Wheel offset calculator

Discount Tire defines offset as the distance from the mounting surface to the wheel centerline, measured in millimeters. Positive offset places the mounting surface toward the wheel face; zero aligns it with the centerline; negative places it toward the back.

Backspacing measures from the mounting surface to the back lip, in inches, according to Discount Tire. In this calculator, poke describes the outer edge’s outward position, while inner clearance change describes movement toward or away from the suspension.

The calculator uses backspacing (in) = (wheel width + 1 in) / 2 + offset (mm) / 25.4. Wheel width here means stated width, not overall width across the flanges.

Forgeline says overall width is almost always 1 in larger than stated width. The formula therefore adds 1 in for the rim flanges, about 0.5 in each; the wheel maker’s spec sheet remains the reference for a particular wheel.

• 8 in wide, +35 mm offset: 5.88 in (149 mm) backspacing by the formula.
• 8 in wide, 0 mm offset: 4.50 in (114 mm) backspacing by the formula.
• 9 in wide, −12 mm offset: 4.53 in (115 mm) backspacing by the formula.
• 8.5 in wide, +18 mm offset: 5.46 in (139 mm) backspacing by the formula.
• 10 in wide, −18 mm offset: 4.79 in (122 mm) backspacing by the formula.

By the formula, changing from 17x8 +35 to 17x9 −12 moves the outer edge 2.35 in (59.7 mm) outward. The inner edge moves 1.35 in (34.3 mm) away from the suspension, increasing nominal inner clearance.

Outer change (mm) = (W2 − W1) × 25.4 / 2 − (ET2 − ET1); inner change (mm) = (W2 − W1) × 25.4 / 2 + (ET2 − ET1). W1 and W2 are current and replacement widths in inches; ET1 and ET2 are their offsets in millimeters. A positive outer result means more poke, while a negative inner result means movement away from the suspension.

Check brake-caliper, suspension and fender clearance on the vehicle before buying. Fit depends on the vehicle, wheels and suspension; the calculator compares edge positions rather than confirming fit.

Les Schwab warns that too much positive offset can cause suspension or bodywork rubbing and brake interference. It also warns that too much negative offset can increase steering kickback, stress the suspension and impair handling. Use the wheel maker’s spec sheet as the reference, and consider width alongside wheel offset.