Wheel Backspace to Offset Calculator
Convert your rim's backspacing measurements into millimeters offset.
Understanding Backspace vs. Offset
When selecting aftermarket wheels for your vehicle, understanding the relationship between backspacing and offset is critical to prevent suspension interference or tire rubbing. While both measure the wheel's mounting position, they use different reference points.
Backspacing is the distance from the back edge of the wheel (the flange) to the mounting surface of the hub. It is typically measured in inches. Higher backspacing moves the wheel further into the wheel well.
Offset is the distance from the exact centerline of the wheel to the mounting surface. It is typically measured in millimeters.
- Zero Offset: The mounting surface is even with the centerline.
- Positive Offset: The mounting surface is closer to the front (street side) of the wheel. This is common on modern FWD and many RWD vehicles.
- Negative Offset: The mounting surface is closer to the back (brake side) of the wheel. This creates a "deep dish" look and is common on lifted trucks and older muscle cars.
How This Calculation Works
To convert backspace to offset, we must account for the physical width of the wheel. The "rated width" (e.g., 9 inches) is measured from bead seat to bead seat. However, the physical width includes the flanges, which typically adds about 1 inch to the total width.
The formula used in this calculator is:
Offset (mm) = (Backspace – ((Wheel Width + 1) / 2)) × 25.4
Why It Matters
If you get the offset wrong, two things can happen:
- Too much positive offset (or high backspace): The wheel sits too far inside. The tire may rub against control arms, struts, or the inner wheel well liner.
- Too much negative offset (or low backspace): The wheel sticks out too far. This puts extra stress on wheel bearings and may cause the tire to hit the fender when the suspension compresses.