Wheel Offset Calculator
Compare theoretical wheel positions from width and offset; confirm real vehicle clearances before fitting.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Compare theoretical wheel positions from nominal rim width and offset. Results do not confirm fitment: tire dimensions, rim-lip measurement, brakes, suspension and body clearance must be checked on the vehicle.
Worked example
calculated with the default values below
Wheel width 8 in · Offset 35 mm · New wheel width 9 in · New wheel offset 35 mm Current backspacing (in) 5.38 in · New backspacing (in) 5.88 in · Inner edge position change (theoretical) 12.7 mm · Outer position change 12.7 mm
General background references
Disclaimer
These calculators give estimates for general information only. Real fuel consumption, performance and range vary with driving style, load, terrain, temperature and vehicle condition. Do not rely on these results for towing limits, payload or braking decisions — use the rating plate of your vehicle.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
b = w ÷ 2 × 25.4 + o; Δin = b₂ − b; Δout = (w₂ − w) × 25.4 − Δin
- w
- Current rim width (in)
- o
- Current offset (mm)
- w₂
- New rim width (in)
- b
- Backspacing (mm)
- Δ_in
- Inner clearance change (mm)
- Δ_out
- Outer poke change (mm)
Width is nominal rim width in inches; offset is in mm. Backspacing output is inches: width ÷ 2 + offset ÷ 25.4. Inner position change = new backspacing − current (mm); outer position change = width increase in mm − inner change. Theoretical geometry only; verify actual fitment.
Frequently asked questions
Why can a one-inch wider wheel poke out only a few millimetres?
The model compares nominal wheel geometry: a 1-inch width increase adds 25.4 mm, while a 10 mm offset increase moves the inner edge inward by 10 mm, leaving a 2.7 mm outer-position change. This does not confirm that the wheel fits; check the actual tire, wheel lip, brake, suspension and body clearances on the vehicle.