Wheel offset calculator
Compare your wheel with a new one: enter the width and offset (ET) of both, and a spacer if you’ll run one. The calculator shows how many millimeters the wheel’s outer edge moves out or in, how much closer its inner edge gets to the suspension, and the backspacing, with a side view of both wheels. It’s free and runs in your browser.
| Wheel | Backspacing | Front spacing |
|---|---|---|
| Current, 20 x 9.5 ET45 | 7.02 in. | 3.48 in. |
| New, 20 x 10 ET35 | 6.88 in. | 4.12 in. |
How the math works
A wheel’s offset is how far its mounting face sits from its centerline. Make the wheel wider and half of the extra width goes out, half goes in. Lower the offset and the whole wheel moves out by the same number of millimeters. So:
- Outer edge moves out = (new width − current width) × 25.4 ÷ 2 − (new offset − current offset) + spacer
- Inner edge moves in = (new width − current width) × 25.4 ÷ 2 + (new offset − current offset) − spacer
- Backspacing (in.) = (width + 1) ÷ 2 + offset ÷ 25.4, with the usual 1 in. for the two rim flanges
Example: 20 x 9.5 ET45 to 20 x 10 ET35 is 0.5 in. wider (6.35 mm a side) with 10 mm less offset, so the outer edge moves about 16 mm out and the inner edge about 4 mm away from the suspension. A negative answer means the edge moves the other way.
Poke, tuck and spacers
Poke is a wheel or tire sitting out past the fender; tuck is sitting inside it. Owners chasing a flush look move the wheel out a few millimeters at a time with a lower offset, a wider wheel or a spacer. A spacer moves the whole wheel out by its thickness, the same as lowering the offset by that many millimeters, and it takes the same amount off the backspacing. Spacers need longer bolts or studs to match, and hub-centric ones keep the wheel centered; ask your installer.
Moving the inner edge in brings the wheel closer to the strut, spring and brake caliper, and a bigger brake needs room behind the spokes as well as inside the barrel. Neither shows up in the offset alone, so check with the wheel seller.
Stock wheels of the 3D cars
The calculator’s starting points, as the fitment guides linked below list them. Check the size stamped on the back of your own wheel before you buy.
| Car | Bolt pattern | Front | Rear |
|---|---|---|---|
| Tesla Model Y (2025+), 20 in. T Sportline | 5x114.3 | 20 x 9.5 ET45 | 20 x 9.5 ET45 |
| Tesla Cybertruck Unplugged Performance | 6x139.7 | 20 x 9 ET58.5 | 20 x 9 ET58.5 |
| BMW M3 Competition (G80) FMB | 5x112 | 19 x 9.5 ET20 | 20 x 10.5 ET20 |
| BMW M3 (F80), 19 in. FMB | 5x120 | 19 x 9 ET29 | 19 x 10 ET40 |
See the new wheel on a 3D car
Pick one of the cars above in the calculator and it links to that car in the 3D visualizer with the new size fitted, so you can see the poke from the side and the front. The visualizer’s wheel styles are unbranded, in common aftermarket looks, in the sizes and finishes each car takes. Open it on a BMW M4, a Tesla Model Y or a Cybertruck.
To compare wheel styles and finishes on every 3D car, use the wheel visualizer.
Questions
What is wheel offset?
Offset (ET) is the distance in millimeters from the wheel’s centerline to the face that bolts to the hub. Positive offset puts the mounting face toward the outside of the wheel, so the wheel sits further in; zero is centered; negative offset pushes the wheel out.
How do I work out how much a new wheel will poke?
Take half the change in width in millimeters (1 in. wider is 12.7 mm more on each side), then subtract the change in offset. A spacer adds its thickness. 9.5 in. ET45 to 10 in. ET35: 6.35 + 10 = about 16 mm further out.
Does a lower offset push the wheel out?
Yes. With the same width, every millimeter less offset moves the whole wheel a millimeter out: the outer edge pokes more and the inner edge moves away from the suspension. A higher offset does the opposite.
What is the difference between offset and backspacing?
Offset is measured from the wheel’s centerline, in millimeters. Backspacing is the distance from the hub face to the wheel’s inner edge, in inches. For a 9.5 in. wheel at ET45 it’s about 7.02 in.: (9.5 + 1) ÷ 2 + 45 ÷ 25.4, where the 1 in. is the two rim flanges.
Will a new wheel fit my car?
The calculator shows where the new wheel sits compared with your current one. It can’t see your fenders, brakes or suspension, or the tire you’ll run. Ask the wheel seller or your installer to confirm fitment for your exact car before you buy.
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