Wheel offset vs backspacing: the difference and a conversion chart

Offset is measured from the wheel's centerline in millimeters, backspacing from the inner lip in inches. How to convert one into the other, a chart for common widths, and what a lower offset or a spacer actually moves.

Sep 27, 2026 · By Calcelate Team

Offset and backspacing describe the same thing: where the mounting face sits inside the width of the wheel. They measure it from different places and in different units, which is the whole source of the confusion.

Offset is the distance from the wheel’s centerline to the mounting face, in millimeters, stamped on the wheel as ET and a number. Positive offset puts the mounting face toward the street side of the centerline, so the wheel sits further in. Negative puts it behind the centerline and pushes the wheel out. Zero means the face sits exactly on the centerline.

Backspacing is the distance from the mounting face to the inner lip of the wheel, in inches. You measure it with the wheel face down, a straightedge across the back lip, and a tape from the straightedge down to the mounting face. It is always positive, and more of it means the wheel sits further in.

So a +35 offset means the mounting face is 35 mm outboard of the centerline. Backspacing is not the same number in other units: it also depends on how wide the wheel is.

How to convert offset to backspacing

Backspacing is half the wheel’s overall width plus the offset. The catch is the word overall. The width on the wheel — the 8 in 17×8 — is measured between the bead seats where the tire sits. The flanges add roughly half an inch on each side, so the overall width is about the marked width plus one inch.

Backspacing (in) = (marked width + 1) ÷ 2 + offset (mm) ÷ 25.4

An 8 in wheel at ET+35: (8 + 1) ÷ 2 = 4.5, plus 35 ÷ 25.4 = 1.38, gives 5.88 in. The other direction is the same formula turned round:

Offset (mm) = (backspacing − (marked width + 1) ÷ 2) × 25.4

Published conversion charts are built on this convention. Summit Racing’s offset chart puts an 8 in wheel with 4.5 in of backspacing at zero offset, which is exactly half of 8 + 1. Valor Offroad gives the formula as (wheel width + lip thickness) ÷ 2 + offset and quotes a 15×7 wheel at +62 mm as 6.44 in of backspacing, which is what the formula above returns.

Flanges are not exactly half an inch on every wheel, and charts round differently, so two charts can disagree by a millimeter or a few hundredths of an inch. For an exact figure on a wheel in your hands, measure it.

Wheel offset to backspacing chart

Read across a row to see what offset does: every step of 25.4 mm is one inch of backspacing. Read down a column to see what width does: each extra inch of width adds half an inch of backspacing, because half of the extra width goes inward and half goes outward.

That is why backspacing on its own does not tell you where a wheel sits. Valor’s page makes the point with a 7 in and an 8 in wheel, both at +38: same offset, different backspacing — 5.50 in and 6.00 in by the formula. Both mounting faces sit in the same place relative to the centerline; the 8 in wheel is simply half an inch longer on each side.

Backspacing to offset chart

The questions people actually ask run the other way: what offset is 4.5 in of backspacing? It depends on the width. On an 8 in wheel it is zero. On a 9 in wheel it is −13 mm. On a 10 in wheel 7.5 in of backspacing is +51 mm, and on a 9 in wheel it is +64.

A quarter inch of backspacing is 6.35 mm of offset, so the rows step by about 6 mm, and the half-inch rows by about 13.

What changing offset actually moves

Both measurements tell you where the mounting face is, and the mounting face is bolted to the car. So what changes when you fit a different wheel is where its two edges end up: the outer edge toward the fender, the inner edge toward the suspension.

Every millimeter less offset moves the whole wheel one millimeter outward. Every inch of extra width adds 12.7 mm on each side. The two add up at the outer edge and partly cancel at the inner one.

Take a truck on a stock 8 in wheel at ET+18, going to a 9 in wheel at ET−12, a common look for a wider stance. This is what the wheel offset calculator shows for that swap:

The offset drops by 30 mm and the wheel gets 25.4 mm wider. The outer edge moves out by both at once: 30 + 12.7 = 42.7 mm, about 1.7 in, with the same again on the other side of the axle, so the track is 85.4 mm wider overall. The inner edge moves in by 12.7 from the width but out by 30 from the offset, so it ends up 17.3 mm further from the suspension.

In backspacing terms, 5.21 in becomes 4.53 in. That drop of 0.68 in is the number a truck forum would quote, and it is honest about the inner edge. It says nothing about the outer one. Only offset and width together tell you the outer edge moved 42.7 mm toward the fender, which is where rubbing and poke happen.

The calculator flags anything past 25 mm outward, and this swap is well past it. Whether that fits is decided by the tire, the fender and the suspension travel: measure at full lock and full compression before you buy.

Wheel offset vs spacers

A spacer bolts between the hub and the wheel and moves the whole wheel out by its own thickness. For where the wheel sits, that is exactly the same as a wheel with that much less offset: a 10 mm spacer on an ET+45 wheel puts both edges where an ET+35 wheel would be.

So a 1 inch spacer is 25.4 mm of offset. On an 8 in ET+45 wheel it behaves like ET+19.6, and the effective backspacing drops from 6.27 in to 5.27 in — one inch, as it should. Because the wheel does not get any wider, the inner edge gains exactly as much room as the outer edge moves out: every row moves both by the same amount.

The geometry is identical, so the choice between spacers and new wheels is about everything else. A spacer adds a joint, relies on the studs or bolts being long enough and correctly torqued, and should be hub-centric so the wheel still centers on the hub rather than on the nuts. A wheel with the right offset has none of that to get wrong. Where a spacer earns its place is in small corrections, or clearing a caliper or strut on a wheel you already own.

One thing to check in the calculator

Our calculator’s comparison table shows a backspacing row too. It uses the same formula, flange included, and rounds to a tenth of an inch: 5.9 in for an 8 in ET+35 wheel against 5.88 in in the chart above. The edge positions it reports are measured at the bead seat, where the tire sits, and the movements — outer edge, inner edge, track width — do not depend on the flange at all, because it cancels out when you compare two wheels. Use the chart here when you need a backspacing number to the hundredth to match a shop’s catalog.

If a wider wheel comes with a different tire, check that too: the tire size calculator compares the overall diameter of the two sizes and the speedometer error the change brings.