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Wheel offset calculator

Compare your current wheels with a new set by width and offset (ET) to see how far the outer and inner edges move, poke toward the fender, clearance to the suspension, and the effect of spacers.

in

Width is marked in inches (the 8 in “18″ × 8J”), offset in millimeters. Mixing the two is the industry convention, not a mistake.

mm

Stamped on a spoke or behind the mounting face as ET and a number, always in millimeters.

in
mm
mm

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Preview

Where each wheel sits

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Both wheels drawn on one scale, measured from the hub mounting face. Left is outward toward the fender, right is inward toward the suspension and brakes.

What it means for fitment

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Outer edgeSticks out 22.7 mm more (toward the fender)
Inner edgeMoves 2.7 mm closer to suspension and brakes
Track width change (per axle)+45.4 mm

Only wheel geometry is compared. Tire width, fender shape, suspension travel and steering lock decide whether it actually fits; measure with the wheel on the car at full compression and lock.

Current vs new

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MeasureCurrentNewChange
Wheel width177.8 mm203.2 mm+25.4 mm
Offset (ET)45 mm35 mm-10 mm
Outer edge vs hub face43.9 mm66.6 mm+22.7 mm
Inner edge vs hub face133.9 mm136.6 mm+2.7 mm
Backspacing (approx.)5.8 in5.9 in+2.7 mm

Offset (marked ET on the wheel) is the distance from the wheel’s centerline to its mounting face. Positive offset pulls the wheel in toward the car; negative pushes it out. Change the offset or the width and both edges of the wheel move, one toward the fender and one toward the suspension. This calculator shows exactly how far.

How it is calculated

  1. Convert width to millimeters (inches × 25.4). Wheel widths are measured between bead seats, so the physical rim is about 25 mm wider; this does not affect the comparison.
  2. Outer edge position = width ÷ 2 − offset. Inner edge position = width ÷ 2 + offset. Both are measured from the hub mounting face.
  3. A spacer reduces the effective offset by its thickness: a 10 mm spacer on an ET45 wheel behaves like ET35.
  4. Backspacing = inner edge position + about 12.7 mm (half an inch) of rim flange, converted to inches. It is measured to the lip, so an 8 in ET35 wheel has (8 + 1) ÷ 2 + 35 ÷ 25.4 = 5.88 in.
  5. The changes are the new positions minus the current ones. Outer change positive means more poke; inner change positive means less clearance.

Example

Current 7 in ET45, new 8 in ET35. Current outer edge: 88.9 − 45 = 43.9 mm; new: 101.6 − 35 = 66.6 mm, so the outer edge moves out 22.7 mm. Inner edge: 133.9 mm now, 136.6 new, moving 2.7 mm closer to the suspension. Almost all the extra width went outward, which is typical of a lower offset.

Rules of thumb

Keeping the outer change under about 15–20 mm usually avoids fender rubbing on a stock car; more than that needs measuring or fender work. Inner clearance matters most on cars with strut suspension, where the coil spring sits close to the wheel. Backspacing, used in the US truck world, is the same idea measured from the mounting face to the inner lip.

Frequently asked questions

What does wheel offset mean?

The distance in millimeters from the wheel’s centerline to the hub mounting face. Positive offset (most cars) sets the wheel further in; negative offset pushes it out for a wider stance.

Will a lower offset stick out more?

Yes. Every 1 mm less offset moves the whole wheel 1 mm outward. Combined with a wider wheel, the outer edge moves out by half the width increase plus the offset decrease.

Do spacers change offset?

Effectively yes. A spacer pushes the wheel outward by its thickness, the same as lowering the offset by that amount. Enter the spacer to see the combined result.

What is backspacing?

The distance from the mounting face to the inner edge of the wheel, common in the US. Higher backspacing means the wheel sits further in. It is related to offset: backspacing = width/2 + offset, plus the rim flange.

How many inches is a negative 44 offset?

ET−44 is about 1.73 inches (44 ÷ 25.4), and the minus sign means the mounting face sits that far outboard of the wheel’s centerline, pushing the whole wheel out toward the fender. Offset is always marked in millimeters; inches only show up in backspacing, which is measured from the mounting face to the inner lip.

How do I calculate the offset of my wheels?

Most wheels have it stamped on a spoke or the back of the mounting face as ET followed by a number. To measure it yourself, lay the wheel face down, measure the overall width across both lips, then the distance from the mounting face to the inner lip — that second figure is the backspacing. Offset = backspacing − half the overall width, in millimeters. Positive means the mounting face is inboard of the centerline, negative means outboard.

Read more

Sources

  1. Les Schwab: What is wheel offset — Offset in millimetres, wheel size in inches: “+43mm offset, wheel size 19″ × 8.5″”.
  2. SAE J1204: Wheel offset definitions
  3. Valor Offroad: Wheel offset vs backspacing — Backspacing = (wheel width + lip thickness) ÷ 2 + offset; 15×7 +62 mm = 6.44″.
  4. Summit Racing: Wheel offset chart — An 8″ wheel with 4.5″ backspacing is zero offset.

By Calcelate Team. Formula from the sources above.

  1. 2026-09-27 · Formula and texts checked