Backspacing vs Offset
Both numbers answer the same question — where does the mounting face sit inside the wheel? — from two different starting points. Offset measures from the wheel's centerline, in millimetres. Backspacing measures from the back (inboard) lip, in inches. Same geometry, two habits.
Definitions
- Offset (ET): centerline to mounting face, mm. Positive means the face is toward the street side (wheel tucks in); negative means toward the inboard side (wheel pokes out). The import, car and manufacturer convention. Explained in full in wheel offset explained.
- Backspacing: from the inboard edge of the rim flange to the mounting face, inches. Bigger backspacing = face further back = wheel sits further in. The US truck, Jeep and hot-rod convention, because deep-dish wheels are easier to picture that way.
They move opposite to each other only in name: more backspacing and higher offset both tuck the wheel in. A "high offset" wheel and a "deep backspacing" wheel are the same wheel.
The half inch that trips people up
A wheel sold as 9 inches wide is 9" between the bead seats, where the tire sits. The rim flanges add roughly half an inch on each side, so the overall width — flange to flange — is about 10". Backspacing is measured to the flange, not the bead seat. That is where the extra 0.5" in the formula comes from, and why converting with plain width ÷ 2 lands half an inch off.
The formulas
With W the nominal width in inches and ET the offset in mm:
- Backspacing (in) = W ÷ 2 + 0.5 + ET ÷ 25.4
- Offset (mm) = (Backspacing − W ÷ 2 − 0.5) × 25.4
Half the nominal width gets you to the centerline; the 0.5" gets you from the bead seat out to the flange; the offset term shifts the face from the centerline.
Worked examples
| Wheel | Work | Result |
|---|---|---|
| 17x9, ET +18 | 4.5 + 0.5 + 18 ÷ 25.4 | 5.71" backspacing |
| 8" wide, 4.5" backspacing | (4.5 − 4 − 0.5) × 25.4 | 0 mm — zero offset |
| 15x10, 3.75" backspacing | (3.75 − 5 − 0.5) × 25.4 | -44.45 mm (deep dish, negative) |
The third row is the classic lifted-truck wheel: quoted as "3.75 backspace", it is a −44 mm offset in the other language.
Zero-offset backspacing by width
A quick sanity check: at zero offset the mounting face is on the centerline, so backspacing is simply half the overall width, (W + 1) ÷ 2. Anything more than this number is positive offset; anything less is negative.
| Nominal width | Backspacing at ET0 |
|---|---|
| 7" | 4.00" |
| 7.5" | 4.25" |
| 8" | 4.50" |
| 8.5" | 4.75" |
| 9" | 5.00" |
| 10" | 5.50" |
| 12" | 6.50" |
Backspacing to offset, by wheel width
"What offset is 4.5" backspacing?" has no single answer — it depends on the width. Read down your backspacing and across your nominal width; the cell is the offset in mm. Positive tucks in, negative pokes out.
| Backspacing | 7" wide | 8" wide | 9" wide | 10" wide | 12" wide |
|---|---|---|---|---|---|
| 3.5" | -13 mm | -25 mm | -38 mm | -51 mm | -76 mm |
| 3.75" | -6 mm | -19 mm | -32 mm | -44 mm | -70 mm |
| 4" | 0 mm | -13 mm | -25 mm | -38 mm | -63 mm |
| 4.25" | +6 mm | -6 mm | -19 mm | -32 mm | -57 mm |
| 4.5" | +13 mm | 0 mm | -13 mm | -25 mm | -51 mm |
| 4.75" | +19 mm | +6 mm | -6 mm | -19 mm | -44 mm |
| 5" | +25 mm | +13 mm | 0 mm | -13 mm | -38 mm |
| 5.5" | +38 mm | +25 mm | +13 mm | 0 mm | -25 mm |
| 6" | +51 mm | +38 mm | +25 mm | +13 mm | -13 mm |
| 6.5" | +64 mm | +51 mm | +38 mm | +25 mm | 0 mm |
| 7.5" | +89 mm | +76 mm | +64 mm | +51 mm | +25 mm |
Computed with the same formula as the converter, including the 0.5" flange allowance.
Which one should you use?
Whichever the seller quotes — then convert, so you can compare a wheel listed in backspacing against one listed in offset. Two cautions:
- The 0.5" flange allowance is a convention, and flange widths vary slightly by wheel. If a manufacturer publishes an offset, trust the offset; it is a direct measurement, backspacing is derived.
- A few sellers quote backspacing to the bead seat rather than the flange. If a listing's backspacing and offset do not agree with the formula above by about half an inch, that is usually why — ask.
Tools that use this
Backspacing ↔ offset converter →
Wheel offset calculator →
Wheel offset explained →
Frequently asked
What is the offset for 4.5" backspacing?
It depends on the wheel width — backspacing alone doesn't fix the offset. 4.5" backspacing is 7" wide: +13 mm, 8" wide: 0 mm, 9" wide: -13 mm, 10" wide: -25 mm, 12" wide: -51 mm. Zero offset on an 8" wheel; negative on anything wider.
What does 4.75 backspacing mean?
The mounting face is 4.75" from the inboard flange. In offset terms that is 7" wide: +19 mm, 8" wide: +6 mm, 9" wide: -6 mm, 10" wide: -19 mm, 12" wide: -44 mm.
What offset is 3.5" backspacing?
Deep-dish territory: 7" wide: -13 mm, 8" wide: -25 mm, 9" wide: -38 mm, 10" wide: -51 mm, 12" wide: -76 mm. Every one of these is a negative offset — the wheel pokes well past a factory position.
What is the offset for 7.5" backspacing?
A very deep backspacing that only makes sense on a wide wheel: 7" wide: +89 mm, 8" wide: +76 mm, 9" wide: +64 mm, 10" wide: +51 mm, 12" wide: +25 mm. On a 9" wheel it is +64 mm, far higher than any factory fitment — check that such a wheel exists before assuming the number.
Is more backspacing the same as more offset?
Yes, in direction: more backspacing and a higher (more positive) offset both move the mounting face outboard, tucking the wheel further in. They are the same geometry measured from different reference points — the inboard flange for backspacing, the centerline for offset.