Two wheels of the same diameter and width can sit almost an inch apart on the same hub, and the number that decides it is stamped on the back of the wheel in a form most people never learn to read. This wheel offset calculator converts between the two ways that number is expressed — metric offset and imperial backspacing — and, more usefully, tells you exactly how much a proposed wheel moves inboard toward the suspension and outboard toward the arch compared with what is on the car now.
Arb Digital built it because the two conventions coexist and are constantly mixed up. European and Japanese wheels are specified as an ET number in millimetres from the centreline; American aftermarket wheels are specified as backspacing in inches from the mounting face. Converting between them is straightforward arithmetic once you know which flange allowance the supplier is using, and it goes wrong constantly because that detail is rarely stated.
What This Calculator Does
It takes the width and offset of the wheel currently fitted and of the wheel you are considering, and returns the backspacing of each, the movement at the inner edge, the movement at the outer edge, and the change in track per side. Those four numbers are the whole fitment question in geometric terms: inner movement is your suspension and strut clearance, outer movement is your arch and fender clearance, and track change is what happens to the vehicle’s stance and to its steering geometry.
It also handles spacers, which are arithmetically identical to reducing offset by their thickness, and will convert a backspacing figure to an ET value if that is the form your supplier quoted. This is a different question from decoding a tyre size, which the tire size calculator does, and from working out what a diameter change does to your indicated speed, which belongs to the speedometer error calculator. This page is about where the wheel sits across the axle, not around it.
How to Use It
- Read the current wheel’s markings from the back of the spokes or the inner barrel. You are looking for something like 8J x 18 ET35.
- Confirm which flange convention your supplier uses before entering a backspacing figure. One inch total is the most common, half an inch is used too, and mixing them puts every answer half an inch out.
- Enter the proposed wheel’s width and offset from the specification, not from a forum post about a similar car.
- Read the inner and outer movement separately. They are different numbers whenever the widths differ, and it is entirely possible for a wheel to gain clearance at one edge while losing it at the other.
- Then measure the car. Take the wheel off, measure to the strut and to the arch lip, turn the steering lock to lock, and compress the suspension. The arithmetic tells you what to look for; the car tells you whether it is there.
The Conversion Formula
Offset, described in the standard account of wheel sizing, is the distance from the hub mounting surface to the wheel’s true centreline, in millimetres. Backspacing is the distance from the same mounting surface to the inner lip of the rim, in inches. They measure from the same face to two different places, which is why converting between them needs the rim width.
Let W be the stated wheel width in inches, F the flange allowance and ET the offset in millimetres. The overall rim width is W + F, the centreline sits at half of that from the inner lip, and the mounting face sits ET millimetres outboard of the centreline. So:
Backspacing = (W + F) ÷ 2 + ET ÷ 25.4 and ET = (Backspacing − (W + F) ÷ 2) × 25.4
Frontspacing, the distance from the mounting face to the outer lip, is the remainder: (W + F) − Backspacing. Inner-edge movement between two wheels is the difference in their backspacing, outer-edge movement is the difference in their frontspacing, and the change in track per side is simply the difference in offset, because the centreline moves by exactly that much.
Worked example on the defaults, which you can check by hand. Going from 8 inches wide at ET35 to 9 inches wide at ET15 with a one-inch flange allowance: the old overall width is 9.0 inches, so its backspacing is 4.5 + 35/25.4 = 4.5 + 1.378 = 5.878 inches and its frontspacing is 9.0 − 5.878 = 3.122 inches. The new overall width is 10.0 inches, so its backspacing is 5.0 + 15/25.4 = 5.0 + 0.591 = 5.591 inches and its frontspacing is 10.0 − 5.591 = 4.409 inches. The inner edge therefore moves 0.287 inches away from the suspension, the outer edge moves 1.287 inches toward the arch, and the centreline moves outboard by 35 − 15 = 20 mm per side, widening the track by 40 mm in total. Note that the two edge movements average to the 20 mm centreline shift, which is a useful check that the arithmetic is right.
What Offset Changes Besides Clearance
Moving a wheel outboard is not a cosmetic change to the car, and this is the part that gets least attention. Offset changes the scrub radius, which is the distance in front view between the steering axis and the centre of the tyre’s contact patch. The standard account of scrub radius is explicit that it changes whenever offset changes, that pushing the wheels outward makes it more positive, and that steering effort rises with it. Manufacturers choose a scrub radius deliberately, and a negative one on many modern cars is there to keep the vehicle stable if one front brake circuit fails.
The second consequence is bearing load. The wheel bearing was designed for a load path in which the tyre’s contact patch sits close to the bearing’s centre. Moving the wheel outboard extends that lever arm, so the same cornering force and the same kerb strike apply a larger bending moment to the bearing and to the hub. That does not fail immediately; it shows up as accelerated wear, and a bearing that would have run for well over a hundred thousand miles starts making noise far sooner. Spacers do exactly the same thing, with the added consideration that they change how much thread the studs have to work with.
Third is the tyre and rim pairing. A wheel width has a range of tyre section widths it is designed to carry, and mounting a tyre outside that range changes the sidewall profile and the shape of the contact patch. The standards bodies that define these fitments, such as the European tyre and rim organisation ETRTO, publish the design tables that manufacturers work to, and stepping outside them is not something a fitment calculator can bless.
Where the Clearance Actually Runs Out
Static measurement with the car parked is the least demanding case, and passing it means very little. The three situations that bite are full steering lock, full suspension compression, and both at once.
At full lock the front tyre swings toward the inner arch liner, the sway bar end link and sometimes the chassis rail. On a wide wheel with reduced backspacing the inner edge may clear the strut comfortably and still catch the liner when the wheel is turned. At full compression the tyre rises into the arch and moves through the camber curve, so a wheel that clears the arch lip by half an inch on the drive may not clear it over a speed bump with four people aboard. Suspension geometry also moves the wheel laterally as it travels, which the static arithmetic on this page does not model at all.
Then there are the things that have nothing to do with offset and stop the fitment anyway: the bolt pattern, the hub centre bore, the seat type of the bolts or nuts, brake caliper clearance behind the spokes, and the load rating of the wheel itself relative to the vehicle. This calculator sees none of them. It also cannot see a rubbing tyre, and rubbing is not a cosmetic problem — a tyre worn through by contact with a liner or an arch lip is a tyre that can fail.
Arb Digital builds free tools that show the formula, name the convention and stop where the model stops. Browse the library, or tell us what your audience keeps getting wrong.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Mixing flange conventions. Converting an ET figure with a one-inch allowance and comparing it against a backspacing quoted with a half-inch allowance puts you half an inch out, which is more than the clearance you were checking.
- Comparing offsets between wheels of different widths. ET35 on an 8-inch rim and ET35 on a 9-inch rim do not sit in the same place. The centreline is in the same place; the edges are not.
- Checking clearance only with the car parked and the wheels straight. Full lock and full compression are where fitments fail, and they are free to check.
- Treating a spacer as a neutral part. A spacer moves the wheel outboard exactly as a lower offset does, with the same effect on scrub radius and bearing load, and it changes stud engagement as well.
- Assuming a forum fitment transfers. Suspension revisions, brake options, tyre choice and even production year change the available clearance on nominally identical cars.
Related Free Tools From Arb Digital
Decode a tyre size code into diameter and circumference with the tire size calculator, and see what a rolling-radius change does to your indicated speed with the speedometer error calculator. Torque figures for wheel fasteners belong in the torque calculator, and if you are working out what a loaded vehicle can legally carry, the GVWR payload calculator handles ratings rather than geometry. For angles quoted in unfamiliar units use the angle converter, and for the millimetre-to-inch arithmetic on its own, the length converter. Everything else is in the free online tools hub.
Frequently Asked Questions
Add the flange allowance to the stated wheel width, halve it, and add the offset converted from millimetres to inches. With a one-inch allowance, an 8-inch wheel at ET35 gives 4.5 + 35 divided by 25.4, which is 5.88 inches of backspacing. Reverse the steps to go from backspacing to an ET figure.
It moves the whole wheel outboard by the difference, so the track widens by that amount per side. The outer edge moves toward the arch, the inner edge gains clearance from the suspension, and the scrub radius becomes more positive, which increases steering effort and changes how the car behaves under braking.
Because offset is measured to the centreline, and a wider wheel has its edges further from that centreline. Two wheels at the same ET share a centreline position but the wider one extends further inboard and further outboard, by half the width difference in each direction.
Geometrically yes: a 20 mm spacer moves the wheel outboard exactly as fitting a wheel 20 mm lower in offset would, with the same effect on track, scrub radius and bearing load. It differs in that it also changes how much stud thread is engaged, which is a separate matter to check.
It can. Moving the wheel outboard lengthens the lever arm between the contact patch and the wheel bearing, so cornering loads and kerb strikes apply a larger bending moment and bearings wear faster. Insufficient clearance causes rubbing, and a tyre worn through by contact with an arch or a liner is a safety problem rather than a cosmetic one.
No. It tells you how far the wheel moves relative to the one already fitted. Whether that clearance exists depends on your suspension, brakes, arch shape, tyre choice, ride height and steering lock, and it is confirmed by measuring the vehicle and test-fitting, not by arithmetic.
It is the distance in front view between the steering axis and the centre of the tyre’s contact patch. Manufacturers choose it deliberately; many modern cars use a negative value because it helps the car stay straight if one brake circuit fails. Changing offset changes it, and larger values increase steering effort.
The bolt pattern, the hub centre bore, the seat type of the bolts or nuts, brake caliper clearance behind the spokes and the wheel’s own load rating relative to the vehicle. All of them are independent of offset and any one of them can rule out a wheel that the geometry says would clear.
This page performs a geometric conversion only. It is not a fitment approval and no output from it states that any wheel, tyre, spacer or combination is suitable for a vehicle. Offset and backspacing changes alter scrub radius, steering effort and the load path through the wheel bearing, and the wrong choice causes rubbing and accelerated bearing wear. Confirm fitment on the vehicle itself, at full steering lock and full suspension compression, and have wheel and suspension work checked by a qualified technician.
