Wheel offset calculator
Enter your current and new wheel, see how far each edge moves.
Width and diameter as stamped on the wheel (17×8 = 17 in diameter, 8 in wide). Negative offset — type a minus.
What is wheel offset?
The distance from the hub mounting surface to the centerline of the wheel, in millimeters. Positive offset puts the mounting surface toward the face of the wheel and tucks the wheel in; negative offset pushes it out toward the fenders (Mickey Thompson).
What is backspacing?
The distance from the back of the mounting pad to the back lip of the wheel, usually in inches. The larger it is, the farther inboard the wheel sits.
How do I convert offset to backspacing?
Mickey Thompson’s formula: backspacing = (rim width + 1) ÷ 2 + offset (mm) ÷ 25.4. The extra inch covers the two rim flanges, since a wheel’s nominal width is measured inside them — so the result is approximate. A 10-inch wheel at +25 mm: 5.5 + 0.98 = 6.48 in.
Example
Going from a 17×8 +45 mm wheel to an 18×9 +35 mm wheel: the extra inch of width adds 12.7 mm on each side, and 10 mm less offset moves the wheel 10 mm out. The inner edge comes 2.7 mm closer to the suspension, and the outer edge pokes out 22.7 mm more.
Does this tell me whether the wheel will fit?
Not by itself. It shows how far each edge moves; you still need to measure the room you have to the strut, brakes and fender, and check the tire size and bolt pattern.
What does a wheel spacer do to the offset?
A spacer pushes the wheel away from the hub by its thickness, which lowers the effective offset by that amount. A 15 mm spacer on an 8-inch wheel at +40 mm acts like a +25 mm wheel: both edges move out by 15 mm, which is 0.59 inches: the clearance to the suspension grows by that much and so does the poke. The comparison tool shows the same result if you enter the +40 mm and +25 mm wheels of equal width: 15.0 mm more poke.
Is a lower offset number always more poke?
Yes, for the same width. Offset is measured from the centerline to the mounting surface, so a lower number moves the wheel outward and a negative number moves it further out. A 10-inch wheel at −25 mm has a backspacing of 4.52 inches, against 6.48 inches at +25 mm.
The geometry of offset
Draw a line down the centerline of the wheel, halfway between its two lips. The mounting surface is where the wheel meets the hub. The offset is the distance from that centerline to the mounting surface, in millimeters: positive if the surface is toward the front (street side) of the wheel, zero if it is on the centerline, and negative if it is toward the back. Backspacing measures from the mounting surface to the back lip, usually in inches. Both describe the same position, which is why the calculator converts between them using a wheel width plus one inch for the two rim flanges, so the conversion is approximate.
Worked example: going wider with a lower offset
A wheel 17×8 at +40 mm is replaced by a 17×9 at +20 mm. The extra inch of width adds 12.7 mm to each side of the centerline, and the offset is 20 mm lower, so the whole wheel moves out by 20 mm. The outer edge moves out by 32.7 mm (12.7 + 20), and the inner edge by -7.3 mm (12.7 − 20), which means it moves 7.3 mm away from the suspension. A 9-inch-wide wheel at +20 mm has a backspacing of 5.79 inches; going from backspacing to offset, a 9-inch wheel with 5.5 inches of backspacing has an offset of 12.7 mm. Keeping the width and lowering the offset from +40 to +25 moves both edges out by 15.0 mm.
What is not covered
Whether a wheel fits depends on more than the position of its edges. The room between the tire and the strut, brake caliper, control arms and inner fender; the fender lip on the outside; the tire width and sidewall; the diameter of the center bore; the bolt pattern; the lug seat type; and the load rating all have to match the car. Changing the rim diameter and the tire together changes the overall tire diameter and the speedometer. The calculator reports only how far the wheel edges move, and a measurement on the car decides if there is clearance.
Common mistakes
Treating a positive offset as outward is the usual error: positive offset moves the wheel inward, toward the car. The second is comparing backspacing numbers from different manufacturers who measure the width differently, and then entering the nominal width. A third is forgetting that the rim width does not include the tire: a tire is wider than the rim it is mounted on and sticks out farther than the wheel lip. Last, the unit mix-up: offset is in millimeters and backspacing in inches, so 25.4 mm is one inch.
Sources: Mickey Thompson, "Backspacing and Offset" (definitions and the formula backspacing = (width + 1) ÷ 2 + offset ÷ 25.4); the millimeter-inch conversion of exactly 25.4 mm per inch. Related: tire size calculator, tire load index calculator and stopping distance calculator.