Offset to backspacing chart
Backspacing is not a property of an offset. It is a property of an offset and a width, which is why a single
column of numbers headed “offset = backspacing” is always wrong. This chart is the whole surface: pick your
nominal wheel width down the left, your offset across the top, and read the backspacing in inches. Every cell is
(width + 1) ÷ 2 + offset ÷ 25.4 and nothing else.
| Width | -24 | -18 | -12 | -6 | 0 | +6 | +12 | +18 | +24 | +30 | +35 | +40 | +45 | +50 | +55 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 6.0″ | 2.56 | 2.79 | 3.03 | 3.26 | 3.50 | 3.74 | 3.97 | 4.21 | 4.44 | 4.68 | 4.88 | 5.07 | 5.27 | 5.47 | 5.67 |
| 6.5″ | 2.81 | 3.04 | 3.28 | 3.51 | 3.75 | 3.99 | 4.22 | 4.46 | 4.69 | 4.93 | 5.13 | 5.32 | 5.52 | 5.72 | 5.92 |
| 7.0″ | 3.06 | 3.29 | 3.53 | 3.76 | 4.00 | 4.24 | 4.47 | 4.71 | 4.94 | 5.18 | 5.38 | 5.57 | 5.77 | 5.97 | 6.17 |
| 7.5″ | 3.31 | 3.54 | 3.78 | 4.01 | 4.25 | 4.49 | 4.72 | 4.96 | 5.19 | 5.43 | 5.63 | 5.82 | 6.02 | 6.22 | 6.42 |
| 8.0″ | 3.56 | 3.79 | 4.03 | 4.26 | 4.50 | 4.74 | 4.97 | 5.21 | 5.44 | 5.68 | 5.88 | 6.07 | 6.27 | 6.47 | 6.67 |
| 8.5″ | 3.81 | 4.04 | 4.28 | 4.51 | 4.75 | 4.99 | 5.22 | 5.46 | 5.69 | 5.93 | 6.13 | 6.32 | 6.52 | 6.72 | 6.92 |
| 9.0″ | 4.06 | 4.29 | 4.53 | 4.76 | 5.00 | 5.24 | 5.47 | 5.71 | 5.94 | 6.18 | 6.38 | 6.57 | 6.77 | 6.97 | 7.17 |
| 9.5″ | 4.31 | 4.54 | 4.78 | 5.01 | 5.25 | 5.49 | 5.72 | 5.96 | 6.19 | 6.43 | 6.63 | 6.82 | 7.02 | 7.22 | 7.42 |
| 10.0″ | 4.56 | 4.79 | 5.03 | 5.26 | 5.50 | 5.74 | 5.97 | 6.21 | 6.44 | 6.68 | 6.88 | 7.07 | 7.27 | 7.47 | 7.67 |
| 10.5″ | 4.81 | 5.04 | 5.28 | 5.51 | 5.75 | 5.99 | 6.22 | 6.46 | 6.69 | 6.93 | 7.13 | 7.32 | 7.52 | 7.72 | 7.92 |
| 11.0″ | 5.06 | 5.29 | 5.53 | 5.76 | 6.00 | 6.24 | 6.47 | 6.71 | 6.94 | 7.18 | 7.38 | 7.57 | 7.77 | 7.97 | 8.17 |
| 12.0″ | 5.56 | 5.79 | 6.03 | 6.26 | 6.50 | 6.74 | 6.97 | 7.21 | 7.44 | 7.68 | 7.88 | 8.07 | 8.27 | 8.47 | 8.67 |
How to read a row
Take the 8.5″ row. Its overall width — rim edge to rim edge, which is what backspacing is measured to — is 8.5 + 1 = 9.5″, so its centreline sits 4.75″ from either edge. At ET0 the hub mounting face is on that centreline and backspacing is exactly 4.75″. Every millimetre of positive offset moves the mounting face outboard, which puts more wheel behind it, so backspacing grows by that millimetre — 0.039″ per millimetre. Every millimetre of negative offset shrinks it by the same amount.
That is the whole reason a bare offset figure tells you nothing about clearance. In the chart above, a 7″ wheel at ET35 and a 10″ wheel at ET35 both read “+35” on the spoke, and their inboard edges sit 1.50″ apart. The wider wheel is the one that hits the strut. Reading the two directions separately is what the wheel offset calculator does with any pair of wheels you give it; this chart is the same arithmetic laid out so you can scan it without typing.
Going the other way: backspacing to offset
Used wheels and American-made wheels are often quoted in backspacing only. The inverse is
offset = (backspacing − (width + 1) ÷ 2) × 25.4. The table
below runs it for the backspacing figures wheels are usually advertised at.
| Width | 3.0″ | 3.5″ | 4.0″ | 4.5″ | 5.0″ | 5.5″ | 6.0″ | 6.5″ | 7.0″ | 7.5″ |
|---|---|---|---|---|---|---|---|---|---|---|
| 6.0″ | -13 | 0 | +13 | +25 | +38 | +51 | +64 | +76 | +89 | +102 |
| 6.5″ | -19 | -6 | +6 | +19 | +32 | +44 | +57 | +70 | +83 | +95 |
| 7.0″ | -25 | -13 | 0 | +13 | +25 | +38 | +51 | +64 | +76 | +89 |
| 7.5″ | -32 | -19 | -6 | +6 | +19 | +32 | +44 | +57 | +70 | +83 |
| 8.0″ | -38 | -25 | -13 | 0 | +13 | +25 | +38 | +51 | +64 | +76 |
| 8.5″ | -44 | -32 | -19 | -6 | +6 | +19 | +32 | +44 | +57 | +70 |
| 9.0″ | -51 | -38 | -25 | -13 | 0 | +13 | +25 | +38 | +51 | +64 |
| 9.5″ | -57 | -44 | -32 | -19 | -6 | +6 | +19 | +32 | +44 | +57 |
| 10.0″ | -64 | -51 | -38 | -25 | -13 | 0 | +13 | +25 | +38 | +51 |
| 10.5″ | -70 | -57 | -44 | -32 | -19 | -6 | +6 | +19 | +32 | +44 |
| 11.0″ | -76 | -64 | -51 | -38 | -25 | -13 | 0 | +13 | +25 | +38 |
| 12.0″ | -89 | -76 | -64 | -51 | -38 | -25 | -13 | 0 | +13 | +25 |
A negative figure here is a real, common offset, not an error: deep-dish and off-road wheels frequently run negative ET, which is how they sit proud of the fender. The sign convention is that positive offset puts the mounting face toward the street side of the centreline.
The one assumption in both tables
The flange allowance. A wheel's stamped width is measured bead seat to bead seat; backspacing is measured to the rim edge, and the two bead flanges stand outside the bead seats. The tables above add 1.0 inch total for the pair, which is the usual trade convention, and it is an assumption, not a measurement — the real figure varies with rim profile and is on the wheel manufacturer's drawing. It shifts every backspacing number in the first table by half the error: if your wheel's true flange allowance is 0.8″ rather than 1.0″, every backspacing cell is 0.10″ high. If that margin matters to you, change it in the calculator, which exposes the allowance as an editable field, or measure the wheel: lay a straight edge across the inboard rim edge and measure down to the mounting face.
What this chart cannot tell you
It has no vehicle data. There is no make, model or year anywhere on this site, so nothing here says which offset or backspacing your car came with, or which one will clear your strut, spring perch, control arm, inner liner or brake caliper. Those are measurements on your own car, taken at full suspension compression and full steering lock, and they differ side to side. It also says nothing about bolt pattern, centre bore or load rating: the bolt circle has to match exactly before any of this arithmetic matters, and the bolt pattern guide covers how to measure yours and why a 0.7 mm near-match is a mismatch.
For the reasoning behind each term — why the hub face is the origin, what offset predicts that backspacing does not, and what “poke” actually changes about the steering — read wheel offset vs backspacing. If the tires are changing at the same time, run them through the tire size calculator first: a taller tire and a lower offset together can close a gap that neither closes alone. Every formula on this page is restated with its derivation on the methodology page.