Loading Wheel Offset Calculator…
Bead seat to bead seat, the number before the J.
Stamped on the spokes or behind them. Negative offsets exist.
A spacer subtracts from the offset, one millimetre for one.
Outer +22.7 mm · Inner +2.7 mm
The outer edge sits further out, towards the arch; the inner edge sits further in, towards the suspension.
Nothing here says either edge clears anything. The outer figure is what the arch, the guard lip and the wheel arch liner have to accommodate; the inner figure is what the strut, the brake caliper, the spring perch and the steering components have to accommodate. Both are measured on the car, at full lock and full suspension compression, with the tyre you intend to fit — and what is permitted is set by the vehicle manufacturer’s specification. A millimetre figure from this page is the size of a change, never a verdict on a fit.
A rim’s stated width is measured bead seat to bead seat. The flanges stand outside that on both sides and the mounted tyre is wider again, bulging further still under load, so the true outermost point of the assembly is always further out than the number above. How much further depends on the rim profile and the tyre, neither of which can be read off a size, so no allowance for it is invented here.
Offset moves the wheel; width moves its edges around wherever the offset put it. That is why two changes made together can cancel at one edge and add at the other, and why the two numbers have to be read separately rather than averaged into a single verdict. Spacers, where they are legal at all, change the offset and nothing else: they do not change the width, the load path through the studs or what the hub was designed to carry, and fitting them is a job with torque figures and hub-centricity requirements attached.
Offset is stamped on almost every modern wheel as ET followed by a number, from the German Einpresstiefe, meaning insertion depth. It is the distance in millimetres from the wheel’s own centreline to the flat face that bolts against the hub. A positive figure means that mounting face sits outboard of the centreline, which tucks the wheel under the car; a negative figure means it sits inboard, which pushes the wheel out. Zero means the face and the centreline are the same plane.
The mounting face is the fixed point in all of this. It is clamped to the hub and cannot move, so every consequence of changing a wheel is really a statement about where the rim edges finish up relative to that unmoving face.
Measure outboard from the mounting face and the outer rim edge sits at half the width minus the offset, while the inner edge sits at half the width plus it. Change the width and the two halves move in opposite directions. Change the offset and both edges shift the same way together. Do both at once, which is what most wheel changes involve, and the edges move by entirely different amounts.
Take a 7.5 inch rim at ET45 replaced by an 8.5 inch rim at ET35. The first is 190.5 mm across, so its outer edge is 50.25 mm out from the face and its inner edge 140.25 mm in. The second is 215.9 mm across, putting its outer edge 72.95 mm out and its inner 142.95 mm in. The outer edge has moved 22.7 mm outward; the inner has moved only 2.7 mm inward. One number would have hidden that completely.
The outer figure is the one the arch, the guard lip and the inner liner have to accommodate, and it is the one that decides whether a wheel sits flush, tucked or proud. The inner figure is the one that has to clear the strut body, the brake caliper, the spring perch and the steering components, and it is the one that gets checked at full steering lock rather than standing still.
Track — the distance between the wheels across an axle — moves with offset alone, because track is measured centreline to centreline and width is added evenly either side of the centreline. Widening a rim without touching its offset therefore changes both edges and leaves the track exactly where it was, which is the single most commonly misunderstood part of the whole subject.
No figure on this page says anything clears anything. Clearance is a property of one specific vehicle at one specific ride height with one specific tyre fitted, and it is established by cycling the suspension through its travel with the steering at full lock, not by subtracting millimetres. What is permissible is set by the vehicle manufacturer’s specification, and in many places by the regulations the vehicle is registered under.
A rim’s stated width is measured bead seat to bead seat as well, so the flanges stand outside it on both sides and the mounted tyre is wider still, bulging further under load. The true outermost point of the assembly is always further out than the arithmetic above. How much further depends on the rim profile and the tyre, neither of which is derivable from a size, so no allowance for it is invented here.
Yes. Reducing the offset moves the mounting face inboard relative to the wheel, which for a fixed hub means the whole wheel moves outward by the difference. Going from ET45 to ET35 shifts everything ten millimetres out, and going from ET35 to ET45 pulls it ten back in.
Only at the outer edge, and only by half the extra width if the offset is unchanged. The other half of that width goes inboard towards the suspension. This is why a wider rim is often paired with a smaller offset figure: the pairing is what determines where the wheel actually sits.
Measure it. Lay a straightedge across the inboard flange and measure from the straightedge down to the mounting face — that is the back spacing. Then measure the overall width across both flanges, halve it, and subtract that from the back spacing. The remainder is the offset in whatever unit you measured in.
Geometrically, yes: a spacer moves the mounting face outboard by its own thickness, which is arithmetically identical to reducing the offset by the same amount. Mechanically they are not the same at all, since they change stud engagement and hub centring, and their legality varies by jurisdiction. Fitting them is a job with torque figures attached.