Wheel Offset Calculator

Calculate wheel offset from backspacing, convert between offset and backspacing, and compare wheel fitment changes. Essential for proper wheel installation and clearance verification.

Inside edge to inside edge (not including lip)

Distance from mounting surface to inner edge

Distance from centerline to mounting surface (ET value)

Stock or current wheel offset

Proposed new wheel offset

Leave blank if width stays the same

Wheel Offset Formula: Offset (mm) = (Backspacing - Wheel Width/2) * 25.4 Backspacing (inches) = (Offset / 25.4) + (Wheel Width / 2) Frontspacing = Wheel Width - Backspacing Positive Offset: Mounting surface toward outside (common FWD) Zero Offset: Mounting surface at centerline Negative Offset: Mounting surface toward inside (common RWD/trucks) Track Width Change = Offset Change * 2
Example 1 (Calculate Offset): Wheel Width: 8 inches Backspacing: 5 inches Centerline: 8 / 2 = 4 inches Offset: (5 - 4) * 25.4 = +25.4 mm (ET25) Example 2 (Compare Fitment): Current: ET45, 8" wide New: ET35, 8.5" wide Offset Change: -10mm Width Change: +0.5" Outer edge: Moves out 0.394" + 0.25" = 0.644" Track width: +20mm wider total WARNING Check fender clearance

What is wheel offset and why does it matter?

Wheel offset is the distance from the wheel's mounting surface to its centerline, measured in millimeters. Positive offset: mounting surface is toward the outside (common in FWD cars). Zero offset: mounting surface at centerline. Negative offset: mounting surface toward inside (common in trucks/RWD). Affects handling, tire clearance, and suspension geometry.

What happens if I change wheel offset?

Lower offset (more negative): wheels stick out more, wider stance, may rub fenders, increased scrub radius affects steering feel. Higher offset (more positive): wheels tuck in, may hit suspension/brakes, less aggressive look. Even 5mm difference can affect fitment. Changes also affect wheel bearing loads and turn-in response.

How do I measure wheel offset?

Formula: Offset = (Wheel Width / 2) - Backspacing. Backspacing is measured from mounting surface to inner wheel edge. Example: 8" wide wheel with 5" backspacing = (8/2) - 5 = -1" = -25mm offset. Most wheels have offset stamped on back (ET35, ET45, etc. where ET = offset in mm).

What is backspacing vs offset?

Backspacing: distance from mounting surface to inner edge of wheel (always positive, measured in inches). Offset: distance from mounting surface to centerline (can be positive/negative/zero, measured in mm). Backspacing = (Offset/25.4) + (Width/2). Both describe wheel position but offset is the modern standard.

Can I change offset without problems?

Safe range typically ±5mm from stock. Lowering offset: check fender clearance when turning. Raising offset: verify brake caliper clearance. Always test-fit before mounting tires. AWD/4WD vehicles are more sensitive to offset changes due to drivetrain geometry. Consider hub-centric rings if hub bore changes.

What is the difference between ET and offset?

ET (Einpresstiefe in German) and offset are the same thing - both measure the distance from the mounting surface to the wheel centerline in millimeters. ET35 = +35mm offset. ET0 = zero offset. You may also see it written as "Offset: +35" or just "+35".

How does offset affect track width?

Each 1mm decrease in offset increases track width by 2mm (1mm per side). Example: changing from ET45 to ET35 (-10mm offset) increases total track width by 20mm (0.79 inches). This creates a wider stance but may require fender modifications or wheel spacers to prevent rubbing.