Wheel Offset and Backspacing Calculator
A wheel offset and backspacing calculator turns one fitment number into the other. Enter your wheel width and either offset (in millimeters) or backspacing (in inches). The formula is backspacing = (wheel width + 1) / 2 + offset / 25.4. For example, an 8 inch wide wheel with a +25.4 mm offset has 5.5 inches of backspacing.
Offset is the number most wheel makers stamp on the back of the wheel. Backspacing is the number that is easy to measure with a tape. Use this page to move between the two before you buy. New to fitment? Our wheel fitment guide also covers bolt pattern and hub bore.
Offset to Backspace Calculator
Enter your offset in millimeters and your wheel width in inches. Backspacing shows in inches. Use a minus sign for negative offset, like -12.
Try it: an 8 inch wide wheel with 25.4 mm offset gives 5.5 inches of backspacing.
Backspace to Offset Calculator
Enter your backspacing in inches and your wheel width in inches. Offset shows in millimeters and can be positive or negative.
Try it: an 8 inch wide wheel with 5.5 inches of backspacing gives +25.4 mm of offset.
What Is Wheel Offset?
Wheel offset is the distance, in millimeters, from the wheel’s centerline to its mounting surface. The mounting surface is the flat part that sits against your hub. The centerline is the middle of the wheel, measured from lip to lip.
Offset decides how far the wheel sits in or out compared to your fender and suspension. Many wheels have it stamped on the back, often as “ET” followed by a number. A few millimeters can change how the wheel looks and whether it rubs, so check it before you order.
Negative Offset
A negative offset means the mounting surface sits behind the centerline, toward the back of the wheel. That pushes the wheel out toward the fender. You see negative offset on lifted trucks, deep dish wheels, and wide stance builds. Our guide on how the right truck rims change your ride shows what that look can do. For that style, browse Fuel, Moto Metal, and XD wheels.
Positive Offset
A positive offset means the mounting surface sits in front of the centerline, toward the face of the wheel. That pulls the wheel in toward the suspension. Most factory wheels on modern cars use positive offset, especially front wheel drive cars. It keeps the tire tucked inside the fender. For car wheels, see TSW, Vossen, or Enkei. Our TSW fitment guide for BMW, Audi, and Honda shows real examples.
Zero Offset
A zero offset means the mounting surface sits exactly on the centerline. The wheel has the same amount of rim on each side of the hub. A 9 inch wide wheel with zero offset has 5 inches of backspacing.
Backspacing vs Offset: How They’re Connected
Offset and backspacing describe the same wheel from two starting points. Offset starts at the centerline and uses millimeters. Backspacing starts at the back edge of the wheel and uses inches. If you know the wheel width, you can convert either one into the other.
| Offset | Backspacing | |
|---|---|---|
| Starts at | Wheel centerline | Back edge of the wheel |
| Ends at | Mounting surface | Mounting surface |
| Unit | Millimeters | Inches |
| Can it be negative? | Yes | No |
| Where you find it | Stamped on the wheel as ET | Measured by hand or listed by the seller |
| Needs wheel width to convert | Yes | Yes |
Offset to backspacing: Backspacing (in) = (Wheel Width + 1) / 2 + Offset (mm) / 25.4
Backspacing to offset: Offset (mm) = (Backspacing (in) – (Wheel Width + 1) / 2) x 25.4
The “+ 1” covers the wheel lips. A wheel sold as 8 inches wide measures about 9 inches from outer lip to inner lip, so its centerline sits 4.5 inches from the back. Tire retailers and wheel makers publish this same formula. Lip size varies a little by wheel, so results are close to a quarter inch. Measure the real wheel before you order.
Worked examples
| Wheel width | Offset | Backspacing |
|---|---|---|
| 8″ | +25.4 mm | 5.50″ |
| 8.5″ | +20 mm | 5.54″ |
| 9″ | +12 mm | 5.47″ |
| 9″ | -12 mm | 4.53″ |
| 10″ | 0 mm | 5.50″ |
| 10″ | -19 mm | 4.75″ |
Going the other way: a 9.5 inch wide wheel with 6.25 inches of backspacing has a +25.4 mm offset. An 8 inch wheel with 4.5 inches of backspacing has a 0 mm offset.
One more rule: at the same offset, every extra inch of wheel width adds half an inch (12.7 mm) on the inside and half an inch on the outside.
Wheel Backspacing to Offset Chart
Use this chart when you only have a backspacing number. Offsets are rounded to the nearest millimeter.
| Wheel width | 4″ | 4.5″ | 5″ | 5.5″ | 6″ | 6.5″ |
|---|---|---|---|---|---|---|
| 7″ | 0 | +13 | +25 | +38 | +51 | +64 |
| 8″ | -13 | 0 | +13 | +25 | +38 | +51 |
| 8.5″ | -19 | -6 | +6 | +19 | +32 | +44 |
| 9″ | -25 | -13 | 0 | +13 | +25 | +38 |
| 9.5″ | -32 | -19 | -6 | +6 | +19 | +32 |
| 10″ | -38 | -25 | -13 | 0 | +13 | +25 |
| 11″ | -51 | -38 | -25 | -13 | 0 | +13 |
| 12″ | -64 | -51 | -38 | -25 | -13 | 0 |
Quick answers:
- 4.5 inches of backspacing is 0 mm on an 8 inch wheel, -13 mm on a 9 inch wheel, and -25 mm on a 10 inch wheel.
- 5 inches of backspacing is +13 mm on an 8 inch wheel, 0 mm on a 9 inch wheel, and -13 mm on a 10 inch wheel.
- 5.5 inches of backspacing is +25 mm on an 8 inch wheel, +13 mm on a 9 inch wheel, and 0 mm on a 10 inch wheel.
Convert Offset From Millimeters to Inches
Offset is listed in millimeters. To see it in inches, divide by 25.4. To go back, multiply inches by 25.4.
| Offset (mm) | Inches |
|---|---|
| 12 mm | 0.47″ |
| 18 mm | 0.71″ |
| 25.4 mm | 1.00″ |
| 30 mm | 1.18″ |
| 38 mm | 1.50″ |
| 45 mm | 1.77″ |
| 50 mm | 1.97″ |
Do not mix up offset in inches with backspacing. Offset in inches is how far the mounting surface sits from the centerline. Backspacing is how far it sits from the back edge.
How Wheel Spacers Change Offset
A spacer moves the wheel outward. It lowers your effective offset by the spacer thickness. The offset stamped on the wheel stays the same, but the wheel sits differently on the car.
Effective offset = wheel offset – spacer thickness (mm)
A +35 mm wheel with a 20 mm spacer sits like a +15 mm wheel. Backspacing drops by the same amount: 20 / 25.4 is about 0.79 inches. A 1 inch spacer is 25.4 mm. A 2 inch spacer is 50.8 mm. Thick spacers add stress, so read our wheel spacers guide before you pick a size.
How to Measure Wheel Backspacing Yourself
Measuring backspacing takes two minutes and a tape measure. Checking the ET stamp on the back of the wheel is faster, but not every wheel has one.
- Take the wheel off the car.
- Lay it face down on a flat surface.
- Place a straightedge across the back lip of the wheel.
- Measure from the straightedge down to the mounting pad.
- That number is your backspacing in inches.
If you measure the full width from lip to lip, subtract about 1 inch to get the listed wheel width. Then use the calculator above to find your offset.
Why Getting Offset and Backspacing Right Matters
The wrong number can cause rubbing, damage, or a wheel that will not fit. Here is what to watch for:
- Too much backspacing or too much positive offset: the wheel sits too far in. It can hit control arms, struts, or brake calipers.
- Too little backspacing or too much negative offset: the wheel sits too far out. It can rub the fender or stick out past the body. Some states also limit how far a tire can extend past the fender.
- A wider wheel changes both sides: more width means less room on the inside and the outside.
Getting the numbers right before you buy saves you a return. Running different widths front and rear? Check each axle on its own. Our staggered vs square wheel setup guide explains how that works.
Once you know your target, shop all wheels or ask about a ready-to-fit wheel and tire package. Not sure what your vehicle needs? Our customer service team can check fitment before you order. Remember to match your lug nuts to your aftermarket wheels too.
Frequently Asked Questions
What is the difference between offset and backspacing?
Offset is the distance from the wheel’s centerline to its mounting surface, in millimeters. Backspacing is the distance from the mounting surface to the back edge of the wheel, in inches. They describe the same wheel from two starting points, so you can convert between them with the wheel width.
How do I convert backspacing to offset?
Use this formula: Offset (mm) = (Backspacing – (Wheel Width + 1) / 2) x 25.4. For an 8 inch wide wheel with 5.5 inches of backspacing, the answer is +25.4 mm. Or enter your numbers in the calculator above.
What is a good backspacing for my wheels?
There is no single number. It depends on your vehicle, wheel width, and the look you want. Your factory backspacing is the safest place to start. Less backspacing pushes the wheel out. More backspacing pulls it in toward the suspension and brakes. Our team can help you pick a range.
Can I use offset and backspacing interchangeably?
Not directly, because they use different units and starting points. But with the wheel width you can convert either one into the other. So a seller who lists only one number can still tell you both.
How do I measure the offset on my current wheels?
Look at the back of the wheel near the hub for “ET” followed by a number, like ET35. That is the offset in millimeters. If there is no stamp, measure your backspacing and convert it with the formula above.
Is backspacing the same as offset?
No. Offset starts at the wheel’s centerline and uses millimeters. Backspacing starts at the back edge and uses inches. Offset can be negative. Backspacing cannot.
How much does a spacer change my offset?
A spacer lowers your effective offset by its thickness. A 20 mm spacer on a +35 mm wheel gives an effective offset of +15 mm. A 1 inch spacer changes it by 25.4 mm.
How do I convert offset in mm to inches?
Divide the offset by 25.4. A 38 mm offset is 1.5 inches. An 18 mm offset is about 0.71 inches.
Is the backspacing formula exact?
It is the standard shop formula, and it is close but not perfect. The “+ 1” allows for the wheel lips, which differ slightly between wheels. Treat the result as accurate to about a quarter inch, and measure the real wheel before you buy.
What offset and backspacing do I need for a lifted truck?
It depends on your lift, tire size, and wheel width. Lifted trucks often run negative offset to fit wider tires, but too much can rub the fender or stress the bearings. Start with your stock numbers, then call us with your truck and tire size.