gains half the extra width on each side: 1 inch wider (25.4 mm) moves both the inner and outer edges by 12.7 mm.

Lowering the offset moves the whole wheel outwards by the same amount: going from ET40 to ET30 moves both edges 10 mm further out, giving 10 mm more inner clearance and 10 mm more poke. Raising the offset does the opposite.

A spacer between the hub and the wheel has the same effect as lowering the offset by its thickness, which is why the calculator shows the new wheel's effective offset.

What this calculator cannot tell you

It works out geometry from the nominal wheel width and offset. It does not know your car, so it cannot say whether a wheel will fit. In particular it cannot account for:

  • brake caliper clearance, which depends on the spoke design and the wheel's inner profile;
  • the shape of the suspension, and clearance at full steering lock or under compression;
  • clearance to the wheel arch and bodywork;
  • the width and shape of the tire fitted to the wheel, which usually extends beyond the rim;
  • alignment settings and the manufacturer's approved wheel specifications.

Frequently asked questions

How do I calculate wheel offset changes?

For each wheel, halve its width in millimetres, then add the offset to get the inner edge and subtract it to get the outer edge. Compare the two wheels edge by edge. Going from 8" ET40 to 9" ET35: compared with the current wheel, the new wheel has 17.7 mm more outward poke and 7.7 mm less inner clearance.

What does ET mean on a wheel?

ET is the wheel's offset in millimetres, from the German Einpresstiefe. ET40 means the hub mounting face is 40 mm outboard of the wheel's centre line.

Does a lower offset push the wheel out?

Yes. Every millimetre of offset you remove moves the whole wheel a millimetre further out, increasing poke and inner clearance by the same amount.

Can wheel offset be negative?

Yes. A negative offset such as ET-12 means the mounting face is behind the centre line, so the wheel sits further out. The calculator accepts positive, zero and negative offsets.

How does a spacer change the offset?

Subtract the spacer thickness from the wheel's offset. A 10 mm spacer behind an ET35 wheel gives an effective offset of ET25, moving the wheel 10 mm further out.

Will the new wheels fit my car?

The calculator cannot tell you. It shows how the wheel's position changes, but fit also depends on brakes, suspension, bodywork, the tire and the spoke design. Check the manufacturer's approved wheel specifications and ask a wheel or tire specialist.

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