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Wheel Offset Calculator

Your numbers

Change any value to update the result.

Nominal outer movement per wheel

+27.70mm

Nominal outer movement per wheel: +27.70 mm
Nominal inner clearance gain: +2.30 mm; Axle centreline track change: +30.00 mm; Current effective ET: +35.00 mm; New effective ET: +20.00 mm; Current wheel-face backspacing: Enter valid measured overall width; symmetric lips required.; New wheel-face backspacing: Enter valid measured overall width; symmetric lips required.; Current wheel-face frontspacing: Enter valid measured overall width; symmetric lips required.; New wheel-face frontspacing: Enter valid measured overall width; symmetric lips required.; Current nominal inboard reach from hub: 142.95 mm; New nominal inboard reach from hub: 140.65 mm; Current nominal outboard reach from hub: 72.95 mm; New nominal outboard reach from hub: 100.65 mm
Nominal inner clearance gain
+2.30 mm
Axle centreline track change
+30.00 mm
Current effective ET
+35.00 mm
New effective ET
+20.00 mm
Current wheel-face backspacing
Enter valid measured overall width; symmetric lips required.
New wheel-face backspacing
Enter valid measured overall width; symmetric lips required.
Current wheel-face frontspacing
Enter valid measured overall width; symmetric lips required.
New wheel-face frontspacing
Enter valid measured overall width; symmetric lips required.
Current nominal inboard reach from hub
142.95 mm
New nominal inboard reach from hub
140.65 mm
Current nominal outboard reach from hub
72.95 mm
New nominal outboard reach from hub
100.65 mm
Wheel cross-section comparison
Inboard / vehicle sideOutboard / fender side
Wheel cross-section comparisonHorizontal positions share one scale. Bead seats and symmetric measured rim lips are shown, without a tyre, brake or suspension envelope. This diagram does not establish vehicle clearance.12

1: Current wheel · 2: New wheel

  • Nominal bead seats
  • Wheel centre
  • Vehicle hub
  • Wheel mounting face

Horizontal positions share one scale. Bead seats and symmetric measured rim lips are shown, without a tyre, brake or suspension envelope. This diagram does not establish vehicle clearance.

Examples

Compare nominal wheel positions from width and effective ET: offset minus spacer. Positive means farther out, more inner room, or wider axle track. Measured overall widths enable back/frontspacing for symmetric rim lips; 0 means unknown. Geometry does not approve vehicle fit.

Worked examples

Case 1

Current wheel width
8.5in
Current wheel offset (ET)
35mm
Current spacer per wheel
0mm
New wheel width
9.5in
New wheel offset (ET)
20mm
New spacer per wheel
0mm
Current overall width (0 = unknown)
0in
New overall width (0 = unknown)
0in

+27.70mm

Nominal inner clearance gain
+2.30mm
Axle centreline track change
+30.00mm
Current effective ET
+35.00mm
New effective ET
+20.00mm
Current wheel-face backspacing
Enter valid measured overall width; symmetric lips required.
New wheel-face backspacing
Enter valid measured overall width; symmetric lips required.
Current wheel-face frontspacing
Enter valid measured overall width; symmetric lips required.
New wheel-face frontspacing
Enter valid measured overall width; symmetric lips required.
Current nominal inboard reach from hub
142.95mm
New nominal inboard reach from hub
140.65mm
Current nominal outboard reach from hub
72.95mm
New nominal outboard reach from hub
100.65mm
Open with these values

Case 2

Current wheel width
8in
Current wheel offset (ET)
35mm
Current spacer per wheel
0mm
New wheel width
9in
New wheel offset (ET)
45mm
New spacer per wheel
0mm
Current overall width (0 = unknown)
0in
New overall width (0 = unknown)
0in

+2.70mm

Nominal inner clearance gain
-22.70mm
Axle centreline track change
-20.00mm
Current effective ET
+35.00mm
New effective ET
+45.00mm
Current wheel-face backspacing
Enter valid measured overall width; symmetric lips required.
New wheel-face backspacing
Enter valid measured overall width; symmetric lips required.
Current wheel-face frontspacing
Enter valid measured overall width; symmetric lips required.
New wheel-face frontspacing
Enter valid measured overall width; symmetric lips required.
Current nominal inboard reach from hub
136.60mm
New nominal inboard reach from hub
159.30mm
Current nominal outboard reach from hub
66.60mm
New nominal outboard reach from hub
69.30mm
Open with these values

Case 3

Current wheel width
8.5in
Current wheel offset (ET)
18mm
Current spacer per wheel
0mm
New wheel width
9in
New wheel offset (ET)
-12mm
New spacer per wheel
0mm
Current overall width (0 = unknown)
0in
New overall width (0 = unknown)
0in

+36.35mm

Nominal inner clearance gain
+23.65mm
Axle centreline track change
+60.00mm
Current effective ET
+18.00mm
New effective ET
-12.00mm
Current wheel-face backspacing
Enter valid measured overall width; symmetric lips required.
New wheel-face backspacing
Enter valid measured overall width; symmetric lips required.
Current wheel-face frontspacing
Enter valid measured overall width; symmetric lips required.
New wheel-face frontspacing
Enter valid measured overall width; symmetric lips required.
Current nominal inboard reach from hub
125.95mm
New nominal inboard reach from hub
102.30mm
Current nominal outboard reach from hub
89.95mm
New nominal outboard reach from hub
126.30mm
Open with these values

How it's calculated

Δhalf-width = (new width − old width) × 25.4 ÷ 2; ΔET = new effective ET − old effective ET

  1. StepEnter both nominal widths in inches and signed offsets in millimetres.
  2. StepEnter spacer thickness per wheel, not the combined axle thickness.
  3. StepRead outer movement, inner clearance gain, axle track and effective ET together; negatives reverse direction.
  4. StepFor symmetric rim lips, enter measured lip-to-lip widths in inches; leave 0 if unknown.
  5. ResultBack/frontspacing uses the wheel's mounting face; spacers shift that face away from the vehicle hub.+27.70mm

What this number means

Effective ET is stamped offset minus spacer thickness. Nominal outer movement is Δhalf-width − ΔET; inner clearance gain is −Δhalf-width − ΔET, per wheel. Track change is −2 × ΔET for equal changes on both sides and unchanged hubs.

Width alone leaves centreline track unchanged. From 8.5-inch ET35 to 9.5-inch ET20 without spacers, the nominal outer edge moves 27.70 mm outward and inner clearance gains 2.30 mm. The diagram compares these bead-seat positions; it models no tyre, brake or suspension envelope.

For back/frontspacing, measure each actual lip-to-lip overall width; the calculator never adds a guessed flange allowance. The lips must be symmetric about the bead-seat centreline: backspacing = half overall width + stamped ET; frontspacing = half overall width − stamped ET, in millimetres. These distances start at the wheel mounting face and do not change with a spacer; the hub-relative diagram translates that face by spacer thickness.

Leave width at 0 when unknown. Width smaller than the nominal width, or a mounting face outside the lip envelope, makes that wheel's spacing unavailable. Use manufacturer dimensions or direct measurements for asymmetric rims.

A negative nominal reach means the reference edge lies beyond the hub in the opposite direction. None of these results establishes remaining vehicle clearance.

A change is not a remaining gap

The result does not know your existing clearance. A positive gain does not prove that a wheel, tyre or spacer fits or is approved.

Measured width and symmetric lips

Overall width includes both rim lips; nominal width is measured between bead seats. Back/frontspacing requires your measured overall width and symmetric lips, with no universal flange allowance. Enter 0 when unknown; use manufacturer dimensions for asymmetric rims.

Track is measured at wheel centres

The distance between outer wheel edges also changes with width. Do not use that distance as axle track.

Reference table

Without spacersOuter movementInner clearance gain
8 ET35 → 9 ET45+2.70 mm−22.70 mm
8.5 ET18 → 9 ET−12+36.35 mm+23.65 mm
8.5 ET35 → 9.5 ET20+27.70 mm+2.30 mm

Questions

Does a lower offset move the wheel outward?

For the same width and spacer, reducing ET moves the wheel outward by the same number of millimetres. Changing width at the same time also moves its edges.

How do spacers affect the calculation?

Subtract the spacer thickness from the stamped ET for that wheel. Enter current spacers as well as new ones so the comparison does not count an existing spacer twice.

What does negative inner clearance gain mean?

The nominal inner edge moves toward the suspension-side reference and reduces the gap. It is a change, not an actual measured clearance.

Does this calculate backspacing?

Yes, when you enter the measured overall lip-to-lip width and the lips are symmetric about the bead-seat centreline. The output is measured from the wheel mounting face, so spacers do not change it. With unknown widths or asymmetric lips, use direct measurement or manufacturer dimensions.

Will these wheels fit my car?

This tool cannot approve fitment. Check the vehicle and wheel specifications, tyre dimensions, brake clearance and full steering and suspension movement with a qualified fitter.

Sources and last check

  1. TireCalculator worked fitment examplesFormula / method
  2. ArrayKit wheel comparison exampleFormula / method
  3. Tire Rack: wheel offset and backspacingFormula / method
  4. Discount Tire: measuring rim widthMeasured data
  5. Speedway WheelWise instructions, page 8Measured data · Calculation convention
  6. Selecting Wheels, page 5Measured data · Calculation convention

Information, not professional advice.