Wheel Offset Calculator
Convert wheel offset (ET, in mm) to backspacing and back, and compare two wheels to see how far the inner and outer rim edges move in or out.
Result…
How it's calculated
Overall width = stamped width + 1 in (0.5 in flange each side), or the measured lip-to-lip width Backspacing (in) = overall width ÷ 2 + offset (mm) ÷ 25.4 Front spacing = overall width − backspacing Offset (mm) = (backspacing − overall width ÷ 2) × 25.4 Inner lip move = new backspacing − old · outer lip move = new front spacing − old
Offset (ET, from the German Einpresstiefe) is the distance from the wheel's centreline to its mounting face, in millimetres: positive when the face sits toward the outside. Backspacing is the distance from the mounting face to the inner edge of the rim, in inches. The stamped width (the 8 in 8Jx18) is measured between the bead seats, and the flanges add about half an inch each side, so this calculator adds 1 inch for the overall width unless you enter a measured lip-to-lip width. Comparing two wheels shows how far the inner lip moves toward the suspension and the outer lip toward the arch.
Your numbers, step by step
About the Wheel Offset calculator
This wheel offset calculator converts offset (ET, in millimetres) to backspacing (in inches) and back, and compares two wheels so you can see how a new set will sit. Offset is the distance from the wheel's centreline to its mounting face; backspacing is the distance from that face to the inner edge of the rim. Enter the width stamped on the wheel, such as the 8 in 8Jx18, and the calculator adds the usual half inch of flange on each side to get the overall width, or enter a width you measured lip to lip. In compare mode it shows how far the inner lip moves toward the suspension and how far the outer lip moves toward the wheel arch, in millimetres, with every step of the arithmetic shown.
How to use it
- Choose what to “Work out”: “Backspacing from offset”, “Offset from backspacing” or “Compare two wheels”.
- Under “Width you enter”, pick “Nominal (as stamped, e.g. 8J)” or “Measured lip to lip”.
- Enter the “Wheel width” in inches and the “Offset (ET)” in millimetres, or the “Backspacing” in inches.
- To compare, add the “New wheel width” and “New wheel offset (ET)”.
- Read the backspacing, front spacing and the inner and outer lip moves.
Worked example
Going from a 7.5 in wheel at ET 45 to an 8.5 in wheel at ET 35. The current wheel is 8.5 in overall, so its backspacing is 4.25 + 45 ÷ 25.4 = 6.02 in (152.95 mm). The new one is 9.5 in overall: 4.75 + 35 ÷ 25.4 = 6.13 in. The extra inch of width adds 12.7 mm each side and the 10 mm lower offset pushes the face out, so the outer lip moves 12.7 + 10 = 22.7 mm out and the inner lip moves 12.7 − 10 = 2.7 mm in.
Updated
Questions people ask
How do I convert wheel offset to backspacing?
Backspacing = stamped width ÷ 2 + 0.5 in + offset in mm ÷ 25.4. AmericanTrucks gives the same formula, with the 0.5 in for the flange beyond the bead seat.
What is the difference between positive and negative offset?
With positive offset the mounting face sits toward the outside of the wheel, so the wheel tucks in; with negative offset it sits toward the inside, so the wheel sticks out further. Zero offset puts the face on the centreline.
How much does a lower offset push the wheel out?
Each millimetre less offset moves both lips about 1 mm outward on the same width of wheel. A wider wheel then adds half the extra width on each side.
Why does my backspacing measurement not match the formula?
The stamped width is measured between the bead seats, and flange shapes differ, so a measured wheel can be a few millimetres off the half-inch rule. Choose “Measured lip to lip” if you measured the whole wheel.
Does a new tyre size change the fit too?
Yes. A wider or taller tyre needs more room at the inner and outer edges; check it with the tire size calculator alongside the wheel figures.
What can this offset comparison not tell me?
It cannot see brake calipers, suspension parts or the wheel arch shape, and rules on wheels sticking out past the body differ by country. A test fit or a wheel and tyre specialist is the safe check.