GarageCalc How offset and width work together
Two things decide where your wheel sits: width and offset. Width grows the wheel symmetrically: half an inch wider means about 6.4 mm more wheel on each side. Offset then shifts the whole wheel: every millimeter of reduced offset moves everything 1 mm outward, toward the fender.
- Outer edge movement = (new width - old width) / 2 - (new offset - old offset)
- Inner edge movement = (new width - old width) / 2 + (new offset - old offset)
- Backspacing = (width + 1 in) / 2 + offset
The backspacing formula uses the industry convention of nominal width plus one inch, which accounts for the wheel flanges. That is why measured backspacing is always a bit more than half the nominal width.
Reading the results
Outer edge positive: the wheel pokes further out. Check fender clearance, especially with suspension compressed and at full steering lock. Inner edge positive: the wheel sits closer to the strut, control arm or brake components. Most cars have limited inner clearance, so more than 5 to 10 mm of inner movement deserves a physical measurement before you buy.
Changing tires at the same time? Verify the new overall diameter with the tire size comparison calculator, and make sure the wheel even bolts on with the bolt pattern calculator.
Frequently asked questions
What is wheel offset (ET)?
Offset is the distance in millimeters between the wheel mounting face (the part that bolts to the hub) and the true centerline of the wheel. Positive offset means the mounting face sits toward the street side of centerline, tucking the wheel inward. Negative offset pushes the wheel outward. It is often stamped on the wheel as ET, for example ET35.
What is the difference between offset and backspacing?
Both describe the same wheel position in different ways. Offset measures from the wheel centerline; backspacing measures from the inner wheel lip to the mounting face. This calculator shows both, so you can use whichever number your wheel or fender specialist expects.
How much offset change is safe?
As a rule of thumb, staying within about 5 mm of the factory offset is nearly always safe. Changes of 10 to 15 mm are common with aftermarket wheels but should be checked against fender and suspension clearance, especially with wider wheels. Large changes also affect scrub radius and steering feel, and can add load to wheel bearings.
Will lower offset make my wheels stick out?
Yes. Lowering the offset moves the whole wheel outward: every 1 mm less offset is 1 mm more poke. Increasing the width moves both edges outward and inward at the same time: half the added width goes to each side. This calculator combines both effects and shows exactly how far each edge moves.