Wheel Offset Calculator

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Walk-through

How to Use This Calculator

4 steps

1

Measure the hub-to-mounting-face distance

Lay the wheel face-down on a flat surface. Place a straightedge across the back lip of the wheel (the barrel), then measure from the straightedge down to the wheel's hub mounting surface — this is the hub-to-mounting-face distance, in millimeters. Enter it above.

2

Enter the wheel width

Enter the wheel's width in inches (the measurement stamped on the wheel, e.g. 8" or 9.5" — not the tire width). The calculator converts it to millimeters and halves it to find the wheel's centerline.

3

Read the offset (ET) and its polarity

The hero result shows the calculated offset (ET) in millimeters, with a positive or negative sign. The Positive vs Negative tab explains what that sign means for how the wheel sits relative to the fender.

4

Compare two offsets to see the fit change

On the Offset Change Effect tab, enter a current offset and a new offset (e.g. when comparing a stock wheel to an aftermarket wheel) to see how far the wheel shifts in or out, and the resulting change in track width.

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Reference

Formula & Methodology

2 formulas

Offset (ET) from hub-to-mounting-face

ET = hubFace − (width × 25.4 / 2)

The wheel's centerline sits at half its total width. Offset (ET) is the distance, in millimeters, between that centerline and the wheel's hub mounting face. Since wheel width is conventionally measured in inches, it's converted to millimeters (× 25.4) before halving. A positive ET means the mounting face sits toward the outside of centerline (wheel tucks in); a negative ET means it sits toward the inside (wheel pokes out).

Offset change → position shift and track width

shift = currentET − newET; trackWidthChange = shift × 2

When comparing a current offset to a new one, the position shift (per wheel) equals the current ET minus the new ET — a positive shift means the new wheel pokes further outward toward the fender. Because both wheels on an axle move the same amount, the total track width change is double the per-side shift.

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Glossary

Key Terms Explained

7 terms

Offset (ET) The distance, in millimeters, between a wheel's hub mounting face and its centerline. "ET" comes from the German "Einpresstiefe" (insertion depth), the industry-standard term stamped on most wheels.

Hub mounting face The flat surface on the back of the wheel that bolts flush against the vehicle's brake hub. This is the reference point offset is measured from.

Centerline The midpoint of the wheel's total width — halfway between the inner and outer lips of the barrel. Offset is measured relative to this line, not to either edge of the wheel.

Positive offset A hub mounting face positioned toward the outside (street side) of the wheel's centerline. Positive-offset wheels sit further inboard, tucked closer to the suspension — the norm on most modern factory wheels.

Negative offset A hub mounting face positioned toward the inside (brake side) of the wheel's centerline. Negative-offset wheels sit further outboard, closer to (or past) the fender — the classic "deep dish" look.

Backspacing A related measurement: the distance from the wheel's hub mounting face to its inner lip (rather than to the centerline). Backspacing and offset describe the same physical wheel from two different reference points, so they aren't interchangeable without knowing the wheel's total width.

Poke Slang for how far a wheel or tire extends outward past the fender's edge. Lower (or more negative) offsets increase poke; higher (or more positive) offsets reduce it.

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Scenarios

Real-World Examples

3 worked examples

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Measuring a used wheel

Unmarked take-off wheel

Wheel width 8 in Hub-to-mounting-face 178 mm

Half of 8 inches in millimeters is 101.6 mm. Subtracting that from the 178 mm hub-to-mounting-face measurement gives an offset of ET 76.4 mm — but a real 8" wheel typically reads ET 35-40, so this hypothetical measurement would actually indicate the wheel is closer to 10-10.5" wide or the mounting-face reading needs to be re-checked. Always double-check the hub-to-mounting-face measurement against the wheel's stamped width before trusting an unmarked wheel's offset.

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Swapping wheels

Going from ET35 to ET20

Current offset 35 mm New offset 20 mm

Position shift = 35 − 20 = +15 mm, so the new wheel pokes 15 mm further outward on each side. Total track width increases by 30 mm (15 mm × 2 sides) — enough that fender clearance and rubbing risk should be checked before installing, especially on lowered vehicles.

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Choosing a look

Positive vs. negative offset diagram

Offset A +35 mm (tucked) Offset B −12 mm (poking)

Picture the wheel's centerline as a vertical line splitting its width in half. A +35 mm offset places the mounting face 35 mm outboard of that line, pulling the wheel's face — and its visual profile — inward, flush or tucked. A −12 mm offset places the mounting face 12 mm inboard of centerline, pushing the wheel's face outward past the fender for a wider, "poked" stance.

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Deep Dive

Understanding Wheel Offset (ET)

3 chapters · 4 min read

Wheel offset — often stamped as "ET" followed by a number — determines how far a wheel sits toward or away from the vehicle's suspension and fender. Getting it wrong can mean rubbing tires, uneven tire wear, or added stress on wheel bearings, so it's worth understanding before buying a new set of wheels or measuring a used one.

How offset is calculated

Offset is the distance between a wheel's hub mounting face and its centerline — the midpoint of its total width. To find it from a physical measurement, take the hub-to-mounting-face distance (measured with the wheel face-down and a straightedge across the back lip) and subtract half the wheel's total width (converted from inches to millimeters). The result is the offset in millimeters: a positive number means the mounting face sits toward the wheel's outside face, a negative number means it sits toward the inside.

Why offset matters for fit

A wheel with too little offset (a very negative ET, or an ET much lower than stock) pushes the tire outward, increasing the chance it rubs the fender lip or fender liner, especially at full steering lock or over bumps. Too much offset (a high positive ET) pulls the wheel inward, which can cause the tire or wheel to contact suspension components, brake calipers, or the inner fender well. Manufacturers engineer a specific offset range for each vehicle to balance fender clearance, steering feel, and bearing load — deviating far from it (more than roughly ±10-15 mm on most cars, though this varies by vehicle) is where problems tend to start.

Offset vs. backspacing — don't confuse the two

Offset and backspacing both describe where a wheel's hub mounting face sits, but from different reference points: offset is measured from the wheel's centerline, while backspacing is measured from the wheel's inner lip. Two wheels with the same backspacing can have different offsets if their widths differ, and vice versa. When comparing wheels or planning a fitment change, make sure any spec sheet or forum post is describing offset (ET) and not backspacing before you use it in this calculator — mixing the two will produce a fender-fit estimate that's off by an amount depending on the wheel's width.

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Questions

Frequently Asked Questions

6 questions

What does a positive offset mean?+

A positive offset means the wheel's hub mounting face sits toward the outside (street side) of the wheel's centerline. This pulls the wheel's overall position inward, tucking it closer to the vehicle's suspension components — the standard setup on most modern factory wheels.

What does a negative offset mean?+

A negative offset means the wheel's hub mounting face sits toward the inside (brake side) of the wheel's centerline. This pushes the wheel further outward, toward or past the fender — the "deep dish" look common on trucks, muscle cars, and widened aftermarket setups.

Does a lower ET number always mean more poke?+

Relative to a given stock offset, yes — lowering the ET number (moving toward zero or negative) pushes the wheel further outward, increasing poke. Comparing two different offsets directly tells you the exact shift: use the Offset Change Effect tab to see how many millimeters a specific change moves the wheel.

Can the wrong offset cause fender rubbing?+

Yes. An offset that's too low (too negative, or well below the vehicle's stock spec) pushes the tire outward and increases the risk of rubbing the fender lip, especially during suspension travel or at full steering lock. Always check fender clearance — and consider a test-fit — before committing to a significant offset change.

Is offset the same as backspacing?+

No. Offset is measured from the wheel's centerline (the midpoint of its total width), while backspacing is measured from the wheel's inner lip to the hub mounting face. They describe related but different reference points, and converting between them requires knowing the wheel's total width. See the Wheel Backspacing calculator if you have a backspacing measurement instead of an offset.

What units does this calculator use?+

Wheel width is entered in inches (the industry-standard unit for wheel width), while hub-to-mounting-face distance and offset are both in millimeters (the industry-standard unit for offset/ET). The calculator handles the inch-to-millimeter conversion internally.

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