Wheel Offset Calculator: Backspacing, Poke and Clearance

Wheel Offset Calculator

Compare current and new wheels to calculate offset, effective offset with spacers, backspacing, frontspacing, inner-clearance change, outer poke and track-width change.




Backspacing is measured from the mounting face to the outermost inner lip, so nominal width needs a lip allowance.

Current wheel
Use the stamped nominal width unless overall width is selected.
Positive ET moves the wheel centerline inward.
A spacer reduces effective offset.
Typical approximation: 1 inch total. Measure the actual overall width for precision.

New wheel

Optional tire and measured-clearance check
Set both tire widths to zero to compare wheels only.
Smallest measured gap to suspension or body on the inside.
Measured fender clearance at the relevant steering and suspension position.

Wheel fitment comparison

Current effective offset
New effective offset
Track-width change
Current backspacing
New backspacing
New frontspacing
Wheel inner-edge movement
Wheel outer-edge movement
Tire outer-edge movement

mounting face
current
new

MeasurementCurrentNewChange
Overall wheel width
Backspacing
Frontspacing
Tire section width
Estimated inner clearance
Estimated outer clearance

Wheel offset and backspacing formulas

Effective offset = wheel offset − spacer thickness.

Backspacing = overall wheel width ÷ 2 + effective offset. Convert the offset to the same unit as the width before adding it.

Frontspacing = overall wheel width − backspacing.

Track-width change = 2 × (current effective offset − new effective offset). A positive result means the left-to-right wheel-center distance increases.

Nominal width is not overall width. A wheel marked 18×8 is measured between bead seats. Backspacing uses the outermost inner lip, so this calculator adds an editable total lip allowance—1 inch by default. Measuring actual overall width gives the most accurate backspacing.

Example

Changing from an 8-inch ET35 wheel to a 9-inch ET25 wheel, with 1 inch of total lip allowance and no spacers, moves the wheel’s inner edge about 2.7 mm inward and its outer edge about 22.7 mm outward. Track width increases by 20 mm.

Fitment limits

This is a geometric comparison, not a fitment approval. Confirm brake-caliper clearance, hub bore, bolt pattern, stud engagement, load rating, tire-to-rim compatibility, steering lock, suspension travel, camber, bodywork and manufacturer requirements. Tire section width varies with the actual tire model, rim width and inflation; the optional tire result assumes the entered section width is centered on the wheel.

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