Wheel Offset & Backspacing Calculator
Offset, backspacing and bolt patterns — where the wheel flange adds exactly half an inch to every backspacing figure, whatever the wheel.
The flange allowance is a convention rather than a measurement — an inch of total extra width is the usual figure, and a particular wheel can differ a little. Whether a wheel actually fits also depends on brake caliper clearance, strut shape and how much the tyre bulges, none of which is arithmetic. Nothing is uploaded.
The flange costs exactly half an inch, on every wheel there is
Rim width is quoted bead seat to bead seat, because that is where the tyre sits. Backspacing is measured to the outer flange, which stands about half an inch proud of each bead seat. So backspacing is (width + 1)/2 + offset — and the flange term contributes a flat half inch that does not depend on the width, the offset, or anything else. Work it out as width/2 + offset instead and every wheel you check comes back half an inch shallower than it really is.
| Rim | Offset | Backspacing | Without the flange | Lost |
|---|---|---|---|---|
| 7" | ET45 | 5.772" | 5.272" | 0.500" |
| 8" | ET35 | 5.878" | 5.378" | 0.500" |
| 8" | ET45 | 6.272" | 5.772" | 0.500" |
| 9" | ET20 | 5.787" | 5.287" | 0.500" |
| 10" | ET-12 | 5.028" | 4.528" | 0.500" |
| 12" | ET-44 | 4.768" | 4.268" | 0.500" |
Widening a wheel at a fixed offset grows it evenly: two inches of extra width is one inch further in and one inch further out, because the offset is measured from the centreline. And a change of offset moves the outer edge by that much on any width of wheel, since the width term cancels when you take a difference.
| Offset change | Poke gained |
|---|---|
| ET45 → ET35 | 10 mm |
| ET45 → ET20 | 25 mm |
| ET35 → ET0 | 35 mm |
| ET35 → ET-12 | 47 mm |
| ET20 → ET-44 | 64 mm |
And a bolt circle is a chord you can reach with a ruler
A pitch circle is hard to measure because its centre is a hole. The gap between two adjacent studs is not — it is a chord, and a chord is PCD·sin(π/n). That fraction changes with the stud count, so the same 100 mm circle measures 70.7 mm across four studs and 58.8 mm across five. A four-stud chord is the PCD over root two; a six-stud chord is exactly the radius, which is why six-stud patterns are the easy ones to identify.
| Pattern | Studs | Bolt circle | Adjacent studs measure |
|---|---|---|---|
| 4×100 | 4 | 100 mm | 70.71 mm |
| 5×100 | 5 | 100 mm | 58.78 mm |
| 5×112 | 5 | 112 mm | 65.83 mm |
| 5×114.3 | 5 | 114.3 mm | 67.18 mm |
| 6×139.7 | 6 | 139.7 mm | 69.85 mm |
| 8×165.1 | 8 | 165.1 mm | 63.18 mm |
The 5×114.3 — the commonest pattern there is — measures 67.18 mm between neighbours, close enough to a 5×112's 65.83 mm that a tape measure will not separate them and a wheel that almost fits will destroy your studs. The 6×139.7 measures 69.85 mm, which is 139.7 halved to the last decimal place.
How to use
- Enter the rim width in inches, as stamped 8J or 9.5J.
- Enter the offset in millimetres.
- Enter your stud count and bolt circle if you want the pattern.
- Read the backspacing, the poke, and what a ruler should measure.
Frequently asked questions
How do I convert offset to backspacing?
Backspacing = (rim width + 1)/2 + offset in inches. The added inch is the flange allowance, which contributes exactly half an inch to the answer on every wheel there is.
Why is there an extra inch in the formula?
Rim width is quoted bead seat to bead seat, because that is where the tyre sits. Backspacing is measured to the outer flange, which stands about half an inch proud of each bead seat.
What happens if I forget the flange?
Every wheel comes back half an inch shallower than it really is — enough to turn one that fouls the strut into one that looks fine on paper. The error is a flat half inch and never shrinks on a narrow wheel.
What does a wider wheel do at the same offset?
It grows evenly. Two inches of extra width is one inch further in and one inch further out, because the offset is measured from the wheel centreline rather than from either edge.
How do I measure a bolt pattern without the centre?
Measure between two adjacent studs. That gap is a chord, and a chord is the bolt circle times the sine of pi over the stud count — so a 5x114.3 measures 67.18 mm between neighbours.
Why is a six-stud pattern easier to identify?
Because the chord across two adjacent studs is exactly the radius. A 6x139.7 measures 69.85 mm between neighbours, which is 139.7 halved to the last decimal place.
Is 5x114.3 close to 5x112?
Dangerously. They measure 67.18 mm and 65.83 mm between adjacent studs, a difference of 1.35 mm that a tape measure will not separate — and a wheel that almost fits will destroy the studs.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.