Wheel offset calculator
Enter the rim width in inches and the offset (ET) in millimetres for your current wheel and the new one. The calculator shows the backspacing of each and how far the inner and outer edges of the wheel move. It helps you judge clearance to the suspension and the arch before buying wheels.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Enter your current rim width in inches and its offset (ET) in millimetres.
- Enter the same figures for the new wheel.
- Read how far the outer and inner edges move, and the backspacing of each wheel.
How it works
Measured from the hub mounting surface:
- Inner distance (backspacing) = width ÷ 2 + ET.
- Outer distance = width ÷ 2 − ET.
- The change is the new distance minus the current one. One inch is 25.4 mm.
Worked example
An 8 in wheel with ET35 has inner 101.6 + 35 = 136.6 mm and outer 66.6 mm. A 9 in wheel with ET25 has inner 114.3 + 25 = 139.3 mm and outer 89.3 mm. The outer edge moves out 22.7 mm and the inner edge moves 2.7 mm closer to the suspension.
Tips
- Lower ET pushes the wheel out. Each 10 mm is about 0.4 in.
- Check the brake caliper and strut clearance when the inner edge moves toward the suspension.
Frequently asked questions
What is wheel offset (ET)?
Offset is the distance in millimetres from the wheel's centreline to the surface that bolts to the hub. Positive offset moves the wheel inward, toward the car. Negative offset pushes it outward.
What is backspacing?
Backspacing is the distance from the hub mounting surface to the inner lip of the rim. It equals half the rim width plus the offset. This tool shows it in inches, as it is usually quoted.
How do I read the results?
A positive outer change means the outer edge of the wheel sticks out farther, toward the arch. A positive inner change means the inner edge comes closer to the suspension. Negative numbers mean more room.
Does this include the tyre?
No. It uses the rim edges only. The tyre bulges beyond the rim, so leave some extra clearance, and check against the wheel arch and the fender lip.