Wheel offset calculator
Offset ET, backspacing and how far the wheel moves.
Fill in the fields and the result will appear here automatically.
Offset ET is the distance from the mounting face to the centre of the rim, and it can be negative: on deep-dish wheels the face sits inboard. The practical question is nearly always the same — how far will the wheel move in or out with a different offset. The sign is counter-intuitive: a SMALLER offset pushes the wheel OUTWARDS — so the direction is spelled out in words, not just as a number.
How it works
Formula and logic
Backspacing = width/2 + ET + 12.7 mm; shift = old ET − new ET. Width is nominal bead-seat width in inches and is multiplied by 25.4 first. The 12.7 mm addition assumes 0.5 inch per rim edge and does not measure a particular wheel. The shift comparison holds width fixed and changes only ET. Width is positive and ET may have either sign; a negative calculated inner-edge distance is outside this model’s geometry.
Example
A 7-inch rim at ET 35 gives 136.6 mm backspacing; swapping to ET 45 pulls the wheel 10 mm inwards.
Fields and units
- Rim width, inches — in
- Offset ET, mm — mm
- New offset ET, mm — mm
How to use
- — Rim width in inches from the marking — 7J means 7 inches.
- — The ET figure is stamped on the wheel and can be negative.
- — The 12.7 mm addition is a nominal edge allowance, not a measurement of actual overall width.
- — Check actual clearance and permitted use separately; changing ET does not establish compatibility.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- Mickey Thompson: signed ET, backspacing and nominal width allowance, not fitment approval
- Limitation
- The 12.7 mm addition assumes 0.5 inch per rim edge and does not measure a particular wheel. The shift comparison holds width fixed and changes only ET.
FAQ
Why does a smaller offset push the wheel out?
Offset is measured from the mounting face, the surface that clamps to the hub. That face stays put, so reducing the offset moves the rim centre further from the hub — outwards.
How much can the offset change?
There is no universal ±5 mm allowance. Manufacturer-approved dimensions and checks of clearance, fasteners, loads and applicable rules are required. This calculation does not establish wheel-to-vehicle compatibility.
How does ET differ from backspacing?
ET is measured from the rim centre, backspacing from the inner edge. The first is stamped on European wheels, the second appears in American tables; half the rim width connects them.
Do spacers help?
A spacer’s thickness reduces effective ET in the geometric comparison, but does not guarantee compatibility. Mounting type, engagement length and permitted use depend on the particular system; follow its instructions rather than universal bolt advice.