Convert between wheel offset (ET, mm) and backspacing (inches) in either direction, using wheel width as the shared reference — then check common width/offset combinations for reference.
Wheel & conversion direction
Pick which measurement you already have, enter the wheel width, then fill in that measurement to convert to the other.
Positive offset pulls the wheel face toward the vehicle centerline (inward); negative offset pushes it outward (more poke).
Distance from the wheel's mounting (hub) face to the back edge of the rim.
Backspacing
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Enter a wheel width and offset to see the backspacing.
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Backspacing (in)
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Offset / ET (mm)
Enter a wheel width and either an offset or a backspacing measurement to convert between the two.
Two ways to describe the same wheel
Offset and backspacing both describe where a wheel's mounting face sits relative to the rim — they just measure from different reference points, so a wheel only ever needs one of the two numbers to fully define its mounting position (given its width).
Offset (ET)
Measured in millimeters, from the wheel's true centerline (the exact midpoint of the wheel's width) to the mounting/hub face.
Positive offset — mounting face is toward the outside of the wheel (front of centerline). Pulls the wheel in, toward the vehicle. Common on modern OEM wheels.
Zero offset — mounting face is exactly at centerline.
Negative offset — mounting face is toward the inside of the wheel (behind centerline). Pushes the wheel out, away from the vehicle (more "poke" or "dish").
Backspacing
Measured in inches, from the mounting/hub face to the back edge of the rim (the inner lip, closest to the suspension/brakes) — not from the centerline.
Always a positive value — it's a physical distance, not a signed direction.
Smaller backspacing (mounting face closer to the back edge) = more of the wheel sits outboard = similar effect to a more negative offset.
Larger backspacing = more of the wheel tucks inboard, toward suspension components = similar effect to a more positive offset.
Because backspacing is measured from the back edge (not the centerline), you need the wheel's width to convert it to offset, and vice versa — that's exactly what the Convert tab does: backspacing = wheel width / 2 + offset / 25.4.
Common wheel width / offset combinations
Example fitments across common wheel widths, with the offset converted to backspacing using the same formula as the Convert tab. Always verify the exact backspacing or offset stamped on your specific wheel — these are illustrative, not manufacturer specs.
Measured overall width
Offset (ET)
Backspacing
6.5 in
+40 mm
4.83 in
7.0 in
+42 mm
5.15 in
7.5 in
+35 mm
5.13 in
8.0 in
+35 mm
5.38 in
8.5 in
+30 mm
5.43 in
9.0 in
+20 mm
5.29 in
9.5 in
+15 mm
5.34 in
10.0 in
+25 mm
5.98 in
Note how backspacing stays in a fairly narrow band (roughly 4.8–6.0 in) across a wide range of widths — because street-fitment offsets are chosen to keep the tire roughly centered under the fender as width grows.
Offset and backspacing are two different ways of measuring the same thing: where a wheel's mounting face sits relative to its rim.
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Walk-through
How to Use This Calculator
3 steps▸
1
Pick a conversion direction
Measure overall measured overall wheel width from the outer rear flange edge to the outer front flange edge. Enter this measured width in inches, not the stamped nominal bead-seat width. Flange projection varies; do not assume the stamped width is overall width.
2
Enter measured overall wheel width and the known measurement
Enter the wheel's width in inches (needed for both directions, since backspacing is measured from the back edge of the rim, not the centerline) and the offset (mm) or backspacing (in) you already have. The result updates instantly as you type.
3
Read the converted value
The result card shows the converted measurement as the headline number, with the original measurement repeated underneath for reference, plus both values side by side in the stat row so you can double-check either direction at a glance.
Backspacing is measured from the wheel's mounting face to its back edge, while offset (ET) is measured from the true centerline. Since the centerline sits at half the wheel's width, adding the offset (converted from mm to inches) to half the width gives the equivalent backspacing. A negative offset subtracts from half the width, shrinking backspacing. Width means measured overall flange width. Backspacing is rear flange edge to mounting pad. The nominal stamped bead-seat width must not be substituted.
This is the same formula rearranged to solve for offset: subtract half the measured overall wheel width from the measured backspacing, then convert the inch difference to millimeters. A backspacing smaller than half the width yields a negative offset (more poke/dish). Width means measured overall flange width. Backspacing is rear flange edge to mounting pad. The nominal stamped bead-seat width must not be substituted.
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Glossary
Key Terms Explained
6 terms▸
Backspacing ↗The distance, in inches, from the wheel's mounting (hub) face to the back edge of the rim — the inner lip closest to the vehicle's suspension and brakes. It's always a positive number since it's a physical measurement, not a signed direction.
Offset (ET) ↗The distance, in millimeters, from the wheel's true centerline (the exact midpoint of its width) to the mounting face. Offset is signed: positive pulls the wheel inward toward the vehicle, negative pushes it outward. "ET" comes from the German Einpresstiefe ("insertion depth").
Measured overall wheel width ↗The distance across the wheel from bead seat to bead seat, in inches (e.g. 8 in a "17x8" wheel). Needed to convert between offset and backspacing because it defines where the centerline sits relative to the back edge.
Centerline ↗The imaginary line running through the exact middle of the wheel's width — the reference point offset is measured from. It does not move regardless of where the mounting face is positioned.
Mounting face ↗The flat surface of the wheel that bolts up against the vehicle's hub. Both offset and backspacing describe how far this face sits from a fixed reference (the centerline for offset, the back edge for backspacing).
Hub face ↗The vehicle-side surface the wheel's mounting face bolts to. "Hub-to-mounting-face" measurements (a separate hands-on method for finding offset) are taken from this surface, not from the wheel itself.
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Scenarios
Real-World Examples
2 worked examples▸
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Measured overall width
Measured overall width
Result
5.50 in backspacing
Overall flange width 9 inOffset +25.4 mm
An 8-inch nominal wheel may measure 9 inches overall, but the allowance must be verified; the equation uses the measured 9 inches.
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Inverse conversion
Inverse conversion
Result
+12.7 mm offset
Overall flange width 9 inBackspacing 127 mm (5 in)
Offset is 127 - 9 × 25.4 / 2 = 12.7 mm. No nominal-width or hidden flange assumption is used.
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Reference
Cite This Calculator
APA & MLA▸
Use either format to cite this calculator in a paper, report, or resource list.
Offset and backspacing are two different ways of measuring the same thing: where a wheel's mounting face sits relative to its rim. Wheel manufacturers almost always publish offset (ET, in mm); people measuring an existing wheel by hand usually end up with backspacing (in inches) instead. This calculator converts between the two in either direction so you can compare a spec sheet number against a hand measurement, or check whether a new wheel will clear your fender and brakes before you buy it.
Why two measurements exist for the same thing
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Offset uses the wheel centerline; backspacing uses the outer rear flange. Measure overall flange width for the centerline calculation. For an illustrative 8-inch nominal wheel measured 9 inches overall, +25.4 mm offset corresponds to 5.5 inches backspacing. At 127 mm backspacing and 9-inch overall width, offset is +12.7 mm. Actual flange projection must be measured or supplied by the manufacturer.
The formula that connects them
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Because the centerline sits at exactly half the wheel's width, backspacing and offset are related by backspacing = measured overall wheel width / 2 + offset / 25.4 (converting millimeters to inches). Flip it around to solve for offset: offset = (backspacing - measured overall wheel width / 2) x 25.4. The measured overall wheel width is required in both directions — it's the only way to translate a centerline-based measurement (offset) into a back-edge-based one (backspacing), or vice versa.
Using this to check fitment
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A larger backspacing (or more positive offset) tucks the wheel further inboard, toward suspension and brake components — risking rubbing on struts, control arms, or big brake calipers. A smaller backspacing (or more negative offset) pushes the wheel further outboard, toward the fender lip — risking rubbing on the fender itself or sticking out past it ("poke"). Because shops and forums mix offset and backspacing depending on the wheel's origin, converting both numbers to the same unit before comparing is the fastest way to check whether a candidate wheel will actually clear your specific setup.
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Questions
Frequently Asked Questions
6 questions▸
What's the difference between backspacing and offset?+
Use measured overall flange width, not nominal stamped bead-seat width. Backspacing is the distance from the outer rear flange edge to the mounting pad. Confirm the manufacturer specification and actual fitment.
What is the backspacing formula?+
Use measured overall flange width, not nominal stamped bead-seat width. Backspacing is the distance from the outer rear flange edge to the mounting pad. Confirm the manufacturer specification and actual fitment.
Which measurement do wheel shops actually use?+
Both are common — offset (ET) is the standard on new OEM and most aftermarket wheels because it's stamped directly on the wheel and printed in manufacturer spec sheets. Backspacing shows up more often on older, custom, or unbranded wheels where no offset spec exists and someone measured it by hand.
What does more backspacing mean for fitment?+
More backspacing means the wheel's face sits further from the back edge relative to the mounting point, which pulls the wheel further inboard — toward the vehicle's suspension and brake components. Less backspacing pushes the wheel further outboard, toward (or past) the fender.
Can a wheel with negative offset still have a normal backspacing number?+
Use measured overall flange width, not nominal stamped bead-seat width. Backspacing is the distance from the outer rear flange edge to the mounting pad. Confirm the manufacturer specification and actual fitment.
What units does this calculator use?+
Use measured overall flange width, not nominal stamped bead-seat width. Backspacing is the distance from the outer rear flange edge to the mounting pad. Confirm the manufacturer specification and actual fitment.
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