Wheel Offset Calculator

The Wheel Offset Calculator compares a new wheel and tire package against your current setup and shows exactly how the fitment changes — how far the wheel pokes toward the fender, how far it tucks toward the suspension, the new backspacing, and the total track-width change. Add your available fender and inner clearances and it returns a plain-English fitment verdict, plus an animated top-down cross-section so you can see the shift at a glance.

What Is Wheel Offset (ET)?

Offset, stamped on the wheel as ET (from the German Einpresstiefe, "insertion depth"), is the distance in millimetres between the wheel's mounting face — the flat surface that bolts to the hub — and the wheel's true centreline. A higher positive offset (e.g. ET45) pushes the mounting face outward, which tucks the whole wheel inboard toward the car. A lower or negative offset (e.g. ET15 or ET−10) pulls the wheel outboard toward the fender for a wider, more aggressive stance.

How Offset and Width Change Fitment

Because the mounting face is bolted fixed to the hub, everything comes down to where the wheel's edges land relative to that face. Measuring outboard-positive from the mounting face:

Here effective width is the tire section width when you provide it, because the tire sidewall — not the wheel lip — is what actually contacts the fender or strut. Comparing two setups is then a simple subtraction of these expressions, giving poke (outer-edge movement) and tuck (inner-edge movement).

Backspacing vs Offset

Backspacing is the distance from the mounting face to the wheel's inner lip, traditionally measured in inches. It is directly tied to offset:

Backspacing uses overall width, not nominal bead-seat width. Blank overall width is estimated as nominal + 1 in; enter a measurement for your wheel. Assumes symmetric rim lips. This field only changes backspacing, not the tire/nominal-width clearance model.

Offset Change = Spacer Thickness

Lowering offset has exactly the same outward effect as bolting on a wheel spacer of equal thickness. Going from ET45 to ET25 moves each wheel 20 mm further out — identical to a 20 mm spacer — and widens the total track by 40 mm. This calculator makes that trade-off explicit through the track-width and poke figures.

Reading the Fitment Verdict

Remember these are static figures. Suspension compression, steering lock, and increased negative camber all reduce real-world clearance, so leave a safety margin above the minimums.

What Affects Real Clearance

How to Use This Calculator

1. Enter your current setup: Wheel width in inches, offset (ET) in millimetres, and optionally the tire section width (e.g. 225) and wheel diameter.
2. Enter your new setup: The width, offset, and tire width of the wheels you are considering.
3. Add your clearances (optional): Measure how much room you have to the fender and to the strut/suspension in millimetres for a fitment verdict.
4. Click Compare: Review poke, tuck, backspacing, track-width change, the verdict, the animated cross-section diagram, and the step-by-step math.

Frequently Asked Questions

What is wheel offset (ET)?

Wheel offset, marked ET (from the German Einpresstiefe), is the distance in millimetres between the wheel's mounting face and its true centreline. A higher positive offset tucks the wheel further inboard toward the car, while a lower or negative offset pushes it outboard toward the fender for a more aggressive stance.

How do offset and width affect fender clearance?

The outer edge that faces the fender sits at half the effective width minus the offset. So a lower offset or a wider wheel or tire both push the outer edge outward and reduce fender clearance. This tool differences the two setups to show exactly how many millimetres the outer edge moves toward the fender, which it calls poke.

What is backspacing and how is it related to offset?

BS = W_overall / 2 + ET / 25.4 (W_overall: in; ET: mm; BS: in). Backspacing uses overall width, not nominal bead-seat width. Blank overall width is estimated as nominal + 1 in; enter a measurement for your wheel. Assumes symmetric rim lips. This field only changes backspacing, not the tire/nominal-width clearance model.

What offset change is like adding a wheel spacer?

Reducing offset has the same outward effect as adding a spacer of equal thickness. A wheel that goes from ET45 to ET25 sits 20 mm further out per side, exactly like a 20 mm spacer, and widens the total track by 40 mm.

How much fender clearance do I need to avoid rubbing?

As a rough guide, more than 10 mm of remaining clearance is comfortable, 3 to 10 mm is tight but usually workable, under 3 mm is very tight and may rub over bumps or at full lock, and a negative value means the setup will interfere. Suspension travel, camber, and load all reduce clearance in motion, so leave a margin.

Does a wider tire change offset?

A wider tire does not change the wheel's offset, but because the tire section width is what actually rubs, a wider tire pushes both the outer and inner edges outward and inward equally. That is why this calculator uses the tire section width, when provided, to measure real fender and suspension clearance.

Additional Resources

• Wheel Sizing - Wikipedia