Wheel Offset and Backspacing Calculator

How we calculate

Enter the original and replacement wheel widths in inches and their offsets in millimeters. Use the nominal bead-seat widths marked on the wheels. Offset, often marked ET, is the distance between the wheel centerline and the mounting face. Positive offset places the mounting face toward the outside of the wheel. This tool compares wheel edges relative to the same hub mounting face, without spacers or changes to the hub.

How we calculate: half the width difference in millimeters equals the new width minus the old width, times 25.4, divided by two. Inward movement equals that half-width difference plus the offset difference. Outward movement equals the half-width difference minus the offset difference. A positive inward value means the inner edge is closer to the suspension. A positive outward value means the outer edge extends farther from the vehicle centerline.

Backspacing is estimated as half the overall wheel width plus the offset converted to inches. Because a nominal wheel width is measured between bead seats, we assume overall width is nominal width plus one inch, allowing half an inch for each flange. Real flange profiles vary. Measure the actual wheel when a precise backspacing figure is needed. The estimate is not a manufacturer drawing or an inspection of the wheel.

For example, replacing an eight-inch wheel at ET35 with a nine-inch wheel at ET20 adds 12.7 mm to each half-width and reduces offset by 15 mm. The inner edge moves away from the suspension by 2.3 mm, while the outer edge extends 27.7 mm farther outward. Our table shows the inward change as negative 2.3 mm so that the signs retain the same definition for narrower wheels and offset changes in either direction.

These values describe wheel geometry only. They exclude tire bulge, rim-protector shape, brake-spoke clearance, steering travel, suspension movement, and bodywork. A tire's section width can change with wheel width and cannot be inferred from wheel edges alone. Do not treat a small inward change as a clearance certificate. Confirm bolt pattern, center bore, wheel load rating, mounting hardware, and actual clearances separately with an installer before ordering or fitting parts.

Frequently asked questions