What Is Wheel Offset?
Wheel offset, often labeled ET (from the German Einpresstiefe), is the distance between the wheel's mounting hub face and its true centerline, measured in millimeters. A positive offset puts the hub mounting surface toward the front (street) side of the wheel, pulling the wheel inward. A negative offset pushes the wheel outward for a deep-dish, aggressive stance. Zero offset means the hub sits exactly on the centerline.
How to Use This Calculator
Enter your wheel's overall width in inches and its backspacing in inches (the distance from the back lip of the wheel to the hub mounting surface). The calculator converts both to millimeters and returns the offset in mm, the standard unit stamped on most wheels.
The Formula Explained
The centerline of a wheel sits at exactly half its overall width. Backspacing measures from the rear flange to the mounting face. So the offset is simply how far the mounting face lies from that centerline:
$$\text{Offset (mm)} = \text{Backspacing (mm)} - \left( \frac{\text{Wheel Width (mm)}}{2} \right)$$
Both inputs are multiplied by 25.4 to convert inches to millimeters before the subtraction.
Worked Example
Suppose you have a 9-inch-wide wheel with 4.5 inches of backspacing. Convert: width = \(9 \times 25.4 = 228.6\) mm; backspacing = \(4.5 \times 25.4 = 114.3\) mm. Half the width = 114.3 mm. Offset = \(114.3 - 114.3 = 0\)... but note that the overall stated width usually excludes the lip allowance. Using exact values: $$114.3 - \left( \frac{228.6}{2} \right) = 114.3 - 114.3 = 0 \text{ mm}$$ for the bare math; a 9in / 4.45in pairing gives \(-1.27\) mm, a slightly negative (outward) offset.
FAQ
What's the difference between offset and backspacing? Backspacing is a physical distance from the back edge of the wheel; offset is measured from the centerline. They are related but not the same number.
Is a negative offset bad? Not at all — negative offsets create a wider, more aggressive track and deep-dish look, but they can increase fender clearance and steering load.
Why is offset in mm but width in inches? Wheel widths are traditionally quoted in inches, while offset (ET) is an industry standard expressed in millimeters.
