About this tool
Wheel Offset & Backspacing Calculator is a free, in-browser tool that converts a wheel's width and offset into backspacing, and shows how a new wheel will sit relative to your current one. It is the quick check to run before ordering rims so you know whether they will poke out past the fender or tuck in toward the suspension. Everything is computed locally in your browser.
Backspacing is the distance from the wheel's mounting face to its inner lip. In inches it is width ÷ 2 + 0.5 + offset ÷ 25.4, where 0.5 in accounts for the two half-inch bead humps and 25.4 converts millimetres of offset to inches. A higher (more positive) offset pulls the wheel inboard and increases backspacing; a lower or negative offset pushes it outward.
Turn on the compare option to enter your old wheel's width and offset. The tool then reports two changes in inches: how much farther the outer edge pokes out — (newWidth ÷ 2 − newOffset ÷ 25.4) minus the same for the old wheel — and how much farther the inner edge moves toward the suspension, using the plus form of the same expression. Positive poke means more fender clearance needed; positive inboard movement means watch strut and control-arm clearance.