Wheel Offset Calculator
Wheel offset and backspacing determine where the wheel sits relative to the vehicle's hub and suspension. Getting this right is essential for proper tire clearance, correct track width, and safe wheel bearing loads. This calculator converts between wheel offset (ET, in millimeters) and backspacing (in inches) using the standard geometric relationship. Enter the wheel width in inches and the offset in millimeters to find the backspacing, or use the reverse calculation. Understanding these measurements before purchasing aftermarket wheels prevents fitment problems and costly returns.
Offset and backspacing formulas
Backspacing (in) = (Width (in) / 2) + (Offset (mm) / 25.4) Front Space (in) = Width (in) - Backspacing (in) Offset (mm) = (Backspacing (in) - Width (in) / 2) * 25.4
Backspacing measures from the inside lip to the mounting face. Front space measures from the mounting face to the outer lip. Both must clear suspension, brake, and body components.
Typical offset ranges by vehicle type
- Front-wheel drive cars: typically +35 to +50 mm offset (high positive, wheel tucked inward for suspension clearance).
- Rear-wheel drive sports cars: typically +20 to +40 mm offset on front, +15 to +35 mm on rear.
- Trucks and SUVs: typically +10 to +30 mm offset; lifted trucks often run negative offsets (-12 to +10 mm) for wider stance.
- Track/race cars: often use zero or negative offsets to maximize track width for cornering stability.
Frequently asked questions
What is wheel offset (ET)?
Wheel offset (ET from German 'Einpresstiefe') is the distance in millimeters from the wheel's hub mounting face to its center line. Positive offset pushes the wheel inward (toward the car); negative offset pushes it outward. Zero offset means the mounting face is at the wheel's center.
What is wheel backspacing?
Backspacing is the distance from the wheel's rear edge (inside lip) to the mounting face, measured in inches. It determines how much of the wheel sits behind the hub, affecting clearance with the brake caliper, spring, and inner fender.
How are offset and backspacing related?
Backspacing (in) = (Width (in) / 2) + (Offset (mm) / 25.4). For a 9-inch wide wheel with +25 mm offset: backspacing = (9/2) + (25/25.4) = 4.5 + 0.984 = 5.484 inches.
How does offset affect handling?
More positive offset (deeper backspacing) moves the wheel inward, narrowing the track width and reducing the scrub radius. This can improve steering response but may increase steering kickback. More negative offset (less backspacing) widens the track, improving cornering stability but stressing wheel bearings more.
What happens if I use the wrong offset?
Incorrect offset causes tire rub on inner fenders or suspension components (too negative/wide), or reduces available brake and steering clearance (too positive/narrow). Both conditions are unsafe. Always verify clearance on the physical vehicle before driving.
Official sources
- SAE International: sae.org - wheel and tire fitment standards.
- NHTSA: nhtsa.gov - vehicle safety and wheel standards.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.