Wheel Offset Calculator
Compare two wheels and see exactly where the edges move: poke at the fender, clearance at the strut, and backspacing - drawn to scale from the hub mounting face.
Enter Two Wheels
Wheel Comparison Results
| Measurement | Current | New | Difference |
|---|
Offset, backspacing, and the missing inch
Offset (the ET number stamped on the wheel) is the distance from the hub mounting face to the wheel's centerline, in millimetres - positive means the face sits outboard of center, pulling the wheel in toward the car. Backspacing is the distance from the mounting face to the inner flange edge, in inches, and it is the number American spec sheets quote. Converting between them trips people up because of a missing inch: a wheel's stated width is measured between the tire bead seats, but the overall width including both flanges runs about one inch wider. So backspacing = (stated width + 1) / 2 + offset in inches. Most of the confusion between the two systems is that inch.
What moves where
Lowering the offset or adding a spacer moves the whole wheel outboard - more poke at the fender, more room at the strut. A wider wheel at the same offset grows both directions equally: half the extra width goes out, half goes in. The two numbers this calculator reports are exactly those edge movements. The outer edge governs fender clearance and stance; the inner edge governs strut, spring, and control-arm clearance. The centerline movement matters too: it widens or narrows the track, and it changes the scrub radius - the lever arm between the steering axis and the contact patch - which is why a big offset change alters steering feel and wheel-bearing load, not just looks. A spacer is just an offset reduction you bolt on afterward: a 5 mm spacer turns an ET+45 wheel into an effective ET+40. Anything beyond a few millimetres of spacer needs correspondingly longer bolts or studs and should be hub-centric - check thread engagement before driving on them.
Why there is no will-it-fit verdict
Whether a wheel fits is a property of the car, not the wheel: fender lip shape, camber, how much the suspension compresses, and the tire's section width all decide it, and none of them are in this calculator's inputs. Any tool that stamps "fits" on a wheel from width and offset alone is guessing. What the numbers here can do is tell you exactly how much closer to trouble each edge moves, so you can measure the gap you actually have. The tire side of the same question - how the tire's diameter and width change - is the tire size calculator's job, and a diameter change there also moves your gearing, which the gear ratio calculator can quantify.