Wheel Offset vs. Backspacing: One Number, Two Languages
Offset measures the hub face's distance from the wheel's centerline in millimeters; backspacing measures it from the back flange in inches. Convert with: backspacing = (width + 1) ÷ 2 + offset ÷ 25.4. Most trailer wheels run zero offset.
Two conventions, one measurement
Where the mounting face sits inside the wheel decides whether the wheel tucks in or pokes out.
- Offset (mm): distance from the wheel centerline. Positive = toward the street side (tucks in). Negative = toward the brakes (pokes out, deep dish). Passenger and import catalogs speak offset.
- Backspacing (inches): distance from the back flange to the face. Trailer and off-road catalogs speak backspacing.
The conversion
Backspacing = (wheel width + 1) ÷ 2 + offset ÷ 25.4 — the +1" covers the flanges.
| 8" wide wheel | Offset | Backspacing |
|---|---|---|
| deep dish | −25 mm | 3.52" |
| centered | 0 mm | 4.50" |
| modern truck | +18 mm | 5.21" |
Or skip the math: the offset ↔ backspacing calculator converts both directions with a live cross-section diagram that moves as you type.
Rules that keep you out of trouble
Trailer wheels are almost always zero offset — load centered over the bearings. On vehicles, staying within about ±5 mm of stock avoids steering and bearing surprises; going more negative widens the stance but loads bearings and can rub. Always check brake-caliper clearance on the backspacing side.
Pattern next: measure it right with the bolt pattern guide, then browse wheels.