Offset & Backspacing Converter
Offset and backspacing describe the same thing in two different units. Enter your wheel width and one value to get the other instantly.
Convert Your Numbers
How It Breaks Down
Need the Full Fitment Picture?
This converter handles the offset-to-backspacing math. To see how a new offset changes poke, clearance, and speedometer reading against your current setup, run the complete fitment calculator.
Open Wheel Fitment CalculatorWheel Offset Calculator
This wheel offset calculator converts between offset and backspacing in seconds. Enter your wheel width and either number, and you get the other instantly, using the same industry-standard formula wheel shops use.
Offset and backspacing measure the same thing, just from different reference points and in different units. If you have one and need the other to check a wheel will sit right, this is the fastest way to get it.
How to Use This Calculator
Three steps, under thirty seconds:
Pick your direction. Choose offset to backspacing, or backspacing to offset, depending on which number you already have.
Enter your wheel width. Use the stamped width in inches (for example, 9 for a 9-inch wheel). The calculator adds the standard 1-inch lip allowance for you.
Enter your known value. Type your offset in millimeters or your backspacing in inches. The result and a full breakdown appear instantly.
Offset vs Backspacing: What's the Difference?
Offset is the distance from the wheel’s mounting face to its true centerline, measured in millimeters and usually stamped as “ET” (for example, ET35). Backspacing is the distance from the mounting face to the inner lip of the wheel, measured in inches.
They describe the same mounting surface from opposite directions, so they always move together. More positive offset means more backspacing and a wheel that tucks further under the fender. More negative offset means less backspacing and more poke toward the fender edge.
For the full breakdown of how offset shapes stance, clearance, and fitment, read our guide to wheel offset and ET.
The Offset to Backspacing Formula
This calculator uses the automotive industry-standard formula, which includes a 1-inch allowance for the wheel lip so the result matches a physical measurement taken with a straightedge:
Backspacing (in) = ((Wheel Width + 1) ÷ 2) + (Offset ÷ 25.4)
Offset (mm) = (Backspacing − ((Wheel Width + 1) ÷ 2)) × 25.4
The extra inch accounts for roughly half an inch of lip on each side of the wheel, the difference between the stamped bead-seat width and the true overall width. Skipping it is the most common reason two calculators disagree by about half an inch.
Want to See How New Wheels Will Actually Sit?
This calculator handles the offset-to-backspacing conversion. If you want to see how a new wheel and tire setup changes poke, fender clearance, and your speedometer reading against your current wheels, use the full comparison tool.
And before you commit to numbers, run through the full fitment research checklist so nothing gets missed.
Frequently Asked Questions
How do you convert offset to backspacing?
Add 1 inch to your wheel width, divide by 2, then add the offset divided by 25.4. For a 9-inch wheel at +35mm offset, that is (10 ÷ 2) + (35 ÷ 25.4) = 6.38 inches of backspacing.
Is offset the same as backspacing?
They measure the same mounting-face position but from different points and in different units. Offset is from the centerline in millimeters; backspacing is from the inner lip in inches. Convert between them with the calculator above.
What does a negative offset mean?
A negative offset puts the mounting face behind the wheel's centerline, which pushes the wheel outward for a more aggressive, poked stance and reduces backspacing.
Why do some calculators give different backspacing numbers?
Most differences come from the 1-inch lip allowance. Calculators that use the true overall width (stamped width plus 1 inch) match a physical measurement; those that use only the stamped width read about half an inch low.
Does wheel diameter affect offset or backspacing?
No. Offset and backspacing depend on wheel width and mounting-face position, not diameter. A 9-inch-wide wheel has the same offset-to-backspacing relationship whether it is 18 or 20 inches.