What is wheel offset (ET)?

Offset, labeled ET (from the German Einpresstiefe, meaning "insertion depth"), is the distance in millimeters from the wheel's mounting hub face to its true centerline. A positive ET means the hub face is toward the street side of the wheel (common on front-wheel-drive cars). A negative ET means the hub face is toward the car, pushing the wheel outward.

Backspacing is the distance in inches from the wheel's mounting hub face to the inner lip. It tells you how much room exists between the hub and the inner barrel. Formula: backspacing = (wheel width / 2) + (offset / 25.4) + 0.5 inches (for the flange).

How does changing offset affect fitment?

Reducing the ET by 10 mm (e.g. from +45 to +35) pushes the entire wheel 10 mm outward, reducing inner clearance by 10 mm and increasing outer clearance by 10 mm. Increasing ET does the opposite — the wheel moves inward.

What is scrub radius and why does it matter?

Scrub radius is the distance between the steering axis and the tire contact patch center at ground level. Changing offset shifts the wheel laterally and alters this radius, which changes steering feel, torque steer tendency, and the load on steering components.

How much offset change is safe without other modifications?

Most vehicles tolerate ±10 mm of ET change without clearance problems, but this depends heavily on the wheel width, suspension design, and brake caliper size. Always measure your actual inner and outer clearances before buying.

Does a wheel spacer change the effective offset?

Yes. A spacer pushes the wheel outward, effectively reducing the ET by the spacer thickness. A 10 mm spacer on a +40 ET wheel makes it behave like a +30 ET wheel at the hub face.

Wheel Offset Calculator

Name: Wheel Offset Calculator
Availability: InStock
Author: Nham Vu

Enter your wheel width and offset (ET) to calculate backspacing, and compare two offsets to find the clearance shift and scrub radius change.

Wheel Dimensions

Wheel Width (inches)

Stamped on the wheel barrel (e.g. 8J = 8 inches)

Stock Wheel

Stock Offset ET (mm)

Positive = hub face closer to street side

New / Aftermarket Wheel

New Offset ET (mm)

Enter wheel dimensions on the left and click Calculate.

Summary

Enter your wheel width and offset (ET) to calculate backspacing, and compare two offsets to find the clearance shift and scrub radius change.

How it works

Enter the wheel width in inches (the measurement stamped on the barrel, e.g. 8.0).
Enter the stock offset (ET) in millimeters — found on the wheel face or in your vehicle spec sheet.
Enter the new (aftermarket) offset you are considering.
Click Calculate to see backspacing for both offsets and how the wheel position shifts.
Read the inward/outward shift values to confirm caliper and fender clearance.
Check the scrub radius change to understand the steering and handling impact.

Use cases

Verify that an aftermarket wheel clears the brake caliper before purchasing.
Confirm a wider wheel will not rub the fender or suspension components.
Understand how a lower-ET (more negative) offset pushes the wheel outward for a flush look.
Calculate backspacing when a supplier lists ET instead of backspacing (or vice versa).
Estimate the scrub radius change when upgrading to a spacer or different-offset wheel.
Compare two aftermarket offsets side by side to choose the better fitment.
Plan a staggered fitment setup with different front and rear offsets.
Check legal fender clearance requirements before a track day or inspection.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu