Offset Wheel Calculator
Wheel Offset and Backspacing Calculator works out Inner rim edge moves inward by and Outer rim edge moves outward by from Current wheel width, Current offset (ET) and New wheel width. The default figures on this page give inner rim edge moves inward by of 12.70 mm; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
5 steps
- Width term = (8 - 7) x 25.4 / 2 = 12.7 mm added to each rim edge.
- Offset change = 40 - 40 = 0 mm; a positive change pulls the whole wheel inboard.
- Inner edge change = 12.7 + 0 = 12.7 mm (positive = toward the suspension).
- Outer edge change = 12.7 - 0 = 12.7 mm (positive = further out past the fender).
- Backspacing goes from 7/2 + 1.575 + 0.5 = 5.575 in to 6.075 in.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the Wheel Offset and Backspacing Calculator calculates
Compares two wheel widths and offsets to show how far the inner and outer rim edges move, converts between offset (ET) and backspacing, and reports the change in track width.
How to use this Wheel Offset and Backspacing Calculator
Enter Current wheel width, Current offset (ET), New wheel width and New offset (ET). Inner rim edge moves inward by updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Inner rim edge moves inward by, Outer rim edge moves outward by and Inner edge.
The method follows NIST Guide to the SI, Appendix B.9 conversion factors, GTSparkplugs, wheel backspace and offset conversion and AmericanTrucks, wheel offset and backspacing reference.
One inch wider at the same offset pushes both edges out by 12.7 mm example
With Current wheel width: 7, Current offset (ET): 40, New wheel width: 8, New offset (ET): 40, the calculator returns Inner rim edge moves inward by: 12.7, Outer rim edge moves outward by: 12.7, Offset change: 0. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Inner rim edge moves inward by, Outer rim edge moves outward by and Inner edge. The inputs you can vary are Current wheel width, Current offset (ET), New wheel width, New offset (ET), Measured inner clearance now and Measured outer clearance now.
Understanding Inner rim edge moves inward by, Outer rim edge moves outward by and Inner edge
This is a geometric estimate of rim-edge movement only. It does not certify tire, brake, suspension, fender, hub or bolt-pattern clearance, or load rating. Check physical clearance at full lock and full suspension travel before fitting wheels.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How do I know what offset I need for my rims?
Compares two wheel widths and offsets to show how far the inner and outer rim edges move, converts between offset (ET) and backspacing, and reports the change in track width. Open “How this is worked out” beside the result to see each step for your own figures.
How much offset is 2 inches?
On the figures this page opens with — current wheel width 7 in, current offset (ET) 40 mm, new wheel width 8 in and new offset (ET) 40 mm — the inner rim edge moves inward by is 12.7 mm. No single number fits everyone: change any figure above and the answer updates as you type, with each step of the arithmetic beside it.
What does +35 offset mean on rims?
Compares two wheel widths and offsets to show how far the inner and outer rim edges move, converts between offset (ET) and backspacing, and reports the change in track width. On this page it is computed from Current wheel width, Current offset (ET) and New wheel width and shown as Inner rim edge moves inward by, Outer rim edge moves outward by and Inner edge.
How many inches is a negative 44 offset?
It needs Current wheel width, Current offset (ET), New wheel width, New offset (ET), Measured inner clearance now and Measured outer clearance now. Nothing else is required, and the figures stay in your browser.
How do I calculate the offset of a wheel?
The result gives Inner rim edge moves inward by, Outer rim edge moves outward by, Inner edge, Outer edge, Offset change and Width change, each labelled with its unit so the figure can be read on its own.
What information do I need for the Wheel Offset and Backspacing Calculator?
You need Current wheel width, Current offset (ET), New wheel width, New offset (ET), Measured inner clearance now and Measured outer clearance now. The page opens with default figures that give inner rim edge moves inward by of 12.70 mm, so you can see the scale before entering your own.