Offset (marked ET on the wheel) is the distance from the wheel’s centreline to its mounting face. Positive offset pulls the wheel in toward the car; negative pushes it out. Change the offset or the width and both edges of the wheel move, one toward the fender and one toward the suspension. This calculator shows exactly how far.
How it is calculated
- Convert width to millimetres (inches × 25.4). Wheel widths are measured between bead seats, so the physical rim is about 25 mm wider; this does not affect the comparison.
- Outer edge position = width ÷ 2 − offset. Inner edge position = width ÷ 2 + offset. Both are measured from the hub mounting face.
- A spacer reduces the effective offset by its thickness: a 10 mm spacer on an ET45 wheel behaves like ET35.
- The changes are the new positions minus the current ones. Outer change positive means more poke; inner change positive means less clearance.
Example
Current 7 in ET45, new 8 in ET35. Current outer edge: 88.9 − 45 = 43.9 mm; new: 101.6 − 35 = 66.6 mm, so the outer edge moves out 22.7 mm. Inner edge: 133.9 mm now, 136.6 new, moving 2.7 mm closer to the suspension. Almost all the extra width went outward, which is typical of a lower offset.
Rules of thumb
Keeping the outer change under about 15–20 mm usually avoids fender rubbing on a stock car; more than that needs measuring or fender work. Inner clearance matters most on cars with strut suspension, where the coil spring sits close to the wheel. Backspacing, used in the US truck world, is the same idea measured from the mounting face to the inner lip.
Frequently asked questions
What does wheel offset mean?
The distance in millimetres from the wheel’s centreline to the hub mounting face. Positive offset (most cars) sets the wheel further in; negative offset pushes it out for a wider stance.
Will a lower offset stick out more?
Yes. Every 1 mm less offset moves the whole wheel 1 mm outward. Combined with a wider wheel, the outer edge moves out by half the width increase plus the offset decrease.
Do spacers change offset?
Effectively yes. A spacer pushes the wheel outward by its thickness, the same as lowering the offset by that amount. Enter the spacer to see the combined result.
What is backspacing?
The distance from the mounting face to the inner edge of the wheel, common in the US. Higher backspacing means the wheel sits further in. It is related to offset: backspacing = width/2 + offset, plus the rim flange.
Sources
By Calcelate Team. Formula from the sources above.
- 2026-09-14 · Formula and texts checked