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Wheel offset and backspacing calculator

Compare your current wheel with a new one to see the backspacing and how far the inner and outer edges move toward the suspension or the fender.

Your numbers

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Results

Check before you buy. Results are calculated from size codes and standard formulas; real tires vary by brand and model. Always check your vehicle placard and owner’s manual for approved sizes and load ratings, and ask a tire professional about fitment and clearance.

Offset and backspacing explained

Two numbers describe where a wheel sits relative to the hub:

  • Offset is the distance, in millimeters, from the wheel's centerline to its mounting face (the surface that bolts to the hub). Positive offset means the mounting face is toward the outside of the wheel, tucking the wheel in. Negative offset pushes the wheel out. Offset is often stamped as ET, for example ET35.
  • Backspacing is the distance, in inches, from the mounting face to the wheel's inner edge. More backspacing puts the wheel closer to the suspension and brakes.

They describe the same thing from different reference points, so you can convert between them if you know the wheel width.

The formulas

Overall wheel width ≈ nominal width + 1 in Backspacing (in) = overall width ÷ 2 + offset (mm) ÷ 25.4 Offset (mm) = (backspacing − overall width ÷ 2) × 25.4 Front spacing (in) = overall width − backspacing

A wheel's nominal width (for example "8 inches") is measured between the bead seats. The outer lips (flanges) add roughly half an inch on each side, so the overall width is about 1 inch more. That is the convention used here. For an exact answer, measure your wheel: lay it face down, place a straightedge across the back lip, and measure down to the mounting face.

Worked example

You have 8 in wide wheels with +30 mm offset and are considering 9 in wheels with 0 mm offset.

Current: backspacing = 9 ÷ 2 + 30 ÷ 25.4 = 4.5 + 1.18 = 5.68 in; front spacing = 9 − 5.68 = 3.32 in.

New: backspacing = 10 ÷ 2 + 0 = 5.00 in; front spacing = 5.00 in.

The inner edge moves 0.68 in further away from the suspension, and the outer edge moves 1.68 in further out toward the fender.

What the changes mean

  • Inner edge closer: less room for brake calipers, control arms, struts and sway-bar links. Check clearance with the wheel turned to full lock.
  • Outer edge further out ("poke"): wider stance and sometimes more room for a wide tire inside, but more chance of rubbing the fender, flares or bumper, more mud and stones thrown, and extra load on wheel bearings. Some places require tires to be covered by the bodywork.
  • Scrub radius: moving the wheel outward changes steering feel and can increase kickback over bumps.

Tires add their own width

This calculator compares wheel positions. The tire sidewall usually bulges beyond the rim, so a wider tire moves both its inner and outer faces further than the wheel edges do. If you change tire width too, add about half of the width increase to each side. Compare tire sizes with the tire size comparison calculator.

Before you buy

Also check bolt pattern, center bore (hub-centric rings if needed), load rating of the wheel, brake caliper clearance behind the spokes, and lug nut seat type. Wheel retailers and tire shops can confirm fitment for your vehicle. We do not include rim-width recommendations here because the approved rim widths for each tire size come from tire manufacturers and industry standards bodies; check the specification for your exact tire.

Frequently asked questions

What is the backspacing of an 8 inch wheel with 0 offset?

About 4.5 inches. With 0 offset the mounting face is at the center, and the overall width is about 9 inches including the flanges.

Does lower offset make the wheel stick out more?

Yes. Lowering offset (or going negative) moves the wheel outward. With the same width, each 25.4 mm of offset reduction moves the whole wheel out by 1 inch.

How do I measure backspacing?

Lay the wheel face down, put a straightedge across the back rim lip, and measure from the straightedge down to the mounting surface.

Is offset the same as ET?

Yes. ET (from the German Einpresstiefe) is the offset in millimeters, so ET35 means +35 mm.

Do wheel spacers change offset?

A spacer effectively reduces offset by its thickness: a 25 mm spacer on a +35 mm wheel acts like a +10 mm wheel. Use only quality hub-centric spacers installed to the correct torque.

Last reviewed November 2026. Spotted an error? Tell us.