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06 / WHEEL TOOLS

Backspacing Calculator

Offset ↔ backspacing.

GEOMETRY CHECK

Your setup

Input in millimeters. Offset may be positive, zero, or negative.
Millimeters, measured outside edge to outside edge.

Nominal wheel width alone cannot establish backspacing. The example uses an explicitly assumed overall width.

What changes

GEOMETRY ESTIMATE

Example calculation. Replace these values with your setup.

Backspacing149.3 mm

Backspacing from offset

Overall width assumes symmetric lips.

Where backspacing is measured

Backspacing

Your setup
149.3 mm

From the inside rear lip of the wheel to the face that bolts onto the car.

01Your setup

Your setup: BackspacingBackspacing: 149.3 mm. Backspacing runs from the physical rear lip to the hub mounting face. The solid orange line is the mounting face; the dashed blue line, when shown, is the wheel center. Overall width defines both physical edge planes under the stated symmetric-flange assumption. Band height is illustrative. Negative oriented values retain their signs.Rear lipMounting faceBackspacing149.3 mm
← Inside · toward carOutside · toward fender →

Overall width assumes symmetric lips.

Orange arrows show the selected measurement. Change your inputs to update the drawing. Values describe static geometry.
Geometry results
MeasurementValue
228.6 mm
35.0 mm
149.3 mm
Assumptions & limits
  • The overall-width model assumes the inner and outer lips are symmetric around the nominal center plane.
  • Backspacing refers to the physical rear lip, not the nominal bead-seat plane. Verify the source of your width measurements.

Convert offset and backspacing

Convert between signed wheel offset and backspacing with an explicit width measurement. Choose symmetric overall width or nominal width plus a measured inner flange.

Example · 228.6 mm overall width, ET35

With symmetric flanges and an assumed overall width of 228.6 mm, +35 mm offset gives 149.3 mm of backspacing, approximately 5.88 inches. The inverse calculation returns ET35.

The example width is an assumption for this calculation, rather than a standard allowance for every wheel.

Backspacing questions

Can I calculate backspacing from nominal wheel width alone?

No. Nominal width is measured between the bead seats; backspacing ends at the physical inboard edge. Supply overall width with symmetric flanges, or nominal width with the measured inner-flange extension.

Should I always add one inch to the nominal width?

No universal flange allowance is used here. Wheel construction and edge dimensions vary. Use an actual measurement or a manufacturer specification appropriate to the wheel.

Which edge is used for backspacing?

Backspacing runs from the hub mounting face to the physical inboard wheel edge. Measure the same reference consistently; a tire sidewall bulge is a separate envelope and does not define wheel backspacing.

Methodology & limits

How this tool works

With symmetric flanges: backspacing = measured overall width / 2 + offset. With a measured inner flange: backspacing = nominal width / 2 + inner flange extension + offset.