Wheels / Geometry
Wheel Offset Calculator: Clearance, Poke and Backspacing
Compare current and new wheel width and ET, or enter backspacing to calculate ET. See inner clearance and outer position changes with a wheel cross-section diagram.
Wheel Offset Calculator: Clearance, Poke and Backspacing: Calculates each wheel's inner and outer edge from its width, offset and an editable total lip allowance. It shows the change relative to a shared hub face, plus backspacing in millimetres and inches. It does not establish tyre or brake fitment. Runs 100% locally in your browser with zero server file uploads.
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Positive poke-in means less inner clearance. Positive poke-out means the wheel extends farther outward; negative values mean the reverse.
1: current wheel. 2: new wheel. Diagram shows wheel edges, not tyre sidewalls or brake clearance.
| Wheel | ET (mm) | Backspacing (mm) | Backspacing (in) |
|---|---|---|---|
| Current | 40.00 | 141.60 | 5.575 |
| New | 35.00 | 149.30 | 5.878 |
Backspacing (in) = (width + lip allowance) ÷ 2 + ET ÷ 25.4. ET (mm) = [backspacing − (width + lip allowance) ÷ 2] × 25.4.
The default 1-inch total lip allowance assumes 0.5 inch per flange; edit it to match measured overall width. This convention is approximate. Results do not confirm fitment, tyre clearance or brake clearance.
Geometry and units
Overall width W = (nominal width + total lip allowance) × 25.4 mm. Backspacing = W/2 + ET; outer reach = W/2 − ET. New minus current inner reach is poke-in; new minus current outer reach is poke-out. Inner clearance change is the negative of poke-in.
Rim dimensions and assumption
ISO 4000-2:2021 describes passenger-car rim designations, contours and dimensions. Official catalogue: https://www.iso.org/standard/78725.html . The calculator's one-inch flange allowance is an approximation, not an ISO requirement. Measure overall width for a more appropriate allowance.
How to use it
- Choose offset or backspacing as the known measurement.
- Enter current and new wheel widths and the matching measurements.
- Adjust the total flange allowance and read the changes and diagram.
Privacy & limitations
Wheel measurements stay in your browser.
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Frequently asked questions
What does positive poke-in mean?
The inner edge moves farther towards the suspension and inner clearance decreases. The clearance-change value is the negative of poke-in. Positive poke-out means the outer edge moves farther outward.
Why add one inch to wheel width?
Wheel width is measured between bead seats. The default assumes another half inch per flange, one inch overall. It is an editable convention, not an exact dimension for every rim.
Does the diagram prove that a wheel fits?
No. It compares wheel edges at a shared mounting face. Tyre bulge, suspension travel, brake shape, wheel load capacity and bodywork are not modelled.
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