Zerdly

Wheel Offset & Backspacing Calculator

Convert between wheel offset and backspacing and check fitment clearance for a new wheel setup, free in your browser.

100% FreeRuns in your browserIntermediate
How to use this calculator

Inputs

Inner clearance gain, outer poke and track change between your current and a new wheel, measured from the hub face.

Current wheel
in
mm
New wheel
in
mm
mm

Blank means no spacer.

Overall width
mm

25.4 mm (1 in) is an APPROXIMATION. Enter measured overall widths below to replace it.

More options
Measured widths
mm
mm
Tyres (approximate)
mm
mm

Assumes each tyre is centred on its rim; real sidewall shape differs.

Measured clearances (optional)
mm

Smallest gap from the inner tyre/rim edge to the nearest suspension part, measured on the car.

mm

Gap from the outer tyre/rim edge to the guard lip (at the point you measured).

Compare candidates

Enter the values and press Calculate.

Or load an example to see a worked result.

Your wheels on the hub face
Schematic
Your wheels on the hub faceSchematic current and new wheel sections on a fixed hub face.hub face (fixed)← vehicle (inner)kerb (outer) →currentnewinner gainouter poke
Calculate to draw both wheels to scale from the hub face.

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 the method works

Inner clearance and outer poke from the hub face
Illustrative method
Inner clearance and outer poke from the hub faceCross-section of a current 8 in ET 0 wheel and a new 9 in ET −12 wheel, both aligned on the fixed hub face, with the inner-edge and outer-edge movements dimensioned.hub face (fixed)← vehicle (inner)kerb (outer) →currentnewinner gain −0.7 mmouter poke +24.7 mm
Measure both wheels from the fixed hub face. With a 1 in lip allowance, the 8 in ET 0 wheel reaches 114.3 mm inboard and the 9 in ET −12 wheel 115.0 mm, so inner clearance gain = 114.3 − 115.0 = −0.7 mm (0.7 mm LESS room inside); the outer edge moves out by 139.0 − 114.3 = +24.7 mm. Positive inner gain means more room.

Fixed example values for illustration — not your result.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the OEM and new wheel (bead-seat) widths in inches and offsets in mm. Add a spacer on the new wheel if you're running one.
  2. Keep the 25.4 mm lip allowance as an approximation, or enter each wheel's measured overall width.
  3. Press Calculate and read the inner clearance gain (+ = more room), outer-poke change, track change and backspacing, with both wheels drawn on the fixed hub face. The current wheel can also be entered by its measured backspacing.
  4. Under More options add tyre section widths (an approximation, tyre centred on the rim), your measured current inner and guard clearances to see projected clearances, and more candidates as label, width, ET[, spacer][, tyre] lines — the fitment worksheet downloads as CSV.

How it works

Overall width W = bead width + lip allowance (or your measured width). From the hub face, the inner extent (backspacing) is W ÷ 2 + offset and the outer extent is W ÷ 2 − offset; a spacer subtracts its thickness from the offset.

Inner clearance change = old inner extent − new inner extent, so a positive number means more room on the suspension side. Outer poke change = new outer − old outer, so positive means further out.

Track width for the axle changes by 2 × (old offset − new offset), which is not the same as rim poke when the widths differ.

Worked example

8″ ET0 to 9″ ET−12. Inner extent 114.3 → 115.0 mm, so 0.7 mm LESS inner clearance, and 24.7 mm more poke. The same 8″ wheel on a 10 mm spacer gains 10 mm inside, pokes 10 mm more, and widens the track by 20 mm.

Common mistakes

  • Reading backspacing growth as clearance gain — more backspacing means the wheel reaches further in.
  • Confusing wheel width with overall width.
  • Ignoring tyre width — a wider tyre adds its own overhang beyond the rim (enter both tyre widths for an approximate tyre-edge change).
  • Treating a projected clearance as a fitment approval — steering lock, suspension travel and brake callipers are not modelled.

Frequently asked questions

How do I convert offset to backspacing?

Backspacing = (overall width ÷ 2) + offset, with overall width ≈ wheel width + 1 in unless measured. 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, so the whole wheel moves out: more poke and MORE 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.

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Cite this tool

Zerdly. (2026). Wheel Offset & Backspacing Calculator. Retrieved from https://www.zerdly.com/tools/wheel-offset-backspacing-calculator/

Tip: Enter any known values to calculate the remaining results.

Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.