Offset, backspacing and frontspacing
- Wheel width: the rated width, the 9 in a 17x9 wheel. It is measured between the bead seats, inside the flanges.
- Offset: the distance from the wheel's centreline to the hub mounting face, in millimetres. Positive offset puts the mounting face outboard of the centreline, which pulls the wheel in toward the suspension. Negative offset pushes it out.
- Backspacing: the distance from the mounting face to the inner edge of the wheel, in inches. It is what you measure with a straight edge across the back of the wheel, and it tells you how close the wheel comes to the suspension.
- Frontspacing: the distance from the mounting face to the outer edge.
The formula
The rated width does not include the flanges. The common convention in the trade adds 0.5 in for each flange, so the overall width is the rated width plus 1 in, and the centreline sits half of that from each edge:
backspacing (in) = width ÷ 2 + 0.5 + offset (mm) ÷ 25.4
offset (mm) = (backspacing − width ÷ 2 − 0.5) × 25.4
frontspacing = width + 1 − backspacingWorked examples
A 17x9 wheel at −12 mm offset: 9 ÷ 2 = 4.5, plus the 0.5 in flange = 5.0 in, and −12 ÷ 25.4 = −0.47 in. The backspacing is 5.0 − 0.47 = 4.53 in (115 mm) and the frontspacing 10 − 4.53 = 5.47 in.
The other way: an 8 in wide wheel with 5 in of backspacing. 5 − 4 − 0.5 = 0.5 in, and 0.5 × 25.4 = +12.7 mm offset. A 9 in wheel with 4.5 in of backspacing works out to −12.7 mm.
About the 0.5 in flange
The flange allowance is a convention, not a law of nature. Most wheel makers quote backspacing this way, but some measure to the very edge of a flange that is a little thicker or thinner, so a catalogue figure can differ from this calculator by a few hundredths of an inch. When clearance is tight, measure the actual wheel: lay it face down, put a straight edge across the back flange and measure down to the mounting pad.
Offset vs backspacing on a wider wheel
Keeping the same offset on a wider wheel adds half the extra width on each side. Keeping the same backspacing puts all of the extra width on the outside. The wheel offset calculator compares two setups side by side. Whether a wheel clears your brakes, suspension and body is something only a test fit or careful measurement can confirm.
Questions
How do I convert offset to backspacing?
Divide the width by 2, add 0.5 in for the flange, then add the offset in mm divided by 25.4. A 9 in wheel at −12 mm has 4.5 + 0.5 − 0.47 = 4.53 in of backspacing.
What offset is 4.5 inches of backspacing?
It depends on the width. On an 8 in wheel, 4.5 in of backspacing is 0 mm offset. On a 9 in wheel it is −12.7 mm, and on a 10 in wheel −25.4 mm.
Is more backspacing the same as more positive offset?
For the same width, yes. Every 25.4 mm of extra offset adds 1 in of backspacing and moves the wheel 1 in closer to the suspension.
Why do I get a slightly different number from a wheel catalogue?
This calculator adds 0.5 in per flange, the usual convention. A maker that measures to a different point on the flange can publish a figure a few hundredths of an inch off.