Wheel Offset Calculator

What is wheel offset?

Offset is the distance, in millimetres, between the wheel's mounting surface (where it bolts to the hub) and the centreline of the wheel. It is stamped on most wheels as 'ET' – from the German Einpresstiefe – for example ET35.

• Positive offset (ET +): the mounting face sits outboard of the centreline, pulling the wheel in under the car. Most modern front-wheel-drive cars use high positive offsets.
• Zero offset (ET 0): the mounting face is exactly on the centreline.
• Negative offset (ET −): the mounting face sits inboard, pushing the wheel out – common on lifted trucks and 'deep dish' styles.

Offset vs backspacing

Backspacing is the distance from the mounting surface to the inner edge of the wheel, usually quoted in inches by US wheel makers. It depends on both offset and width, so two wheels with the same offset but different widths have different backspacing.

• Backspacing (in) = (rim width + 1) ÷ 2 + offset ÷ 25.4
• Front spacing (in) = (rim width + 1) − backspacing

The '+1' covers the two bead flanges: a wheel sold as 8 inches wide measures about 9 inches overall.

Worked example: 8×ET35 to 9×ET20

The wider, lower-offset wheel gains only 2.3 mm of clearance on the inside but sticks out 27.7 mm further. Whether that fits depends on how much space there is between the current tyre and the fender lip.

How to calculate clearance change by hand

1. Convert both offsets to inches (mm ÷ 25.4).
2. Work out backspacing for each wheel: (width + 1) ÷ 2 + offset.
3. Inner change = old backspacing − new backspacing (positive = more room inside).
4. Outer change = new front spacing − old front spacing (positive = wheel sits further out).
5. Compare the outer change with the gap you measured between tyre and fender.

How to measure offset and backspacing at home

Lay the wheel face-down, place a straight edge across the inner lip and measure from the straight edge down to the mounting surface – that is backspacing. Measure the overall width the same way across both lips. Offset (in) = backspacing − overall width ÷ 2; multiply by 25.4 for millimetres.

How offset affects handling

Moving the wheel outward widens the track, which can add stability, but it also changes the scrub radius – the distance between the steering axis and the tyre's contact patch. Large changes can make steering heavier, increase tramlining over ruts and add load to wheel bearings. Staying within about ±5 mm of the original effective position keeps the car's steering feel close to factory.

Fitment checklist

• Check brake-caliper clearance – larger brakes need more spoke clearance, especially with low-offset wheels.
• Check strut and spring clearance at full steering lock.
• Measure fender clearance with the suspension compressed, not just at ride height.
• Match the bolt pattern and centre bore (use hub-centric rings if the bore is larger).
• Keep tyre load rating at least equal to the original.
• Many countries require tyres to be covered by bodywork – too much poke can fail inspection.

How to read wheel markings

A typical marking such as 18x8.5 ET35 5x114.3 CB73.1 means: 18-inch diameter, 8.5-inch width, +35 mm offset, five studs on a 114.3 mm circle, and a 73.1 mm centre bore. All five numbers must suit your car – offset and width decide where the wheel sits, while bolt pattern and centre bore decide whether it fits the hub at all.

Wheel spacers and offset

A spacer adds its thickness between hub and wheel, reducing effective offset by the same amount: an ET40 wheel on a 15 mm spacer behaves like ET25. Use hub-centric spacers with suitable studs or bolts, re-torque after the first drive, and check that enough thread engagement remains.

Tyre width and offset together

A wider tyre on the same wheel grows equally on both sides. If you fit a tyre 20 mm wider than before, the sidewall moves about 10 mm closer to the suspension and 10 mm closer to the fender – before any change in offset. Add that to the calculator's result when you check clearance.

Lowered and lifted vehicles

Lowering a car brings the fender down over the tyre and often adds negative camber, so poke that cleared at stock height may rub when lowered. Lifted trucks commonly use negative offsets and wider tyres; check that the wheels stay within local rules on bodywork coverage.

Choosing an offset for a wider wheel

• Keep the inner edge where it was: lower the offset by 12.7 mm for every extra inch of width – all the extra width goes outward.
• Keep the outer face where it was: raise the offset by 12.7 mm per extra inch – all the extra width goes inward toward the suspension.
• Split the difference: keep the same offset and the extra width is shared equally, half an inch each side per extra inch.