Wheel offset calculator
Enter your current wheel and the one you are looking at: width, offset and any spacer, plus tire sizes if you want the tire edges too. You see how far the wheel and tire move out toward the fender and in toward the suspension. The second tab converts offset to backspacing and back.
Inputs
Result
Where the wheel sits
Seen from above, hub face in the middle. Nominal sizes, drawn to scale. The dashed lines are the wheel centrelines.
Side by side
What offset and backspacing are
Offset is the distance in millimetres from the wheel’s centreline to the face that bolts to the hub. Positive offset (+35, ET35) puts the mounting face toward the outside of the wheel, so the wheel sits further in. Negative offset (−12, −44) puts it toward the inside, so the wheel sticks further out. Zero offset means the mounting face is right on the centreline.
Backspacing is the distance in inches from the mounting face to the inner lip of the wheel, the edge nearest the suspension. It is the same idea measured a different way, and the one most truck and off-road wheels are sold by.
The formulas
Worked example
Going from a 17x8 +35 to a 17x9 −12. The old wheel has 5.88 in of backspacing and 3.12 in in front of the hub; the new one has 4.53 in and 5.47 in. So the outer lip moves 2.35 in (60 mm) further out, the inner lip moves 1.35 in outward, and the centreline moves 47 mm out. With 265/70R17 going to 285/70R17 tires, the tire’s outer edge moves 57 mm out and its inner edge 37 mm out.
Offset to backspacing chart
Backspacing in inches, by offset (mm, rows) and wheel width (in, columns). Assumes the usual 1 in of flange.
| Offset | 7 in | 7.5 in | 8 in | 8.5 in | 9 in | 10 in | 12 in |
|---|---|---|---|---|---|---|---|
| −44 | 2.27 | 2.52 | 2.77 | 3.02 | 3.27 | 3.77 | 4.77 |
| −38 | 2.50 | 2.75 | 3.00 | 3.25 | 3.50 | 4.00 | 5.00 |
| −25 | 3.02 | 3.27 | 3.52 | 3.77 | 4.02 | 4.52 | 5.52 |
| −18 | 3.29 | 3.54 | 3.79 | 4.04 | 4.29 | 4.79 | 5.79 |
| −12 | 3.53 | 3.78 | 4.03 | 4.28 | 4.53 | 5.03 | 6.03 |
| 0 | 4.00 | 4.25 | 4.50 | 4.75 | 5.00 | 5.50 | 6.50 |
| +12 | 4.47 | 4.72 | 4.97 | 5.22 | 5.47 | 5.97 | 6.97 |
| +18 | 4.71 | 4.96 | 5.21 | 5.46 | 5.71 | 6.21 | 7.21 |
| +25 | 4.98 | 5.23 | 5.48 | 5.73 | 5.98 | 6.48 | 7.48 |
| +35 | 5.38 | 5.63 | 5.88 | 6.13 | 6.38 | 6.88 | 7.88 |
| +45 | 5.77 | 6.02 | 6.27 | 6.52 | 6.77 | 7.27 | 8.27 |
Backspacing to offset chart
Offset in mm, rounded to the nearest whole mm, by backspacing (in, rows, in quarter inches) and wheel width (in, columns).
| Backspacing | 7 in | 8 in | 8.5 in | 9 in | 10 in | 12 in |
|---|---|---|---|---|---|---|
| 3 | −25 | −38 | −44 | −51 | −64 | −89 |
| 3.25 | −19 | −32 | −38 | −44 | −57 | −83 |
| 3.5 | −13 | −25 | −32 | −38 | −51 | −76 |
| 3.75 | −6 | −19 | −25 | −32 | −44 | −70 |
| 4 | 0 | −13 | −19 | −25 | −38 | −64 |
| 4.25 | +6 | −6 | −13 | −19 | −32 | −57 |
| 4.5 | +13 | 0 | −6 | −13 | −25 | −51 |
| 4.75 | +19 | +6 | 0 | −6 | −19 | −44 |
| 5 | +25 | +13 | +6 | 0 | −13 | −38 |
| 5.25 | +32 | +19 | +13 | +6 | −6 | −32 |
| 5.5 | +38 | +25 | +19 | +13 | 0 | −25 |
| 5.75 | +44 | +32 | +25 | +19 | +6 | −19 |
| 6 | +51 | +38 | +32 | +25 | +13 | −13 |
| 6.25 | +57 | +44 | +38 | +32 | +19 | −6 |
| 6.5 | +64 | +51 | +44 | +38 | +25 | 0 |
| 6.75 | +70 | +57 | +51 | +44 | +32 | +6 |
| 7 | +76 | +64 | +57 | +51 | +38 | +13 |
How to find your wheel’s offset
- Look inside the wheel. Most wheels have the size and offset cast or stamped on the back of a spoke or the inside of the barrel, such as 17x9 ET-12, +35 or OFFSET 45.
- Measure the backspacing. Take the wheel off, lay it face down, put a straightedge across the back lip, and measure from the straightedge down to the mounting face. Then use the converter tab with the wheel width.
- Check the wheel maker’s listing for the exact part number. Flange thickness varies a little by wheel, so their backspacing figure is more exact than the formula.
Spacers
A spacer pushes the wheel out by its own thickness, so it works the same as that much less offset: a +35 wheel on a 20 mm spacer sits like a +15. It reduces backspacing and adds the same amount of poke. Use hub-centric spacers of a known quality, and make sure the studs or bolts still engage fully.
What a change in offset does
Less offset (or a spacer) widens the track and moves the tire out toward the fender: a more planted look and stance, but more chance of rubbing the fender lip, more scrub radius (heavier steering and more kickback), and more load on wheel bearings. More offset tucks the wheel in, closer to the strut, brake caliper and control arms. A wider wheel at the same offset grows by half the extra width on each side. Inner clearance and outer clearance are both worth checking at full lock and with the suspension compressed.
Questions
What does +35 offset mean?
The wheel's mounting face is 35 mm outboard of its centreline, toward the face of the wheel. Most of the wheel sits inboard, toward the suspension. Positive offsets like +35 to +45 are typical on cars and crossovers.
What does negative 12 offset mean?
The mounting face is 12 mm inboard of the centreline, so the wheel sits slightly further out than a zero offset wheel. On a 17x9 it gives about 4.53 in of backspacing, a common truck and off-road fitment.
What does negative 44 offset mean?
The mounting face is 44 mm inboard of the centreline, so most of the wheel sits outside the hub and the tire sticks well out. A 20x10 −44 has about 3.77 in of backspacing. It is a lifted-truck style that usually needs trimming or fender flares.
How do I find out what my offset is?
Look on the back of a spoke or inside the barrel: most wheels have it stamped, often as ET followed by the number. If not, measure backspacing with a straightedge across the back lip and convert it with the wheel width.
4.5 backspacing equals what offset?
It depends on the wheel width: +13 mm on a 7 in wide wheel, 0 mm on an 8 in wide wheel, −13 mm on a 9 in wide wheel, −25 mm on a 10 in wide wheel.
5.5 backspacing equals what offset?
+38 mm on a 7 in wide wheel, +25 mm on an 8 in wide wheel, +13 mm on a 9 in wide wheel, 0 mm on a 10 in wide wheel.
6 backspacing equals what offset?
+51 mm on a 7 in wide wheel, +38 mm on an 8 in wide wheel, +25 mm on a 9 in wide wheel, +13 mm on a 10 in wide wheel.
4.75 backspacing equals what offset?
+19 mm on a 7 in wide wheel, +6 mm on an 8 in wide wheel, −6 mm on a 9 in wide wheel, −19 mm on a 10 in wide wheel.
What backspacing is 0 offset?
Half the wheel's overall width, because a zero offset wheel is mounted on its centreline: about 4.5 in on an 8 in wide wheel, 5 in on a 9 in, 5.5 in on a 10 in and 6.5 in on a 12 in.
What offset is 3.5 backspacing?
−13 mm on a 7 in wide wheel, −25 mm on an 8 in wide wheel, −38 mm on a 9 in wide wheel, −51 mm on a 10 in wide wheel.
How much offset is 1.5 inches?
1.5 in is 38.1 mm, so about 38 mm of offset. Offset is the distance between the mounting face and the wheel centreline; to compare it with backspacing you also need the wheel width.
Does a wheel spacer change the offset?
Yes, by its thickness. A 25 mm spacer on a +35 wheel makes it sit like a +10: the wheel moves 25 mm out, and backspacing drops by about an inch.
Is higher or lower offset better?
Neither in general. The best offset is the one that clears the suspension and brakes on the inside and the fender on the outside. Staying close to the original offset keeps steering feel and bearing load as designed.