Wheel Offset (ET) Calculator
Compare two wheels to see how far the inner and outer edges move, include a spacer, or convert between offset in millimetres and backspacing in inches.
Backspacing to offset chart
Offset in mm for common wheel widths. Positive = wheel sits further inside the arch.
| Backspacing | 7 in wide | 8 in wide | 9 in wide | 10 in wide | 12 in wide |
|---|---|---|---|---|---|
| 3.5 in | -13 mm | -25 mm | -38 mm | -51 mm | -76 mm |
| 3.75 in | -6 mm | -19 mm | -32 mm | -44 mm | -70 mm |
| 4 in | 0 mm | -13 mm | -25 mm | -38 mm | -64 mm |
| 4.25 in | +6 mm | -6 mm | -19 mm | -32 mm | -57 mm |
| 4.5 in | +13 mm | 0 mm | -13 mm | -25 mm | -51 mm |
| 4.75 in | +19 mm | +6 mm | -6 mm | -19 mm | -44 mm |
| 5 in | +25 mm | +13 mm | 0 mm | -13 mm | -38 mm |
| 5.25 in | +32 mm | +19 mm | +6 mm | -6 mm | -32 mm |
| 5.5 in | +38 mm | +25 mm | +13 mm | 0 mm | -25 mm |
| 5.75 in | +44 mm | +32 mm | +19 mm | +6 mm | -19 mm |
| 6 in | +51 mm | +38 mm | +25 mm | +13 mm | -13 mm |
| 6.5 in | +64 mm | +51 mm | +38 mm | +25 mm | 0 mm |
| 7 in | +76 mm | +64 mm | +51 mm | +38 mm | +13 mm |
Offset to backspacing chart
Backspacing in inches for common offsets (ET) and wheel widths.
| Offset (ET) | 6.5 in wide | 7 in wide | 7.5 in wide | 8 in wide | 8.5 in wide | 9 in wide | 10 in wide |
|---|---|---|---|---|---|---|---|
| ET-25 | 2.77 in | 3.02 in | 3.27 in | 3.52 in | 3.77 in | 4.02 in | 4.52 in |
| ET-12 | 3.28 in | 3.53 in | 3.78 in | 4.03 in | 4.28 in | 4.53 in | 5.03 in |
| ET0 | 3.75 in | 4.00 in | 4.25 in | 4.50 in | 4.75 in | 5.00 in | 5.50 in |
| ET10 | 4.14 in | 4.39 in | 4.64 in | 4.89 in | 5.14 in | 5.39 in | 5.89 in |
| ET15 | 4.34 in | 4.59 in | 4.84 in | 5.09 in | 5.34 in | 5.59 in | 6.09 in |
| ET20 | 4.54 in | 4.79 in | 5.04 in | 5.29 in | 5.54 in | 5.79 in | 6.29 in |
| ET25 | 4.73 in | 4.98 in | 5.23 in | 5.48 in | 5.73 in | 5.98 in | 6.48 in |
| ET30 | 4.93 in | 5.18 in | 5.43 in | 5.68 in | 5.93 in | 6.18 in | 6.68 in |
| ET35 | 5.13 in | 5.38 in | 5.63 in | 5.88 in | 6.13 in | 6.38 in | 6.88 in |
| ET40 | 5.32 in | 5.57 in | 5.82 in | 6.07 in | 6.32 in | 6.57 in | 7.07 in |
| ET45 | 5.52 in | 5.77 in | 6.02 in | 6.27 in | 6.52 in | 6.77 in | 7.27 in |
| ET50 | 5.72 in | 5.97 in | 6.22 in | 6.47 in | 6.72 in | 6.97 in | 7.47 in |
Offset, ET, inset and backspacing explained
How to measure wheel offset
Lay the wheel face-down and place a straight bar across the back edge. Measure from the bar down to the mounting surface: that is the backspacing. Measure the overall width of the wheel from lip to lip and use half of it as the centreline. Offset is the backspacing minus that half-width, converted to millimetres. If the wheel carries an ET number stamped on the back, use that rather than measuring.
How the calculation works
The inner edge of a wheel sits at half the width plus the offset from the mounting face, and the outer edge sits at half the width minus the offset. Comparing two wheels, the change in the inner edge is (new width ÷ 2 + new ET) − (old width ÷ 2 + old ET), and the change in the outer edge is the same calculation with the sign of the offset reversed. A spacer lowers the effective offset by its thickness. For wheel width, the page uses the nominal size in inches (for example 8 for an 8J wheel) and converts it to millimetres.
This tool shows how a wheel change moves the wheel relative to your current setup. It cannot tell you whether a wheel will fit your particular car: that depends on the suspension, brakes and arches, so check with the wheel supplier.
Worked examples
The wheel is an inch wider (25.4 mm) and the offset is 5 mm lower. Half the extra width goes to each side, so each edge moves 12.7 mm, and the lower offset pushes the whole wheel 5 mm further out.
The outer edge moves 12.7 + 5 = 17.7 mm outward. The inner edge moves 12.7 − 5 = 7.7 mm inward, towards the suspension. The track is 10 mm wider overall.
A spacer behaves like a lower offset. A 15 mm spacer on an ET35 wheel gives an effective offset of ET20. Compared with the same wheel without a spacer, the whole wheel sits 15 mm further out, and both edges move with it. Against the original 7J ET40 wheel, the outer edge is now 32.7 mm further out and the inner edge has moved 7.3 mm outward, away from the suspension.
A wider tire changes things too. Going from a 205 to a 225 tire adds 20 mm of width, so each sidewall sits roughly 10 mm further out and 10 mm further in than before (real tires vary a bit with the rim width). Add that to the wheel change above and you have the full picture of where the rubber ends up.
Reading the numbers stamped on a wheel
Most wheels have their specs on the back of a spoke or inside the barrel. Here's how to read a typical one: 8Jx17 ET35 5x112 CB66.6.
| Code | Means |
|---|---|
| 8J | The rim is 8 inches wide. The J is a code for the shape of the rim flange. |
| x17 | The rim diameter is 17 inches. |
| ET35 | The offset is +35 mm. |
| 5x112 | 5 bolts on a circle 112 mm across (the PCD, or pitch circle diameter). |
| CB66.6 | The centre bore, the hole in the middle, is 66.6 mm. |
The offset can't make up for a wrong bolt pattern or centre bore. The PCD must match your hub exactly. If the wheel's centre bore is bigger than your hub, a hub-centric ring fills the gap so the wheel sits centred on the hub, not hanging from the bolts.
How much change is okay?
There's no number that suits every car. As a starting point, staying within about 5 mm of the original keeps things close to what the engineers designed. Beyond that, it's worth checking clearance at full steering lock and over bumps, brake callipers, and any rules about wheels sticking out beyond the arch where you live. The best test is a supplier who knows your car, or a careful test fit.
Mistakes to avoid
- Comparing the offset on its own. The same offset on a wider wheel puts the edges in different places. Always compare the width and the offset together.
- Mixing up the sign. A higher positive offset pulls the wheel in. A lower or negative offset pushes it out. If your edges are moving the wrong way, check the sign.
- Forgetting the tire. A wheel might clear, but a wider tire on it might not.
- Treating spacers as risk-free. Use hub-centric spacers of good quality, make sure the wheel studs or bolts are long enough, and re-check the torque after a short drive.
- Trusting a rough conversion. Backspacing and offset conversions assume a typical rim flange. Real wheels vary a little, so for a tight fit, measure the wheel itself.
A wheel that sits a few millimetres too far in or out can rub the suspension, catch the arch or change how the car steers. Those few millimetres are easy to measure before you buy and expensive to fix afterwards.
Related tools
Frequently asked questions
What is wheel offset (ET)?
Wheel offset, written ET in Europe and South Africa (from the German Einpresstiefe), is the distance in millimetres from the centreline of the wheel to the face that bolts to the hub. A positive offset means the mounting face sits towards the outside of the wheel, which tucks the wheel further in. Zero offset means the mounting face is on the centreline, and a negative offset pushes the wheel further out.
What is the difference between offset and backspacing?
Offset is measured from the centreline of the wheel and is quoted in millimetres. Backspacing is the distance from the mounting face to the rear edge of the wheel, usually quoted in inches. They describe the same thing: for a given wheel width, one can be converted to the other. This calculator uses the common convention that the overall width is the nominal width plus about 1 inch, so an 8-inch wheel with 0 offset has 4.5 inch backspacing.
How do I convert backspacing to offset?
Subtract half of the overall wheel width from the backspacing, then multiply by 25.4 to get millimetres. For an 8-inch wheel with 4.5 inch backspacing: overall width = 8 + 1 = 9 in, half = 4.5 in, 4.5 − 4.5 = 0, so the offset is 0 mm. With 5.5 inch backspacing it is (5.5 − 4.5) × 25.4 = +25.4 mm.
What is the difference between inset and offset?
Inset is another word for positive offset: how far the mounting face is moved towards the outside of the wheel and so the wheel is pulled in towards the car. Some wheel brands quote "inset" for the same measurement as offset or ET. A larger positive number always means the wheel sits further inside the arch.
What does ET35 or ET40 mean on a wheel?
It is the offset in millimetres. A wheel marked ET35 has its mounting face 35 mm outboard of the wheel centreline. The same number on different wheel widths puts the wheel in different positions, so always compare the width and the offset together.
How do wheel spacers change offset?
A spacer pushes the wheel outwards by its thickness, so it lowers the effective offset by that many millimetres. A 10 mm spacer on an ET40 wheel gives an effective offset of ET30. Enter the spacer thickness in the compare tool to include it.