How to use the wheel offset calculator
- Choose whether you know the offset (in mm, often stamped "ET35") or the backspacing (in inches).
- For your current wheel, enter the size (diameter x width, like 18x8) and the offset or backspacing. Add a spacer if you run one.
- Do the same for the new wheel.
- Optionally add the tire size on each wheel to see where the tire sidewalls end up too.
- Read how far the new wheel sits out or in (poke), the change in inner clearance and track width, and the full side-by-side table.
What is wheel offset?
Offset is the distance from the center line of the wheel to its mounting face (the flat surface that bolts to the hub), in millimeters. It sets where the wheel sits in the wheel well:
- Positive offset (+1 to about +55): the mounting face is toward the outside, so the wheel sits in. Most cars, SUVs and newer trucks.
- Zero offset: the mounting face is exactly on the center line.
- Negative offset (below 0): the mounting face is toward the inside, so the wheel pokes out. Common on lifted trucks and "deep dish" styles.
Lower offset always pushes the wheel out; higher offset pulls it in. The number is usually stamped on a spoke or inside the barrel as ET (e.g. ET35 = +35 mm).
Offset vs backspacing
Backspacing is the distance from the mounting face to the inner lip, in inches. It depends on both the offset and the wheel width: backspacing = (width + 1) ÷ 2 + offset ÷ 25.4. The extra inch accounts for the half-inch lip on each side of the nominal width. Truck and off-road wheels are often sold by backspacing; car wheels by offset.
Example: an 18x8 wheel at +45 has 6.27 in of backspacing and 2.73 in of front spacing. Backspacing tells you inner clearance (struts, control arms); front spacing tells you how far the wheel reaches toward the fender.
Example: 18x8 +45 vs 18x9 +35
Moving to a wider wheel with a lower offset is a common upgrade. The new wheel's outer lip sits 22.7 mm further out: 10 mm from the lower offset plus 12.7 mm from the extra half-inch of width on the outside. The inner lip moves 2.7 mm closer to the suspension, and the track widens by 20 mm (+10 mm per side).
On a truck, going from 17x8 +18 to 17x9 −12 pushes the outer lip out 42.7 mm (1.68 in) while gaining 17.3 mm of inner clearance.
Backspacing chart
Backspacing in inches for common wheel widths and offsets.
| Width \ Offset | -44 mm | -25 mm | -12 mm | 0 mm | +12 mm | +25 mm | +35 mm | +45 mm |
|---|---|---|---|---|---|---|---|---|
| 7 in | 2.27 | 3.02 | 3.53 | 4.00 | 4.47 | 4.98 | 5.38 | 5.77 |
| 7.5 in | 2.52 | 3.27 | 3.78 | 4.25 | 4.72 | 5.23 | 5.63 | 6.02 |
| 8 in | 2.77 | 3.52 | 4.03 | 4.50 | 4.97 | 5.48 | 5.88 | 6.27 |
| 8.5 in | 3.02 | 3.77 | 4.28 | 4.75 | 5.22 | 5.73 | 6.13 | 6.52 |
| 9 in | 3.27 | 4.02 | 4.53 | 5.00 | 5.47 | 5.98 | 6.38 | 6.77 |
| 9.5 in | 3.52 | 4.27 | 4.78 | 5.25 | 5.72 | 6.23 | 6.63 | 7.02 |
| 10 in | 3.77 | 4.52 | 5.03 | 5.50 | 5.97 | 6.48 | 6.88 | 7.27 |
| 12 in | 4.77 | 5.52 | 6.03 | 6.50 | 6.97 | 7.48 | 7.88 | 8.27 |
How to measure wheel offset
- Lay the wheel face down. Put a straightedge across the back (inner) lip.
- Measure from the straightedge down to the mounting pad: that's the backspacing.
- Measure the overall width from lip edge to lip edge (about the nominal width + 1 in).
- Offset (mm) = (backspacing − overall width ÷ 2) × 25.4. For a 9 in wheel (10 in overall) with 5 in of backspacing: (5 − 5) × 25.4 = 0 mm. With 6 in: 25.4 mm.
Wheel spacers and offset
A spacer adds its thickness between the hub and the wheel, which lowers the effective offset by the same amount. An 18x8 +45 wheel on a 15 mm spacer sits like an 18x8 +30 wheel: 15 mm further out, with 15 mm more inner clearance. Use hub-centric spacers of a quality brand, and re-torque the wheels after the first drive.
Related tire and wheel tools
- Tire size comparison: how much taller and wider a new tire is, and how it changes your speedometer.
- Tire size calculator: dimensions of any tire size and equivalent sizes for other wheel diameters.
Frequently asked questions
What is wheel offset?
Wheel offset is the distance, in millimeters, from the center line of the wheel to the surface that bolts against the hub. It decides how far in or out the wheel sits in the wheel well. It is stamped on most wheels as ET (from the German Einpresstiefe) followed by the number, for example ET35 or +35.
What is the difference between positive and negative offset?
With positive offset the mounting face is toward the outside of the wheel, so the wheel sits further in (most cars and modern trucks). With zero offset the mounting face is exactly on the center line. With negative offset the mounting face is toward the inside, so the wheel sticks out further (common on lifted trucks with a "deep dish" look).
What does +35 offset mean?
The wheel's mounting face is 35 mm outboard of its center line, so most of the wheel sits inboard of the hub. Going from +45 to +35 on the same width wheel moves the whole wheel 10 mm further out, toward the fender.
How do I convert offset to backspacing?
Backspacing (in) = (wheel width + 1) ÷ 2 + offset (mm) ÷ 25.4. The +1 inch allows for the lips on each side. An 18x8 wheel with +45 offset has (8 + 1) ÷ 2 + 45 ÷ 25.4 = 6.27 in of backspacing. To go the other way, offset = (backspacing − (width + 1) ÷ 2) × 25.4.
How much will a wider wheel stick out?
Each extra inch of width adds about 12.7 mm on each side at the same offset. Going from 18x8 +45 to 18x9 +35, the outer lip moves 22.7 mm out (10 mm from the lower offset plus 12.7 mm from the extra width) and the inner lip moves 2.7 mm closer to the suspension.
Does a wheel spacer change the offset?
Yes. A spacer moves the wheel outward by its thickness, so the effective offset is the wheel offset minus the spacer. An 18x8 +45 wheel on a 15 mm spacer behaves like a +30 wheel: it sits 15 mm further out and gains 15 mm of inner clearance. Enter the spacer thickness in the calculator.
How do I measure wheel offset?
Lay the wheel face down, put a straightedge across the back lip and measure from it down to the mounting pad: that is the backspacing. Measure the overall width lip to lip. Offset (mm) = (backspacing − overall width ÷ 2) × 25.4. A positive result is positive offset. Most wheels also have the offset stamped on a spoke or inside the barrel.
What offset should I choose?
Start from your vehicle's factory offset and width. Staying within about 5 mm of the stock outer position usually keeps the tire under the fender, and staying within a few millimeters of the stock inner position avoids rubbing on struts and control arms. Big changes also affect scrub radius, steering feel and wheel bearing load, so check fitment for your exact vehicle before buying.
Is my data saved or sent anywhere?
Your entries are saved only in your own browser so they are there next time. Nothing is sent to our servers.