Wheel Offset, Backspacing & Tire Diameter Calculator

Getting offset and backspacing right is the difference between a wheel that clears your suspension and one that rubs the fender liner on the first turn. This calculator uses the same formula ANKUSI’s engineering team applies on every fitment page in our fitment guide library, so the numbers here match what you will see quoted for your specific vehicle.

This tool gives you a calculated reference number, not a clearance guarantee. Suspension geometry, brake calipers, and fender lips vary by trim and modification — always confirm final clearance with a physical test fit before production.

Offset & Backspacing Calculator

Enter any two known values and select the corresponding calculation to solve for the third. Wheel width is the nominal stamped width in inches. Offset is measured in millimeters from the wheel centerline to the hub-mounting surface. Positive offset places the mounting surface toward the wheel’s street-side face and positions the wheel farther inboard.

Formulas Used

Given nominal wheel width W (in) and offset ET (mm):

Backspacing(in) = W ÷ 2 + ET(mm) ÷ 25.4 + 0.5

Given backspacing BS (in) and nominal width W (in):

ET(mm) = [BS − W ÷ 2 − 0.5] × 25.4

The 0.5 in constant is a standard single-flange allowance used industry-wide when working from nominal (stamped) width rather than a caliper-measured total outer width.

Tire Diameter Calculator

Compare two tire sizes to check how much the overall rolling diameter changes when you go up or down in wheel diameter — useful for keeping speedometer accuracy and fender clearance in check. Calculated using the standard ETRTO/TRA sidewall geometry formula.

Tire Diameter Formula

Overall Diameter(in) = Rim Diameter(in) + 2 × [Width(mm) × Aspect Ratio(%) ÷ 100] ÷ 25.4

This is the standard ETRTO/TRA sidewall-height calculation used across the tire industry. Actual measured diameter can vary roughly ±1–2% by manufacturer and tread design, so treat this as a close reference rather than an exact spec.

Frequently Asked Questions

What is the difference between offset and backspacing?

Offset is the distance in millimeters from the wheel’s true centerline to its mounting (hub) surface, and can be positive, negative, or zero. Backspacing is the distance in inches from the mounting surface to the back (inner) edge of the wheel, and is always a positive number. They describe the same physical geometry from two different reference points, which is why one can be calculated from the other once wheel width is known.

Does a higher positive offset increase or decrease backspacing?

At the same nominal wheel width, a higher positive offset increases backspacing. The hub-mounting surface moves toward the wheel’s street-side face, so the wheel sits farther inboard: the inner edge moves closer to the suspension while the outer edge becomes more tucked. A lower or negative offset decreases backspacing and moves the wheel outward. These measurements describe lateral wheel position only and do not guarantee spoke-to-caliper, suspension, or fender clearance.

Does this calculator guarantee my wheel will clear my brakes or fenders?

No. It gives you the geometric offset/backspacing or tire diameter number only. Actual clearance also depends on caliper size, suspension travel, camber, and fender lip shape, which this tool does not measure. A physical test fit is recommended before production, especially on aggressive or lowered setups.

About This Calculator

Written by: ANKUSI Engineering Team
Published: August 22, 2026  •  Last updated: August 23, 2026

Formulas cross-checked against ANKUSI’s own fitment-page generation process and standard ETRTO/TRA tire sidewall geometry conventions.