Calculators · Tyres and gearing
Wheel offset, backspacing and PCD calculator
When you fit a wheel of a different width or offset, it moves inward towards the suspension, outward towards the wing, or both. These calculators show exactly how far, convert between the two ways of describing wheel position — offset (ET) and backspacing — and work out a bolt pattern’s pitch circle diameter from a measurement between two bolts.
Offset and width change
Compares a new wheel with your current one and shows how far its inner edge moves towards the suspension and how far its outer edge moves towards the wing. Positive inner values mean less clearance to the strut; positive outer values mean the wheel sticks out further.
inner = (W₂/2 + ET₂) − (W₁/2 + ET₁) outer = (W₂/2 − ET₂) − (W₁/2 − ET₁)
Offset to backspacing
Backspacing is the distance from the mounting face to the inner lip, the measure common in the US. Because the lips add roughly half an inch on each side of the nominal width, it is estimated from width and offset.
backspacing = (W + 1 in) / 2 + ET
Backspacing to offset
Converts a backspacing measurement into metric offset (ET), so you can compare a US-listed wheel with a European one or with your car’s original specification.
ET = backspacing − (W + 1 in) / 2
Pitch circle diameter from bolt spacing
Works out the pitch circle diameter (PCD) of a bolt pattern from the centre-to-centre distance between two neighbouring bolts, which is easy to measure with a ruler or calipers.
PCD = s / sin(180° / n)
Worked example: 7.5J ET45 to 8.5J ET35
- Half widths: 7.5 in = 95.25 mm, 8.5 in = 107.95 mm.
- Inner edge: (107.95 + 35) − (95.25 + 45) = +2.7 mm closer to the suspension.
- Outer edge: (107.95 − 35) − (95.25 − 45) = +22.7 mm further out towards the wing.
- An 8 in wheel at ET35 has a backspacing of about 149 mm (5.9 in).
Neighbouring-bolt spacing of common bolt patterns
| Bolt pattern | Spacing between neighbouring bolts |
|---|---|
| 4×100 | 70.7 mm |
| 4×108 | 76.4 mm |
| 5×100 | 58.8 mm |
| 5×112 | 65.8 mm |
| 5×114.3 | 67.2 mm |
| 5×120 | 70.5 mm |
| 6×139.7 | 69.9 mm |
Related
About these results
Results are theoretical estimates from standard engineering formulas. Real-world figures depend on conditions these formulas do not capture. Always follow the manufacturer's specifications for maintenance, loading and safety decisions.
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