Wheel Offset &Backspacing Calculator
Compare stock vs new wheel fitment to check inner suspension clearance and outer fender poke, or use the conversion charts below to move between offset in millimetres and backspacing in inches.
The inner wheel lip will have 6.0 mm more room away from the strut.
The outer face will extend 44.0 mm further outward toward/past the fender.
Understanding Wheel Offset (ET) & Backspacing
Why Are Units Mixed (Inches & Millimeters)?
In the global automotive industry, wheel dimensions follow a standard hybrid format:
- Wheel Width (Inches): Universal rim bead width (e.g., 7.5", 8.5", 9.0"). Manufacturers worldwide sell rims in inches.
- Offset ET (Millimeters): The distance from the wheel centerline to the hub mounting face (e.g., +45 mm, +20 mm, -12 mm). Precision in millimeters is necessary for exact brake and strut fitment.
- Backspacing (Inches): The distance from the hub mounting pad to the inner rim lip. Standard measurement for US truck and 4x4 lift kits.
Offset Types Explained
The hub mounting face is towards the street/front side of the wheel. The wheel sits further inside the fender (common on modern passenger cars & FWD/AWD SUVs).
The hub mounting face aligns exactly with the wheel's centerline.
The hub mounting face is recessed toward the back/brake side. The wheel pushes aggressively outward past the fender (popular on lifted trucks and deep-dish wheels).
Wheel Offset to Backspacing Conversion Chart
Backspacing in inches for each combination of stated rim width and ET offset. Overall width is the stated width plus one inch, because each flange adds a quarter inch per side. Read down the column for your wheel width.
| Offset (ET) | Inches | 7" wide | 7.5" wide | 8" wide | 8.5" wide | 9" wide | 10" wide |
|---|---|---|---|---|---|---|---|
| -25 mm | -0.98" | 3.02" | 3.27" | 3.52" | 3.77" | 4.02" | 4.52" |
| -12 mm | -0.47" | 3.53" | 3.78" | 4.03" | 4.28" | 4.53" | 5.03" |
| 0 mm | +0.00" | 4.00" | 4.25" | 4.50" | 4.75" | 5.00" | 5.50" |
| +12 mm | +0.47" | 4.47" | 4.72" | 4.97" | 5.22" | 5.47" | 5.97" |
| +20 mm | +0.79" | 4.79" | 5.04" | 5.29" | 5.54" | 5.79" | 6.29" |
| +25 mm | +0.98" | 4.98" | 5.23" | 5.48" | 5.73" | 5.98" | 6.48" |
| +30 mm | +1.18" | 5.18" | 5.43" | 5.68" | 5.93" | 6.18" | 6.68" |
| +35 mm | +1.38" | 5.38" | 5.63" | 5.88" | 6.13" | 6.38" | 6.88" |
| +40 mm | +1.57" | 5.57" | 5.82" | 6.07" | 6.32" | 6.57" | 7.07" |
| +45 mm | +1.77" | 5.77" | 6.02" | 6.27" | 6.52" | 6.77" | 7.27" |
| +50 mm | +1.97" | 5.97" | 6.22" | 6.47" | 6.72" | 6.97" | 7.47" |
Backspacing to Offset Conversion Chart
The same relationship read the other way. Note how one backspacing figure maps to a completely different offset depending on rim width: this is why a backspacing number on its own does not describe a wheel.
| Backspacing | 7" wide | 7.5" wide | 8" wide | 8.5" wide | 9" wide | 10" wide |
|---|---|---|---|---|---|---|
| 4.00" | 0 mm | -6 mm | -13 mm | -19 mm | -25 mm | -38 mm |
| 4.50" | +13 mm | +6 mm | 0 mm | -6 mm | -13 mm | -25 mm |
| 4.75" | +19 mm | +13 mm | +6 mm | 0 mm | -6 mm | -19 mm |
| 5.00" | +25 mm | +19 mm | +13 mm | +6 mm | 0 mm | -13 mm |
| 5.50" | +38 mm | +32 mm | +25 mm | +19 mm | +13 mm | 0 mm |
| 6.00" | +51 mm | +44 mm | +38 mm | +32 mm | +25 mm | +13 mm |
| 6.50" | +64 mm | +57 mm | +51 mm | +44 mm | +38 mm | +25 mm |
Staggered Setups: Front and Rear Are Two Separate Calculations
A staggered fitment runs a wider wheel at the rear than the front, usually on rear-wheel-drive and performance AWD cars. Because backspacing depends on width, the front and rear wheels of a staggered set almost never share an offset, and averaging them tells you nothing useful.
Run the calculator twice
Enter your front stock wheel against your front target wheel, read the result, then do the same for the rear. Each axle has its own clearance limits, so a change that is comfortable at the front can rub at the rear.
- Front axle: limited by the strut on the inside and by steering sweep, the wheel moves through its full lock arc.
- Rear axle: limited by the trailing arm or spring perch inside, and by the quarter panel lip outside. No steering sweep, so the outer edge is usually the tighter constraint.
Why the rear normally takes a lower offset
Adding an inch of rim width moves each lip outward by half an inch. Keeping the same offset therefore pushes the inner lip half an inch closer to the suspension. Dropping the rear offset gives that half inch back to the inside and spends it on the outside instead, which is also why staggered setups tend to sit flush at the rear.
On an AWD car, keep the rolling diameter of the front and rear tires matched even when the wheels differ in width. A diameter mismatch across the axles makes the centre differential work continuously, which is a driveline problem rather than a fitment one.
Frequently Asked Questions About Wheel Offset
Why is wheel width in inches while offset is in millimeters?
This is the universal international standard across the automotive industry (ETRTO, SAE, DIN, and JATMA). Wheel diameters and bead widths have always been manufactured in inches (e.g. 17x8.5J or 18x9J), whereas offset (ET - Einpresstiefe) requires millimeter precision (e.g. +35mm, +20mm, -12mm) to accurately gauge suspension clearance and fender alignment.
What is the difference between Wheel Offset (ET) and Backspacing?
Offset is the distance in millimeters from the wheel centerline to the hub mounting face. Backspacing is the distance in inches from the hub mounting face to the back inside lip of the wheel. Backspacing is commonly used in off-road and custom truck fitments to verify suspension strut clearance.
How do I convert wheel offset in millimeters to inches?
Divide the offset by 25.4, since there are exactly 25.4 millimetres in an inch. A +12 mm offset is 0.47 inches, +20 mm is 0.79 inches, and +45 mm is 1.77 inches. Offset stays in millimetres on the wheel itself because the tolerances that matter for brake and strut clearance are finer than a sixteenth of an inch.
How do I convert backspacing to offset?
Offset in millimetres equals (backspacing minus half the overall wheel width) multiplied by 25.4, where overall width is the stated rim width plus one inch for the two flanges. On an 8 inch wheel, 4.75 inches of backspacing works out to +6 mm of offset. The same 4.75 inches of backspacing on a 10 inch wheel is a very different wheel: -44 mm.
Why does backspacing depend on wheel width but offset does not?
Offset is measured from the wheel centreline, which sits in the middle whatever the width. Backspacing is measured from the inner lip, so widening the rim moves that lip inward and increases backspacing even when the offset has not changed. This is why a backspacing figure means nothing without the wheel width alongside it, and why offset is the more portable number.
How do I calculate offset for a staggered setup?
Work the front and rear separately, because a staggered fitment runs a wider wheel and often a different offset at the rear. Run the calculator once with your front pair and once with your rear pair. The rear usually needs a lower offset to keep the wider wheel from hitting the inner arch, and the front and rear results should be read against different clearances: struts and steering sweep at the front, trailing arm and quarter panel at the rear.
How do I know if a new wheel will rub on my suspension or fender?
Compare your stock wheel width and offset to the new wheel using our calculator above. If the inner clearance shows positive millimeters (e.g. 15mm closer), the wheel moves closer to the strut. If the outer poke shows positive millimeters (e.g. 20mm poke), the wheel face pushes outward closer to or past the fender lip.