ConverterHub

Wheel Offset Change Calculator

Enter your old and new wheel width and ET offset to see how the outer and inner edges of the wheel move.

How it is calculated

A wheel's outer lip sits at width ÷ 2 − ET from the hub mounting face and its inner lip at width ÷ 2 + ET (backspacing). The tool compares old and new wheels. Width is entered in inches and converted: half-width change = (new width − old width) × 25.4 ÷ 2 in mm, and ET change = new ET − old ET in mm. Then outer change (poke) = half-width change − ET change and inner change = half-width change + ET change. A positive outer value means the face sticks out further towards the fender; a positive inner value means the inner lip moves closer to the suspension and brakes. ET may be zero or negative but must be within ±200 mm, and each width must be above 0 and at most 20 in; anything outside those ranges shows no result. Tyre size, camber and wheel spacers are not part of this calculation.

Worked example

Changing from a 7 in wheel at ET 40 to an 8 in wheel at ET 35. Half-width change: (8 − 7) × 25.4 ÷ 2 = 12.7 mm. ET change: 35 − 40 = −5 mm. Outer edge: 12.7 − (−5) = 17.7 mm further out. Inner edge: 12.7 + (−5) = 7.7 mm closer to the suspension. So the new wheel pokes out 17.7 mm more and uses 7.7 mm of inner clearance.

Try this example →

Common mistakes

  • Entering width in millimetres: the width fields are inches, so 203 instead of 8 is rejected and shows no result, as does any width above 20 in.
  • Believing a lower ET moves the wheel inwards; a lower ET pushes the wheel out, which is why the outer value rises when ET drops.
  • Swapping old and new wheels: the signs flip, and a poke of +17.7 mm becomes −17.7 mm.
  • Using the tyre width, or the overall width including the outer flange, when the field wants the rim width the wheel is sold with.
  • Looking only at the outer figure: the inner figure shows the clearance used against suspension and brakes, and both must fit. With the same width and ET 40 to 35 the outer edge moves 5.0 mm out while the inner edge moves 5.0 mm the other way.

About this calculator

Wheel offset (ET) is the distance in millimetres between a wheel’s true centerline and its mounting hub face — a higher ET pulls the wheel’s face inward toward the car, a lower ET pushes it outward. When either wheel width or ET changes, the wheel’s outer edge (how far it “pokes” outward) and inner edge (how close it sits to suspension components) both shift, and the amount is half the width change plus the full offset change in each direction, applied separately to the inner and outer lip.

Car enthusiasts fitting wider wheels or a different offset run this calculation before buying, because getting it wrong means either the wheel rubs the fender at the outer edge or fouls suspension and brake components at the inner edge — problems that only become obvious once the wheel is mounted and the car is back on the ground, by which point a return or refit is expensive and inconvenient.

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