Free Wheel Offset Calculator
More positive offset tucks the wheel in. More width with the same offset pokes both ways.
- Offset (ET) is measured from the wheel centreline to the mounting pad, in millimetres. Positive = the pad sits toward the street.
- Backspacing is measured from the inner lip to the mounting pad, in inches. They start from different points.
- Advertised width is bead seat to bead seat. The total metal is about one inch wider, because of the two flanges.
- That is why the conversion must add that inch — otherwise backspacing comes out half an inch short.
What it does
Compares old vs new advertised width and ET offset and reports how far the inner and outer lips move.
How to use it
Enter current width and offset, then the new width and offset. Width is the stamped size (8, 8.5, 9…), offset is ET in millimeters.
How to read the result
Positive outer movement is poke toward the fender. Positive inner movement is toward the control arm and strut. About 0.25" is where shops start to worry. These are estimates — fender roll, lift and tire bulge are not modeled.
What this does not calculate
Does not know your fender, lift, brake caliper or hub bore. A number in the green band can still rub on a lowered car.
Not a guarantee the wheel will clear. Confirm hub bore, bolt pattern (Vehicle Fit) and brakes separately.
Questions
- Which way is positive offset?
- The mounting pad is toward the street side of the wheel centerline. Most modern cars use positive ET so the wheel sits inboard.
- Does tire width add more poke?
- Yes. This calculator is the wheel only. A 10.5" tire on a 9" wheel sticks out past the lip.
Related tools
- Offset to Backspacing ConverterOffset is from the centerline, in mm. Backspacing is from the back lip, in inches. They need the rim width to talk to each other.
- Wheel Spacer CalculatorA spacer pushes the wheel out. On slip-on hardware it also eats stud thread. Measure; do not guess.
- Rim Width Calculator by Tire SizeMeasuring width is where the tire is spec’d. Acceptable is the full range. Optimal is near measuring.
- Free Bolt Pattern Converter — 5x114.35x114.3 mm is 5x4.5". 5x112 is not. Close is not a fit.
- Vehicle FitLook up bolt pattern, offset, center bore and factory wheel specs by year, make and model.
Estimate — not a fitment approval.
