Offset to Backspacing Converter
Offset is from the centerline, in mm. Backspacing is from the back lip, in inches. They need the rim width to talk to each other.
- 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
Converts both ways using advertised rim width plus the 1" overall-width allowance for the two flanges (~0.5" each).
How to use it
Enter width and either ET or backspacing. Leave the other blank. The unused field is calculated.
How to read the result
If a calculator disagrees with us by about half an inch, it skipped the flanges. Backspacing is measured to the metal back edge, not the bead seat. We match Curva Concepts, GetYourWheels and PowerHouse Wheels on this.
What this does not calculate
The +1" rule is the industry convention, not a measurement of your particular wheel. Forged wheels with unusual lips can differ by a few millimeters. When it matters, measure.
Does not tell you if that backspacing clears the strut. Use the offset calculator with old vs new for that estimate.
Questions
- Why the extra inch?
- Stamped width is bead to bead. Overall width is about 1" more because of the flanges. Backspacing is measured from the flange, so the centerline sits at (width+1)/2.
- Can I measure backspacing at home?
- Yes: lay the wheel face down, rest a straightedge across the inboard flange, and measure to the pad. That number should match this converter.
Related tools
- Free Wheel Offset CalculatorMore positive offset tucks the wheel in. More width with the same offset pokes both ways.
- 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.
