Wheel Offset Calculator
Compare the fitment of two wheels or convert offset (ET) to backspacing and back. Enter rim width and offset for each wheel, plus optional tire section widths, to see clearance, poke and track width changes in mm.
Compare Wheel Fitment or Convert Offset to Backspacing
Frequently Asked Questions
How do I calculate backspacing from offset by hand?
Use the formula \( BS = \frac{W + 1}{2} + \frac{ET}{25.4} \), where \( W \) is rim width in inches, \( ET \) is offset in mm, and 1 in accounts for the two half-inch flanges. Example: an 8-inch wheel with ET35 gives \( \frac{9}{2} + \frac{35}{25.4} \approx 5.88 \) in of backspacing. To go the other way: \( ET = (BS - \frac{W+1}{2}) \times 25.4 \).
What does ET actually mean, and what do positive and negative values do?
ET (from German Einpresstiefe) is the distance in millimeters between the hub mounting face and the wheel centerline. Positive offset tucks the wheel inward toward the suspension; negative offset pushes it outward toward the fender; ET0 puts the mounting face exactly on the centerline. Most front-wheel-drive cars use ET35–ET50, while rear-wheel-drive and truck applications often accept lower or negative values.
Can I keep the same offset when I go to a wider wheel?
Not automatically. At the same ET, a wider wheel splits the extra width evenly: half goes toward the suspension and half pokes out of the fender. To keep the inner clearance unchanged, increase the offset by about half the width gain. To keep the same outer poke, decrease it by half. That is why entering both wheels in the compare tab shows separate inner and outer changes.
How much offset change is safe before I risk rubbing?
There is no universal limit — it depends on fender, strut, and spring clearance. As a rough rule, staying within about 5–10 mm of the stock offset on the same width wheel rarely causes issues, but wider wheels have far less margin. Always verify clearance with the steering at full lock and the suspension compressed, and remember that actual mounted tire widths differ from the sidewall marking.
Why does the same offset give different backspacing on different width wheels?
Backspacing depends on both offset and width, because offset is measured from the centerline, not the inner edge. Two wheels with ET35 sit 7 mm off-center by the same amount, but a 9-inch wheel has its inner edge about 12.7 mm further inboard than an 8-inch wheel. This is a common mistake when buying wheels based on backspacing alone Always pair the width with the offset or backspacing.
Is it better to shop by offset or by backspacing?
Offset is the standard in Europe and Asia and what most wheel markings show; backspacing is more common in the US, especially for trucks and off-road wheels. Front spacing controls fender poke, backspacing controls suspension clearance — you generally need both, which is why the converter shows all three values together. Whichever you use, always state the rim width with it, since neither number is meaningful alone.
How much does tire size change the fitment compared to the bare wheel?
A tire adds roughly its section width, split equally around the wheel centerline, so a 245 mm tire sticks out about 5 mm more than a 235 mm tire even on the identical wheel. Changing tire width by \( \Delta \) moves each sidewall about \( \Delta/2 \). Note that measured tire widths vary by brand and rim width, and can be inflated or squished versus the sidewall number, so treat tire results as estimates.