Wheel Offset Calculator

What wheel offset (ET) means

Offset is the distance from the rim's centreline to the face that bolts to the hub, in millimetres; German makers stamp it as ET (Einpresstiefe). A higher offset pulls the wheel into the arch, a lower or negative one pushes it out. A wider rim grows by half its extra width on each side.

So going from 8J ET35 to 9J ET30 moves the outer lip 17.7 mm out and the inner edge 7.7 mm towards the strut. A spacer moves the whole wheel out by its thickness, which is the same as lowering the offset by that much.

Formulas

• Outer change = (new width × 12.7 − new ET) − (old width × 12.7 − old ET)
• Inner change = (new width × 12.7 + new ET) − (old width × 12.7 + old ET)
• Backspacing (in) = (width + 1) ÷ 2 + ET ÷ 25.4

Before you buy

• Measure the gap from the current tyre to the strut and to the arch lip at full lock and with the suspension compressed.
• A wider tyre on the new rim adds its own width; check the tyre size too.
• Big offset changes alter scrub radius and steering feel, and may need approval for road use in some countries.