Wheel Offset and Backspacing
Converting wheel offset to backspacing, and where the inch hides: offset (ET, mm, mounting face to centerline) and backspacing (in, mounting face to inboard rim edge) describe the same fitment in different units. backspacing = rim_width/2 + 0.5 + offset/25.4, and the rim 'width' is the bead seat -- the wheel is ~1 in wider overall (half an inch per flange), the omission that makes a fitment come out an inch wrong. An 8 in wheel at +45 mm has 6.27 in backspacing and 2.73 in frontspacing; a zero-offset wheel sits nearly 1.8 in further out. A more positive offset pulls the wheel inboard (more fender, less brake/strut clearance). A fitment aid; the wheel, hub, and suspension clearances govern.
Formula and source
overall_width = rim_width + 1; backspacing = rim_width/2 + 0.5 + offset_mm/25.4; offset_mm = (backspacing - rim_width/2 - 0.5) x 25.4; frontspacing = overall_width - backspacing.
Wheel offset / backspacing conversion (Tire & Rim Association wheel dimensions), first-principles geometry, by name; the actual wheel, hub, and suspension clearances govern.
Audience
This tile is built for mechanics and the adjacent professions in the Mechanic - Auto, Marine, Aviation group. The interactive calculator runs entirely in your browser. No account, no fee, no advertising, no tracking.
Related tools
Posture
Rough Logic answers the math question the working professional asks on the job. The site is a calm, fast, ad-free, account-free, ever-free reference. It does not interpret code. It does not replace the licensed professional. It does not store your inputs. The Authority Having Jurisdiction governs all installations and inspections.