Wheel Offset Calculator
Enter the wheel width, offset and spacer. The calculator shows effective offset, backspacing and front spacing in millimetres.
Offset, backspacing and spacer
Width is the wheel width in inches. Offset and spacer are in millimetres. A spacer moves the wheel outward and reduces the effective offset.
Wheel position
These values describe the wheel and spacer dimensions. They do not confirm brake, suspension or fender clearance.
Offset, backspacing and spacers
Offset is the distance from the mounting face to the wheel centreline. Positive offset tucks the wheel inward; negative offset pushes it outward. Backspacing is the distance from the mounting face to the inner lip.
A spacer sits between the hub and the wheel. It moves the wheel outward by the spacer thickness. The effective offset becomes the original offset minus the spacer, while front spacing increases by the same amount.
How to read the result
Backspacing is useful when you are checking inner clearance. Front spacing is useful when you are checking how far the wheel sits toward the fender. Neither number tells you the shape of the car.
Use the offset calculator for one wheel and spacer. If you are comparing a complete new wheel and tire package against the current one, use the fitment calculator instead.
If you searched for a wheel offset visualizer, this is the numeric side of that question: it shows where the wheel sits, while a photo preview shows how the design looks on the car.
What this calculator cannot tell you
It does not measure brake calipers, suspension components, fender clearance or tire stretch. It also does not confirm the wheel is safe to use with a spacer. Check the hardware and the car before driving.
Frequently asked questions
What does ET mean on a wheel?
ET is the offset in millimetres, measured from the wheel mounting face to the centreline. A lower ET moves the wheel outward; a higher ET moves it inward.
What does a spacer do to offset?
A spacer moves the wheel outward and reduces the effective offset by the spacer thickness. A 10 mm spacer on an ET45 wheel gives an effective ET35.
How do I convert offset to backspacing?
Backspacing in millimetres is half the wheel width in millimetres plus the offset. For an 8.5-inch wheel with ET45, that is 107.95 + 45 = 152.95 mm.
Will this calculator tell me if the wheel rubs?
No. It tells you the wheel position, not the shape of the fender or suspension. A physical clearance check is still required.
Standards and sources
This calculator uses the tire and wheel dimension conventions from ISO 4000-1 and ISO 3911; fitment guidance also references ETRTO.