Vehicle · Wheels
Wheel offset and backspacing calculator.
Convert offset (mm) to backspacing (inches) and back, from the wheel width.
Inputs
Offset and backspacing
Enter the details to see the result.
How do you convert wheel offset to backspacing?
Offset is the distance from the wheel centreline to the mounting face, in millimetres. It is positive when the mounting face sits toward the outside of the wheel and negative when it sits toward the inside. Backspacing is the distance from the mounting face to the inner edge of the wheel, usually in inches. The two are linked through the wheel centre: backspacing = (width + 1) / 2 + offset / 25.4. The extra 1 inch is because the overall width of a wheel is almost always about an inch more than its stated width (Forgeline), since the stated width is measured between the bead seats and the lips add to it.
Worked example: a 17x9 wheel with +12 mm offset has an overall width of 10 in, so the centreline is 5 in from either lip. 12 mm is 0.472 in, so backspacing is 5.47 in and frontspacing is 4.53 in. To go the other way, offset (mm) = (backspacing - (width + 1) / 2) x 25.4. A wheel maker's own listed backspacing is more reliable than this estimate, because real lip dimensions vary a little.
Frequently asked questions
What is the formula for backspacing from offset?
Backspacing in inches = (wheel width + 1) / 2 + offset in mm / 25.4. For a 9 inch wide wheel at +12 mm, that is 5 + 0.472 = 5.47 inches.
Is a bigger backspacing more positive offset?
Yes. For the same wheel width, more backspacing means the mounting face is further from the inner lip toward the outside, which is a higher (more positive) offset. About 25.4 mm of offset equals 1 inch of backspacing.
Why add 1 inch to the wheel width?
The stated width is measured between the bead seats. The lips add to it, so the overall width is almost always about 1 inch more than the stated width, and the centreline sits half the overall width from each edge.
What does negative offset mean?
The hub mounting face sits toward the inside of the wheel centreline, which pushes the wheel outward on the vehicle. Positive offset puts the mounting face toward the outside of the centreline.
Sources
Checked
Scope
Fitment depends on brakes, suspension, fenders and the exact wheel. Confirm the wheel and offset with the wheel maker or a tire shop before buying. Estimates only.