Wheel Offset Calculator

Convert backspacing to offset (ET), or compare an old and new wheel's width and offset to see how far the wheel pokes out, how much suspension clearance it loses, and how much the track width changes.

Quick Facts

Formula
Offset (mm) = (Backspacing − Width/2) × 25.4
Positive offset tucks the wheel toward the suspension; negative offset pushes it out toward the fender.

Your Results

Calculated
New backspacing
-
Hub face to inner lip, in inches
Outward shift (poke)
-
Change at the outer, fender-side face
Inward shift (clearance)
-
Change at the inner, suspension-side face
Track width change
-
Combined shift across the axle

Ready

Enter the old and new wheel width and offset, then press Calculate.

About Wheel Offset

Wheel offset (ET, from the German Einpresstiefe, "insertion depth") is the distance in millimeters from a wheel's hub mounting surface to its true centerline — half of the total wheel width. It is stamped on most wheels alongside the diameter and width, for example 18x8.5 ET35. Offset determines exactly where the wheel and tire sit relative to the fender on one side and the suspension and brakes on the other, which is why swapping wheels with a different offset (even at the same diameter) can change how a car fits.

The core formula

Shops commonly measure backspacing — the distance from the mounting face to the wheel's inner (suspension-side) lip — because it is easy to measure with a straightedge. Offset converts from it directly:

  • Offset (mm) = (Backspacing (in) − Wheel width (in) / 2) × 25.4
  • Backspacing (in) = Wheel width (in) / 2 + Offset (mm) / 25.4 (the inverse)

Example: an 8-inch-wide wheel with 5.5 inches of backspacing has offset = (5.5 − 4) × 25.4 = 38.1 mm, a typical positive offset for a modern passenger car.

Comparing an old and new wheel

This calculator's main use is fitment comparison: given a current wheel (width and offset) and a candidate new wheel, it reports how far the wheel's faces move. Because the hub mounting face itself does not move (it is fixed by the vehicle), only the wheel's width and offset determine where the outer and inner faces land:

  • Outward shift (mm) = (New width − Old width) / 2 × 25.4 − (New offset − Old offset). A positive value means the wheel face moves toward the fender (more poke).
  • Inward shift (mm) = (New width − Old width) / 2 × 25.4 + (New offset − Old offset). A positive value means the wheel face moves toward the strut, control arm, or brake caliper (less clearance).
  • Track width change (mm) ≈ 2 × outward shift, since both wheels on the axle move the same amount.

Positive vs. negative offset

Zero offset means the mounting face sits exactly at the wheel's centerline. Positive offset moves the mounting face toward the wheel's outer edge, which pulls most of the wheel's width inboard, toward the suspension — common on front-wheel-drive cars and crossovers. Negative offset moves the mounting face toward the inner edge, pushing more of the wheel outboard, toward the fender — common on trucks, off-road builds, and "deep dish" wheels. Neither is inherently correct; the right offset is the one that clears both the fender and the suspension for a specific vehicle.

Frequently Asked Questions

What is wheel offset (ET)?
Wheel offset, marked ET on the wheel, is the distance in millimeters from the hub mounting surface to the wheel's true centerline. Positive offset means the mounting surface sits toward the outside of the centerline, tucking the wheel in toward the suspension. Negative offset means it sits toward the inside, pushing the wheel out toward the fender.
How do I convert backspacing to offset?
Offset (mm) = (Backspacing (in) − Wheel width (in) / 2) × 25.4. For example, an 8-inch-wide wheel with 5.5 inches of backspacing gives (5.5 − 4) × 25.4 = 38.1 mm. To go the other way, backspacing (in) = width (in) / 2 + offset (mm) / 25.4.
What happens when I switch to a wider wheel or a different offset?
The outward shift is (change in width / 2) × 25.4 minus the change in offset, in millimeters — a positive number means the wheel face moves toward the fender. The inward shift is the same width term plus the offset change, and a positive number means less clearance to the strut, control arm, or brake caliper. Track width change across the axle is roughly double the outward shift.
Is a higher or lower offset better?
Neither is universally better — it depends on the vehicle's factory geometry. More positive offset tucks the wheel toward the suspension and can rub struts, springs, or control arms. Lower or negative offset pushes the wheel outward and can rub the fender lip, especially at full steering lock or suspension travel. Small changes of a few millimeters from factory are usually low risk; larger changes should be checked against your specific vehicle.

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