Wheel Offset and Backspacing Calculator
Backspacing from wheel width and ET offset, and the change when swapping to a different offset.
Results
Backspacing
146.60
mm
Backspacing
5.772
in
Difference
10.0
mm
Difference
-10.0
mm
What this tool does
Offset (ET) is measured from the mounting face to the centreline of the rim, while backspacing is measured from the mounting face to the inner lip — two ways of saying the same thing that rarely appear on the same catalogue. Lowering the ET pushes the wheel outward by exactly that many millimetres, which is what eats clearance at the arch and changes the scrub radius.
Formula
backspacing = width ÷ 2 + ET
Variables
| Symbol | Meaning | Unit |
|---|---|---|
wd | Wheel width | in |
et | Offset (ET) | mm |
et2 | Offset (ET) | mm |
BS | Backspacing | mm |
BI | Backspacing | in |
OU | Difference | mm |
IN | Difference | mm |
Worked example
- Wheel width8 in
- Offset (ET)45 mm
- Offset (ET)35 mm
- Backspacing146.60 mm
- Backspacing5.772 in
- Difference10.0 mm
- Difference-10.0 mm
Limitations
- For work that must comply with a standard or be signed off, check the result against the applicable code and have it reviewed by a qualified engineer.
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.