Wheel Offset Calculator

Compare your current wheels with a new set. Enter each wheel's width and offset to see its backspacing, and how far the new wheel will sit toward the suspension on the inside and toward the fender on the outside.

Current vs New Wheel

Offset and backspacing

Offset is the distance, in millimetres, from the wheel's centerline to the face that bolts against the hub. Positive offset puts the mounting face toward the outside of the wheel, which tucks the wheel in under the car. Negative offset pushes it out. Offset is stamped on most wheels as ET followed by a number, such as ET45.

Backspacing is the distance, in inches, from the mounting face to the inner edge of the wheel. It tells you directly how much room the wheel takes up on the suspension side.

Overall width = rim width + 1 inch (the two flanges) Backspacing = overall width ÷ 2 + offset (mm) ÷ 25.4 Front spacing = overall width − backspacing

The extra inch is for the lips, or flanges, on each side of the rim. Wheel widths such as 8.5 inches are measured between the bead seats, not across the outer edges.

Worked example

Going from a 7.5 inch wheel at ET45 to an 8.5 inch wheel at ET35:

  • Current backspacing: 8.5 ÷ 2 + 45 ÷ 25.4 = 6.02 in
  • New backspacing: 9.5 ÷ 2 + 35 ÷ 25.4 = 6.13 in
  • The inner face moves about 2.7 mm closer to the suspension.
  • The outer face moves about 22.7 mm further out toward the fender.

The wider wheel and the lower offset both push the outside outward, so nearly all of the extra width ends up as poke.

What the numbers don't tell you

  • Tire width. The tire sidewall usually bulges past the rim. A wider tire on the new wheel adds clearance problems of its own.
  • Brake caliper clearance. Offset doesn't capture the shape of the spokes. A wheel can fit by offset and still hit a large caliper.
  • Bolt pattern and center bore. These must match the hub. Hub-centric rings fill a larger bore, but they don't fix a wrong bolt pattern.
  • Load rating. The wheel must be rated for the vehicle's axle load.
  • Suspension travel and steering lock. A wheel that clears when parked can still rub at full lock or under compression.

Spacers

A spacer reduces effective offset by its thickness: a 10 mm spacer on an ET45 wheel behaves like ET35 at the outer face. Use only hub-centric spacers from reputable makers, with correct-length studs or bolts, and re-check torque after the first drive.

Treat these figures as a starting point. Before buying, confirm fitment with the wheel retailer or a tire shop, ideally with a test fit at full steering lock.

Frequently Asked Questions

Offset is the distance in millimetres from the centerline of the wheel to its hub mounting face. Positive offset tucks the wheel in toward the suspension; negative offset pushes it out toward the fender. It is usually stamped on the wheel as ET followed by a number.

Add 1 inch to the rim width for the flanges, halve it, then add the offset converted to inches (offset in mm divided by 25.4). An 8.5 inch wheel at ET35 has a backspacing of 9.5 ÷ 2 + 35 ÷ 25.4 = 6.13 inches.

Yes. Lowering the offset moves the mounting face toward the inside of the wheel, so the whole wheel sits further out. Going from ET45 to ET35 on the same width moves the wheel about 10 mm outward at both the inner and outer faces.

It can, on either side. Extra width splits between the inside and outside according to the offset change. Check both the inner change, toward the suspension, and the outer change, toward the fender, then allow for the tire sidewall and steering lock.