Wheel Offset & Backspacing Calculator
A free, browser-based calculator. Runs entirely in your browser — no sign up, nothing stored.
Wheels
| Width (in) | Offset (mm) | |
|---|---|---|
| OEM | ||
| New |
Width is the wheel (bead-seat) width; backspacing uses the overall lip-to-lip width (≈ width + 1 in). Negative offset and spacers push the wheel outward. Runs in your browser.
Result
The new wheel pokes out further by 24.7 mm, and the inner edge moves away from the suspension by 0.7 mm.
Estimates only — check before you fit
These results are calculated from tyre-size geometry and the values you enter. Real tyre dimensions vary by brand, model, load, pressure and wear. Always check the vehicle placard, tyre manufacturer data, local road rules, and guard / brake / suspension clearance, and get professional fitment advice before changing tyre or wheel sizes. Check guard, suspension, brake and wheel-bearing clearance and legal track-width limits with a professional fitter.
How to use this calculator
- Enter the OEM and new wheel widths (inches) and offsets (mm). Add a spacer on the new wheel if you're running one.
- Read the backspacing, front spacing, inner-clearance change and outer-poke change.
- Use the top-down diagram to see how the new wheel sits relative to the hub face.
How it works
Overall (lip-to-lip) width ≈ wheel width + 1 in; the centreline is half of that. Backspacing = centreline + offset (offset in inches), and front spacing = overall width − backspacing.
A spacer reduces the effective offset by its thickness, pushing the wheel outward. The inner-clearance change is the difference in backspacing; the outer-poke change is the difference in front spacing.
Worked example
8″ wheel, +0 mm offset. Overall width 9″, centreline 4.5″, so backspacing ≈ 4.5″. A −12 mm, 9″ wheel pokes out further and reduces inner clearance.
Common mistakes
- Confusing wheel width with overall width — backspacing uses the overall (≈ +1 in) width.
- Forgetting a spacer effectively lowers offset and increases poke.
- Ignoring tyre width — a wider tyre adds its own overhang beyond the rim.
Frequently asked questions
How do I convert offset to backspacing?
Backspacing = (overall width ÷ 2) + offset in inches, where overall width ≈ wheel width + 1 in. The tool does it for both wheels and compares them.
What does a negative offset do?
It moves the mounting face inboard of centre, so the wheel sits further out (more poke) and has less backspacing.
Do spacers change offset?
Effectively yes — a spacer reduces the effective offset by its thickness, increasing poke and reducing inner clearance.
Are these results safe to fit?
They're geometry estimates only. Real tyres vary by brand, load, pressure and wear. Always check the vehicle placard, manufacturer data, local road rules and guard/brake/suspension clearance, and get professional fitment advice before changing tyre or wheel sizes.
Related tools
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.