Wheel Offset &Backspacing Calculator
Compare stock vs new wheel fitment to check inner suspension clearance and outer fender poke.
The inner wheel lip will have 6.0 mm more room away from the strut.
The outer face will extend 44.0 mm further outward toward/past the fender.
Understanding Wheel Offset (ET) & Backspacing
Why Are Units Mixed (Inches & Millimeters)?
In the global automotive industry, wheel dimensions follow a standard hybrid format:
- Wheel Width (Inches): Universal rim bead width (e.g., 7.5", 8.5", 9.0"). Manufacturers worldwide sell rims in inches.
- Offset ET (Millimeters): The distance from the wheel centerline to the hub mounting face (e.g., +45 mm, +20 mm, -12 mm). Precision in millimeters is necessary for exact brake and strut fitment.
- Backspacing (Inches): The distance from the hub mounting pad to the inner rim lip. Standard measurement for US truck and 4x4 lift kits.
Offset Types Explained
The hub mounting face is towards the street/front side of the wheel. The wheel sits further inside the fender (common on modern passenger cars & FWD/AWD SUVs).
The hub mounting face aligns exactly with the wheel's centerline.
The hub mounting face is recessed toward the back/brake side. The wheel pushes aggressively outward past the fender (popular on lifted trucks and deep-dish wheels).
Frequently Asked Questions About Wheel Offset
Why is wheel width in inches while offset is in millimeters?
This is the universal international standard across the automotive industry (ETRTO, SAE, DIN, and JATMA). Wheel diameters and bead widths have always been manufactured in inches (e.g. 17x8.5J or 18x9J), whereas offset (ET - Einpresstiefe) requires millimeter precision (e.g. +35mm, +20mm, -12mm) to accurately gauge suspension clearance and fender alignment.
What is the difference between Wheel Offset (ET) and Backspacing?
Offset is the distance in millimeters from the wheel centerline to the hub mounting face. Backspacing is the distance in inches from the hub mounting face to the back inside lip of the wheel. Backspacing is commonly used in off-road and custom truck fitments to verify suspension strut clearance.
How do I know if a new wheel will rub on my suspension or fender?
Compare your stock wheel width and offset to the new wheel using our calculator above. If the inner clearance shows positive millimeters (e.g. 15mm closer), the wheel moves closer to the strut. If the outer poke shows positive millimeters (e.g. 20mm poke), the wheel face pushes outward closer to or past the fender lip.