Wheel Offset & Spacer Calculator

Compare an original and proposed wheel setup — width, offset (ET) and optional spacer — to see how much further the new wheel extends outward and how its inner clearance changes. Negative ET values are supported.

Original setup

Wheel width (in)

Offset ET (mm)

Spacer (mm)

Proposed setup

Wheel width (in)

Offset ET (mm)

Spacer (mm)

Wheel position diagram (hub face fixed)

Outward is to the right; the inner edge sits to the left, towards the suspension.

Results

Original

Width

203.2mm

Effective offset

40mm

Outer position

61.6mm

Inner extent

141.6mm

Proposed

Width

228.6mm

Effective offset

35mm

Outer position

79.3mm

Inner extent

149.3mm

Changes

Outward extension

17.7mm

Inner extent change

7.7mm

A positive inner-extent change means the inner edge moves closer to the suspension — reduced inner clearance.

How this calculator works

Wheel offset (ET) is the distance in millimetres between the wheel’s mounting face and its centreline. A spacer pushes the whole wheel outward, which reduces the effective offset. This tool converts wheel width from inches to millimetres, applies any spacer, then works out where the outer and inner edges of each wheel sit relative to the hub face.

Width (mm) = width (in) × 25.4
Effective offset = ET − spacer thickness
Outer position = width ÷ 2 − effective offset
Inner extent = width ÷ 2 + effective offset
Outward extension = new outer − original outer
Inner change = new inner − original inner (positive = reduced clearance)

Worked example: changing from an 8-inch ET40 wheel to a 9-inch ET35 wheel, with no spacers. Original outer = 203.2 ÷ 2 − 40 = 61.6 mm; proposed outer = 228.6 ÷ 2 − 35 = 79.3 mm. The wheel extends 17.7 mm further outward. Original inner = 101.6 + 40 = 141.6 mm; proposed inner = 114.3 + 35 = 149.3 mm, so inner clearance is reduced by 7.7 mm.

What this does not tell you

This compares nominal wheel positions only. It does not account for tyre bulge, tyre width, spoke shape, brake-caliper clearance, suspension arms, or the loaded position of the wheel under compression and steering. A wheel that looks right on paper can still foul a caliper or suspension component in reality.

Always confirm fitment with the wheel and tyre supplier, and check clearance at full steering lock and with the suspension compressed. Spacers must be hub- and wheel-centric, correctly torqued, and legal for your jurisdiction and use.

Frequently asked questions

What does ET mean?

ET (Einpresstiefe, “insertion depth”) is the offset in millimetres — how far the mounting face sits from the wheel’s centreline. Positive ET tucks the wheel inboard; negative ET pushes it outward. Lowering ET (or adding a spacer) moves the wheel outward.

How do spacers affect offset?

A spacer sits between the hub and the wheel, pushing the whole wheel outward by the spacer’s thickness. That reduces the effective offset by the same amount — a 15 mm spacer on an ET40 wheel gives an effective offset of ET25.

Can I enter a negative offset?

Yes. Many deep-dish and concave wheels use negative ET values. Enter the offset exactly as stated, including the minus sign.

Does a wider wheel always stick out more?

Not necessarily — it depends on the offset too. A wider wheel with a higher offset can tuck further in. This calculator shows the combined effect of width, offset and spacers together.

Are the results rounded?

Yes. Calculations use full precision internally and only the displayed values are rounded to one decimal place.

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