Calculate inner/outer protrusion when changing wheel offset. Visualize fender and suspension interference risk.
The Wheel Offset & Backspacing Calculator is a precision fitment simulation tool that calculates exactly how much a new wheel will move inward or outward in millimeters. Choosing the wrong offset can cause tires to protrude past the fenders—violating laws—or strike suspension arms and brake lines internally, potentially leading to catastrophic failure. This tool helps you achieve the perfect wheel 'fitment' by visualizing these risk factors in advance.
Calculations are based on wheel width and offset (ET) values to determine distance changes from the centerline. Decreasing the offset moves the wheel outward, while increasing it moves it inward. It also derives backspacing values to analyze how deep the inner edge of the wheel sits from the hub mounting surface. By comparing this with stock data, the system calculates 'outer protrusion increase' and 'inner clearance reduction' to judge suspension interference and fender flushness.
Enter wheel width (inches) and offset (mm) to calculate backspacing and inner/outer edge positions. Higher offset pushes the wheel inward toward the vehicle.
Positive outer change means the wheel protrudes beyond the fender. Positive inner change means the wheel moves inward, risking suspension or brake caliper interference.
Within ±5mm is generally safe, 5–15mm requires caution, and 15mm+ should be verified by a professional.
Offset (ET, from German Einpresstiefe) is the distance in mm between the wheel's mounting face and its centerline; backspacing is the distance from the mounting face to the inner lip. The link is backspacing = (rimWidth / 2) + offset, where rim width is converted to mm (1 inch = 25.4 mm) and the bead-seat width is the marked width plus ~12.7 mm of flange. Worked example: an 8.5-inch wheel (≈215.9 mm bead seat) with ET35 has backspacing ≈ (215.9 + 12.7)/2 + 35 ≈ 149 mm.
A lower or more negative ET pushes the wheel outward toward the fender; a higher ET tucks it inward toward the suspension. Moving from ET45 to ET35 shifts the wheel exactly 10 mm outboard. A common mistake is changing only wheel width and forgetting the centerline math: a wider wheel at the same ET grows roughly equally inboard and outboard, so half the added width moves each way.
For fitment, the poke (outboard movement) is the dominant fender-clearance risk and the inboard movement threatens strut and brake-line contact. Spacers effectively reduce ET by their thickness (a 15 mm spacer turns ET45 into an effective ET30) but require longer studs and proper hub-centric design. Always cross-check the wheel's hub bore and PCD; offset alone does not guarantee the wheel bolts up safely.