Wheel Offset Calculator
Compare your current wheels against new ones: see exactly how much farther the new wheel pokes out or tucks in, the backspacing of each, how much inner clearance you gain or lose — with a cross-section visualizer. Plus a two-way ET ↔ backspacing converter and offset reference tables.
Compare current vs new wheels
—outer edge change
—inner edge change
Cross-section visualizer
Cross-section through the wheel center, viewed from above. Both wheels are drawn from the hub mounting face (orange line); “+outboard” points toward the fender. Widths are bead-seat widths — see the guide tab for the flange note.
ET ↔ backspacing converter
Backspacing = width ÷ 2 + ET ÷ 25.4. The reverse: ET = (backspacing − width ÷ 2) × 25.4.
Common backspacing by width and offset
Bead-seat backspacing in inches, computed live from the same formula as the calculator. Tape-measured values on a real wheel read about ½″ more (flange note in the guide).
Worked examples
How it works
Offset (ET) is the distance in millimeters between the wheel centerline and the mounting face. Positive ET puts the face outboard of the centerline, so the wheel tucks inward. Backspacing = width ÷ 2 + ET ÷ 25.4 (inches) — the distance from the face to the inner bead seat.
When you change wheels, two things move: the outer edge (poke/flush vs the fender) and the inner edge (clearance to struts and brake calipers). With Δoffset = old ET − new ET and Δwidth in inches:
- Outer change = Δoffset + Δwidth ÷ 2 × 25.4 mm (positive = farther out)
- Inner change = Δoffset − Δwidth ÷ 2 × 25.4 mm (negative = closer to suspension)
Honest scope. This is bead-seat geometry — it cannot see your fenders, liners, struts, or brakes. Warning bands are rules of thumb (~12 mm caution / ~25 mm risk outward; ~8 / ~20 mm inward), not a fitment guarantee. A tape measure on a real wheel reads about ½″ more backspacing than this formula because rim flanges extend ~12 mm past the bead seats on each side. Always test-fit before driving on a big change.
Related tool: changing wheels usually means changing tires too — use our tire size comparator to check overall diameter, speedometer error, and the will-it-fit verdict for the tire side.
Frequently asked questions
What is wheel offset (ET)?
Offset — stamped as ET (from the German "Einpresstiefe") — is the distance in millimeters between the wheel's centerline and its mounting face. A positive offset (ET40) means the mounting face sits 40 mm outboard of the centerline, so the wheel tucks inward. A zero offset puts the face exactly on the centerline, and a negative offset (ET−12) pushes the wheel outward, the classic deep-dish look.
What is backspacing?
Backspacing is the distance from the wheel's mounting face to its inner edge, usually given in inches. For example, an 8-inch wide wheel with ET40 has backspacing of 8 ÷ 2 + 40 ÷ 25.4 = 5.57 inches (141.6 mm). More backspacing means the wheel sits deeper in the wheel well.
How do I convert wheel offset (ET) to backspacing?
Backspacing (inches) = wheel width ÷ 2 + ET ÷ 25.4. For example, a 9-inch wheel with ET30 gives 9 ÷ 2 + 30 ÷ 25.4 = 5.68 inches (144.3 mm). To go the other way: ET (mm) = (backspacing − width ÷ 2) × 25.4. The converter tab above does both directions instantly.
What does a lower offset do to fitment?
Lowering the offset moves the whole wheel outward. On the same 8-inch width, going from ET40 to ET30 pushes the outer edge 10.0 mm farther out (more poke) and also moves the inner edge 10.0 mm outward, giving you more suspension clearance. Rule of thumb: every 1 mm of offset change moves the wheel 1 mm at the same width.
How much does a wider wheel poke out?
Adding width at the same offset splits evenly: half goes outward, half inward. Going from 8 to 9 inches wide at ET40 moves the outer edge 12.7 mm outward and the inner edge 12.7 mm inward toward the suspension. That is why a wider wheel often needs a lower offset to keep inner clearance.
What happens when I change both width and offset?
Both changes add up. Going from a 7.5-inch ET45 wheel to an 8.5-inch ET35 wheel moves the outer edge 22.7 mm farther out, while the inner edge sits 2.7 mm closer to the suspension. Enter your current and new wheels in the compare tab to see your exact numbers and the visual overlay.
What is negative offset, and when is it used?
Negative offset puts the mounting face inboard of the centerline, pushing the wheel outward for a deep-dish stance — common on trucks and classic muscle cars. A 10-inch wide wheel with ET−12 has 4.53 inches of backspacing, so most of the wheel sits outboard of the hub. It increases scrub radius and fender-rub risk, so check clearances carefully.
Will a lower offset cause fender rubbing?
It can. As a rough guide, up to about 12 mm of extra outward movement is usually harmless, 12–25 mm deserves a careful fender-clearance check (especially with wider tires), and beyond 25 mm you should verify with a test fit. This tool flags those bands automatically — but it cannot see your fenders, so treat it as guidance, not a guarantee.
Can a new wheel hit my suspension or brakes?
Yes, on the inner side. If the inner edge moves more than about 8 mm closer to the suspension, check strut and brake-caliper clearance; beyond 20 mm a test fit is strongly advised. Enter your current inner clearance in the compare tab and the tool will compute the remaining gap for your exact wheel change.
Why do tape-measured backspacing numbers differ from calculators?
Wheel width is measured between the bead seats, but the rim flanges stick out roughly 12 mm (about 1/2 inch) on each side. A tape measure from the mounting face to the outer lip of the flange therefore reads about 1/2 inch more than the bead-seat formula used here and by most online calculators. Both are correct — they just measure to different edges. This is documented so you can convert between the two.
Does offset affect handling?
Yes, through scrub radius — the distance between the tire's contact-patch center and the point where the steering axis meets the road. Lower offset (more poke) increases scrub radius, which can make steering feel heavier and transmit more kickback over bumps, while very high offset reduces it. Big changes (25 mm or more) are the ones most likely to be felt.
Is a wheel spacer the same as changing offset?
Effectively, yes: a 10 mm spacer on an ET45 wheel behaves like an ET35 wheel — it pushes the outer edge 10 mm farther out and moves the inner edge 10 mm away from the suspension, with no change in width. Spacers do not change backspacing measured on the wheel itself, but they change the installed position exactly like an offset change.
Free tool by NRTechStudio