Wheel Offset & Fitment
Backspacing from width and offset, how far a new wheel’s inner and outer faces move (clearance and poke), and the bolt circle from the stud spacing.
The inner and outer change in millimetres when you swap wheels, both wheels’ backspacing, and the PCD measured from adjacent studs — geometry only, no fitment promise.
Example: Going from 8 in ET35 to 9 in ET25 moves the outer face 22.7 mm outboard and the inner face 2.7 mm inboard; backspacing 5.88 → 5.98 in.
Two faces move,
not one number.
Where offset and backspacing meet, how the inner and outer faces shift when you change wheels, how the bolt circle is measured, and what fitment still depends on.
Offset and backspacing
Offset (ET) is the distance from the wheel’s centreline to its mounting face, positive when the face is towards the street side. Backspacing is the distance from the mounting face to the inner edge; with the nominal-width convention (rim width is measured bead seat to bead seat and the two flanges add about an inch) backspacing = (width + 1 in) ÷ 2 + offset. The page shows both so you can compare a wheel listed either way.
What moves when you change wheels
With the mounting face fixed at the hub, the inner bead sits at width ÷ 2 + offset inboard and the outer bead at width ÷ 2 − offset outboard. The change in each between two wheels is the page’s answer: more inboard means less clearance to suspension and brakes; more outboard means poke past the fender. A lower offset alone moves the whole wheel outboard; a wider wheel at the same offset grows both ways equally.
Bolt circle from the studs
The pitch circle diameter is easy to measure on 4-, 6- and 8-stud hubs (opposite studs) and awkward on 5-stud ones. From the centre-to-centre distance of two adjacent studs, PCD = d ÷ sin(180° ÷ n) — for five studs that is d × 1.7013. Measure carefully; catalogue patterns are a few millimetres apart.
What the number does not settle
The wheel is treated as a rim; the tire’s section width, sidewall bulge and diameter, the hub bore, stud length, brake caliper shape and the body’s own clearances at full lock and full compression are not modelled. Legal limits on wheels beyond the bodywork vary by country. Fitment is a physical check. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.