Wheel Offset & Backspacing Calculator

What does ET45 mean?

ET45 means the mounting face is 45 mm toward the outside of the wheel from its centreline. It does not mean the wheel sits 45 mm farther outward than your current wheel; that comparison also needs widths and both offsets.

Offset is the signed distance from wheel centreline to the hub mounting face, usually given in millimetres as ET. Positive offset places that face toward the outside of the wheel. Backspacing measures from the same face to the inner flange, usually in inches. They describe the same mounting geometry from different reference points.

Choose the conversion direction, then enter the measured overall width including both flanges. The nominal width stamped on a wheel describes the distance between bead seats; it is not the outside-to-outside measurement. This calculator never adds an assumed inch to turn one into the other.

How the conversion works

With all dimensions in millimetres: backspacing = measured overall width ÷ 2 + offset. The inverse is offset = backspacing − measured overall width ÷ 2. An inch is exactly 25.4 mm. Changing result units changes presentation, not the underlying geometry.

Nominal inner position = half bead-seat width + offset; nominal outer position = half bead-seat width − offset. These are bead-seat references, not flange positions. The measured-width diagram shows flange-to-flange width; the listed nominal edge positions use the separate stamped width.

Compare wheels without mixing conventions

The Compare wheels mode uses current and proposed nominal widths, offsets and optional spacers. It answers how the two edges move, not how to measure a mechanical backspace. Lower effective offset moves the same wheel outward; a wider wheel expands both edges away from its centreline.

A negative signed extent means the mounting plane lies beyond the named edge in the entered geometry. Check measurements and conventions rather than treating a negative number as an installation recommendation.

Example: measured width and ET45 to backspacing

Inputs: Nominal bead-seat width: 7.5 in; Measured overall wheel width: 8.5 in; Stamped offset: 45 mm. Metric results below are computed by the same engine as the live tool.

Using the explicitly entered 215.9 mm overall width, offset is ET+45.0 and signed backspacing is 153.0 mm. Nominal bead-seat width is used only for wheel-edge positions.

Important limitations