Wheel Offset Calculator

Wheel offset is the distance between the wheel's centerline and the hub mounting face, measured in millimeters and stamped on the wheel as ET (from the German Einpresstiefe, insertion depth). It decides where the wheel sits in the arch: how far it reaches toward the suspension and how far it reaches toward the fender.

Positive offset (ET+) puts the mounting face toward the outside of the wheel, pulling the wheel into the arch. Almost every modern front-wheel drive and all-wheel drive car runs ET30 to ET50. Zero offset puts the face on the centerline. Negative offset (ET-) puts the face toward the inside, pushing the wheel out. That is the deep-dish truck look, and it is common on lifted trucks with ET-12 to ET-44 wheels.

Backspacing is the distance from the mounting face to the inner lip of the wheel, usually quoted in inches by truck and off-road brands. It describes the same position as offset, from a different reference point. The conversion is:

backspacing (in) = (width + 1) / 2 + offset (mm) / 25.4

The extra inch is the two wheel flanges: an 8.5-inch wheel is about 9.5 inches from lip to lip. Frontspacing is the remainder, from the mounting face to the outer lip. Use the converter under the calculator to turn a backspacing number into ET.

The two numbers that matter are the inner lip change and the outer lip change. A positive outer number means the wheel face moves toward the fender, which is more poke. A positive inner number means the back of the wheel moves toward the strut, spring and control arms, which is where most wide wheels rub.

Going 1 inch wider at the same offset adds 12.7 mm on each side. Dropping the offset by 10 mm moves the whole wheel 10 mm out: 10 mm more poke and 10 mm more room at the strut. That is why a wider wheel usually comes with a higher offset, so the extra width goes outward without hitting the suspension. Track width across the axle grows by twice the offset drop.

Changing the tire size too? Check the new diameter and speedometer error with the tire size calculator.

With the car on level ground at normal ride height, measure the smallest gap between the inside of the tire or wheel and the strut, spring or control arm. Turn the steering to full lock on both sides for the front wheels, since the gap changes as the wheel turns.

For the fender side, hang a plumb line or hold a straight edge vertically from the fender lip and measure to the tire sidewall or wheel face. Enter both numbers in the calculator to see what is left with the new setup. Under 3 mm is too tight once the suspension compresses.