Wheel offset calculator
Enter the width and offset (ET) of your original wheels, then those of the fitment you are considering: the diagram shows, to the millimetre, how far the wheel moves out at the arch and in at the suspension. Add a spacer in one tap.
Worked example: original 8J ET45 versus 9J ET40
| Wheel arch side | the wheel sits 17.7 mm further out |
|---|---|
| Suspension side | 7.7 mm closer |
| Axle track width | +10.0 mm (+5.0 mm per wheel) |
Indicative geometric calculation. Real-world fitment depends on the brake callipers, the wheel arches and the suspension; at Onyx, every fitment is validated on the measurement sheet submitted to the customer before machining.
What is wheel offset?
Wheel offset (ET) is the distance, in millimetres, from the hub mounting face of the wheel to the centre line of the rim (UN Regulation No. 142, paragraph 2.4). The higher it is, the further the wheel sits in under the arch; the lower it is, the further out.
Positive, zero or negative offset
Offset can be positive, zero or negative (UN Regulation No. 124, paragraph 2.6):
- positive (ET35, ET45…): the mounting face is on the outer side of the centre line, and the wheel sits in towards the suspension and brakes (Michelin);
- zero (ET0): the mounting face is on the centre line;
- negative (−10 mm, for example): the mounting face is on the inner side, and the wheel sits out towards the arch.
The formula
The calculation places each wheel relative to the hub mounting face, counting positive towards the outside of the car:
- the centre line is at −offset;
- the outer lip is at −offset + width / 2;
- the inner lip is at −offset − width / 2.
The width is converted from inches to millimetres (× 25.4). A spacer of e millimetres moves the wheel away from the hub: it works like an offset reduced by e.
Example: from 8J ET45 to 9J ET40, the extra inch of width adds 12.7 mm on each side, and the 5 mm lower offset moves the whole wheel outwards. The outer lip sits 17.7 mm further out, the inner lip 7.7 mm closer to the suspension, and the axle track width grows by 10.0 mm.
The calculation is about the wheel. The tyre bulges slightly beyond the rim lips, and its width follows the rim width by about 5 mm per half-inch: see the tyre size calculator.
Worked examples
Common fitment changes, calculated with the same formula. On the arch side, a positive figure means the wheel sits further out. On the suspension side, a positive figure means the inner lip comes closer.
| Original | New fitment | Wheel arch side | Suspension side | Axle track width |
|---|---|---|---|---|
| 7J ET45 | 8J ET45 | +12.7 mm | +12.7 mm | 0.0 mm |
| 8J ET45 | 8J ET35 | +10.0 mm | −10.0 mm | +20.0 mm |
| 8J ET45 | 9J ET40 | +17.7 mm | +7.7 mm | +10.0 mm |
| 8J ET45 | 8J ET45 + 10 mm spacer | +10.0 mm | −10.0 mm | +20.0 mm |
| 8.5J ET35 | 9.5J ET35 | +12.7 mm | +12.7 mm | 0.0 mm |
| 8.5J ET35 | 10J ET25 | +29.1 mm | +9.1 mm | +20.0 mm |
| 9J ET50 | 10J ET40 | +22.7 mm | +2.7 mm | +20.0 mm |
| 9J ET30 | 10.5J ET20 | +29.1 mm | +9.1 mm | +20.0 mm |
| 10J ET50 | 11J ET45 + 5 mm spacer | +22.7 mm | +2.7 mm | +20.0 mm |
Wheel spacers
A spacer sits between the hub and the wheel. For the position of the wheel, it works like an offset reduced by its thickness: with a 10 mm spacer, the wheel sits 10 mm further out at the arch, 10 mm further from the suspension, and the axle track width grows by 20 mm. It does not change the width of the rim.
We found no rule specific to spacers in Great Britain. At the MOT, a wheel whose fixings are not compatible with it is a major defect (5.2.2), and so is a tyre fouling part of the vehicle (MOT inspection manual, 5.2.3). Changing wheels is a modification that can affect your insurance (Financial Ombudsman Service).
What the calculation does not tell you
It compares two positions, to the millimetre. It does not know where your brake callipers, suspension arms and arch edges are, nor how the wheel moves when it steers and when the suspension compresses. UN Regulation No. 142 (paragraph 5.2.1.3) requires the space in which the wheel turns to allow unrestricted movement with the largest permitted tyres and rim widths, taking into account the minimum and maximum offsets and the suspension and steering limits declared by the vehicle maker.
At Onyx, the customer tells us where they want the wheels to sit: as standard, further out, or flush with the arch. The design office works out the offset and width for their car and submits them to the customer on the measurement sheet before machining. The lesson Wheel offset (ET) explained (page in French) explains how offset works.
Frequently asked questions
What is wheel offset (ET)?
The distance, in millimetres, from the face where the wheel bolts to the hub to the centre line of the rim. An ET45 wheel has its mounting face 45 mm on the outer side of the centre line: the wheel sits further in under the arch.
What does a 10 mm lower offset change?
The whole wheel moves 10 mm outwards: the outer lip sits 10 mm further out, the inner lip 10 mm further from the suspension, and the axle track width grows by 20 mm.
And a wheel one inch wider, with the same offset?
It gains 25.4 mm, split evenly: 12.7 mm more on the arch side and 12.7 mm more on the suspension side. The track width does not change.
Is a 15 mm spacer the same as a 15 mm lower offset?
For the position of the wheel, yes: a 15 mm spacer behind an ET45 wheel places it like an ET30 wheel. It does not change the width of the rim. We found no rule specific to spacers in Great Britain; a tyre fouling part of the vehicle is a major MOT defect.
Where do I find a wheel's offset?
It is usually marked on the wheel, next to the wheel size: 8.5J x 20 ET35 is an 8.5-inch-wide, 20-inch wheel with a 35 mm offset.
Does the calculator tell me which offset to choose?
No. It compares two fitments that you enter. The right offset depends on the brake callipers, the wheel arches and the suspension of the car: at Onyx, the design office calculates it and submits it to the customer on the measurement sheet, before machining.
Further reading
- Tyre size calculator
- Wheel offset (ET) explained (page in French)
- Reading wheel dimensions (page in French)
Sources
- UN Regulation No. 142, installation of tyres, paragraphs 2.4 and 5.2.1.3, EUR-Lex.
- UN Regulation No. 124, replacement wheels, paragraph 2.6, EUR-Lex.
- Michelin, technical bulletin on measuring offset.
- DVSA, MOT inspection manual for cars, section 5: axles, wheels, tyres and suspension, 5.2.2 and 5.2.3, GOV.UK.
- Financial Ombudsman Service, car modifications and insurance.
- UN Regulation No. 30, passenger car tyres, paragraphs 2.20, 6.1.1.1 and 6.1.5, EUR-Lex.
Pages consulted on 10 October 2026. Rules change: your national authority has the final say.
Calculations and text: the Onyx Forged design office. Updated on .
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