Wheel Offset and Backspacing Converter
Wheel Offset and Backspacing Converter
ET in millimetres to backspacing in inches and back, with the half-inch-a-side rim flange allowance that decides the answer — plus what a change of offset or width does to the inboard clearance and the poke, and why a lower ET moves the wheel outward.
Offset and backspacing
an 8-inch wheel at ET35, compared with a 9-inch wheel at ET20
One equation, run in both directions
- stated width
- the number on the wheel, in inches, measured between the bead seats — not across the flanges
- +1 inch
- the flange allowance: about half an inch of rim flange standing proud of each bead seat. Three independent publishers use exactly this figure. Omit it and every backspacing comes out 12.7 mm short
- offset (ET)
- mounting face to wheel centreline, in millimetres, positive when the face is outboard. Raising it pulls the wheel inboard
- 25.4
- millimetres in an inch. The formula converts units mid-equation because the two measurements are conventionally given in different ones
- what it cannot tell you
- whether the wheel will clear anything. That needs the strut, the caliper, the arch and the suspension travel
Worked example
an 8-inch wheel at ET35, compared with a 9-inch wheel at ET20
Width across the flanges = 8 + 1 = 9 in, because the stated 8 is measured between the bead seats and each flange stands about half an inch proud
Half of that is 4.5 in from the centreline to each rim edge
ET35 puts the mounting face 35 mm = 1.3780 in outboard of the centreline, so backspacing = 4.5 + 1.3780 = 5.8780 in
Forget the allowance and you get 5.3780 in instead — half an inch, 12.7 mm, too little. That is more than the strut clearance on plenty of cars
The 9-inch wheel at ET20 has a backspacing of 5.7874 in, so its inboard edge sits 2.3 mm further OUT than the original's — slightly more strut clearance, not less
But its outboard edge moves 27.7 mm further out toward the fender, which is where an extra inch of width and 15 mm less offset both land
The two measurements, and the third quantity they need
| Quantity | Measured from | Measured to | Units | Sign |
|---|---|---|---|---|
| Offset (ET) | the wheel’s mounting face — the flat that bolts to the hub | the wheel’s CENTRELINE, halfway across the rim | millimetres | positive when the mounting face is outboard of the centreline; negative when inboard. Cast into the wheel as ET35, ET-12 and so on |
| Backspacing | the same mounting face | the INBOARD rim edge — the back lip | inches, by convention | always positive on any road wheel |
| Frontspacing | the same mounting face | the OUTBOARD rim edge | inches | positive unless the offset is extreme. Backspacing + frontspacing = the width across the flanges, always |
| Stated width | one bead seat | the other bead seat | inches | the number stamped on the wheel, as in 8J. It is NOT the outside width |
The flange allowance, which is the difference between the two answers you find online
| Source | Formula as published | Allowance |
|---|---|---|
| Mickey Thompson Tires, tech bulletin “Backspacing and Offset” | “(Wheel Width + 1)/2 + (offset × .03937)”, and inverted, “Backspacing − (Wheel Width + 1)/2 × 25.4 = offset” | +1 inch |
| Wheel-size.com | “Backspacing = ((Wheel Width + 1) / 2) + (Offset / 25.4)” | +1 inch |
| Element Wheels | “offset = 25.4 × (backspacing − .5 × full rim width)”, with the warning “Be sure not to use the published width of the wheel found stamped on the wheel. The actual width of the wheel is normally 1 inch more than the published width.” | +1 inch |
| Wikipedia, Wheel sizing | no conversion formula given, but the reason for the allowance: “The width is the inside distance between the bead seat faces” | — |
| A calculator that omits it | backspacing = width/2 + offset/25.4 | 0 — and the answer is half an inch (12.7 mm) too small every time |
Which way things move when the offset changes, at a constant width
| Change in ET | Offset is | Inboard rim edge | Outboard rim edge | Track, per corner |
|---|---|---|---|---|
| -25 mm | lower | 25.0 mm outboard, away from the suspension | 25.0 mm out toward the fender | 25.0 mm |
| -10 mm | lower | 10.0 mm outboard, away from the suspension | 10.0 mm out toward the fender | 10.0 mm |
| +0 mm | unchanged | no change | no change | 0 mm |
| +10 mm | higher | -10.0 mm inboard, toward the suspension | -10.0 mm in, away from the fender | -10.0 mm |
| +25 mm | higher | -25.0 mm inboard, toward the suspension | -25.0 mm in, away from the fender | -25.0 mm |
What this page cannot tell you
| Question | Why not |
|---|---|
| Will this wheel clear my car? | Not computable from offset and width. Inboard clearance depends on the strut or spring body, the brake caliper, the steering knuckle and how much the suspension compresses and steers; outboard clearance depends on the arch lip, the liner and the tyre’s own section width and sidewall bulge. Two cars with the same stud pattern can accept wheels two inches apart in width. The only reliable method is a test fit with the suspension at full compression and full lock. |
| Will the tyre rub? | That is the tyre’s question, not the wheel’s. A wider tyre on the same wheel bulges further, and its section width depends on the rim width it is mounted on. For the tyre dimensions see the tyre size converter. |
| What spacer do I need? | A spacer of thickness s reduces the effective ET by s, so the arithmetic here covers it — set the second wheel’s ET to your ET minus the spacer thickness. Whether a spacer is safe or legal on your vehicle is a different question, and it turns on stud length, load path and local regulation. |
| Does changing offset affect the steering? | Yes, and this page does not quantify it. Moving the wheel’s centre away from the steering axis changes the scrub radius, which changes steering feel, kickback over bumps and the loads on bearings and joints. It needs the suspension geometry, which a wheel spec does not contain. |
| Bolt pattern, centre bore, load rating | None of them is derivable from offset and width, and all three must match the vehicle. The bolt pattern and centre bore are measurements; the load rating is a marking. Get them from the wheel and from the vehicle’s own specification. |
Same face, opposite ends, and an inch of rim flange in between
Offset and backspacing are the same measurement taken to opposite ends of the wheel, which is why one cannot become the other without the width. Both start at the mounting face — the flat that bolts to the hub. Offset (ET, from the German Einpresstiefe, “press-in depth”) runs from that face to the wheel’s centreline and is quoted in millimetres, positive when the face is outboard. Backspacing runs from the same face to the inboard rim edge and is quoted in inches. So backspacing = half the wheel’s width + the offset, and the width is not optional information: it is the bridge between the two. That is this whole vertical’s rule in one page — a conversion whose answer depends on a third quantity is one a search engine’s unit widget cannot do for you.
And the width in that equation is not the width stamped on the wheel. The stated width — the 8 in 8J×17 — is measured between the bead seats, where the tyre sits. The rim flanges stand proud of those seats, conventionally about half an inch on each side, and backspacing is measured to the outside of the inboard flange. So the working formula is backspacing = (stated width + 1) ÷ 2 + offset ÷ 25.4, and three independent publishers — Mickey Thompson, Wheel-size.com and Element Wheels — print exactly that +1 inch, with Element Wheels spelling out the reason: “Be sure not to use the published width of the wheel found stamped on the wheel. The actual width of the wheel is normally 1 inch more than the published width.” A calculator that omits the allowance gives a backspacing 12.7 mm too small, every time, and 12.7 mm is more than the gap between rim and strut on plenty of cars. The allowance is an input above so you can see the size of it.
Which way does the wheel move? Low ET, wheel out. This is the classic error and it is worth deriving rather than memorising. The hub’s face is fixed by the car. The wheel’s centreline sits ET millimetres inboard of the wheel’s own mounting face, so bolting on a wheel with a smaller ET puts its centreline — and both its rim edges — further outboard relative to the hub. More poke at the fender, more room at the strut. A larger ET tucks the wheel in: less poke, and less inboard clearance, which is the direction that fouls a strut body or a spring perch. Widening the wheel is a separate movement again: at a constant ET, an extra inch of width pushes both edges out by 12.7 mm each, so you spend clearance at both ends simultaneously. The calculator works out all three movements for a candidate wheel against the one you have.
What this page cannot tell you is whether the wheel will actually clear. Inboard, the obstacles are the strut or spring body, the brake caliper and the steering knuckle, and the clearance changes as the suspension compresses and the wheel steers — the tightest point is rarely where the car sits on the ground. Outboard, the obstacles are the arch lip and the liner, and the thing that touches them is usually the tyre’s sidewall bulge rather than the rim, which depends on the tyre and on the rim width it is mounted on. Two cars with the same stud pattern can accept wheels two inches apart in width. So use these numbers to compare a candidate against what came off the car, and then test fit one wheel and tyre, at full compression and full lock, before you buy four. Offset also changes the scrub radius and therefore the steering feel and the loads on bearings and joints, and that needs the suspension geometry, which a wheel specification does not contain. For the tyre half of the problem see the tyre size converter, and for the inch fractions the inch fraction converter.
Frequently asked questions
How do I convert offset to backspacing?
Backspacing in inches = (stated width + 1) ÷ 2 + offset ÷ 25.4. An 8-inch wheel at ET35 has a backspacing of 4.5 + 1.378 = 5.878 in. The +1 is the flange allowance and it is not optional: the stated width is measured between the bead seats, and the rim flanges stand about half an inch proud of each seat, so the actual outside width is an inch more than the number on the wheel. You cannot do the conversion at all without the width, because offset and backspacing measure from the same mounting face to opposite ends of the wheel.
Why does the formula add an inch?
Because the stated width is not the outside width. Wikipedia’s Wheel sizing article puts it plainly: “The width is the inside distance between the bead seat faces.” The flanges that hold the tyre bead in place stand proud of those seats, by convention about half an inch each side, and backspacing is measured to the outside of the inboard flange. Element Wheels says the same thing from the other end — “The actual width of the wheel is normally 1 inch more than the published width” — and Mickey Thompson and Wheel-size.com both print the +1 in their formulas. Omit it and every answer is 12.7 mm short, which is more than the strut clearance on many cars.
Does a lower offset push the wheel out or in?
Out. This is the one everybody gets backwards, so it is worth being careful. The hub’s mounting face is fixed by the car. The wheel’s centreline sits ET millimetres inboard of its own mounting face, so when you bolt on a wheel with a smaller ET the whole wheel shifts outboard relative to the hub — more poke at the fender, more room at the strut. A higher ET tucks the wheel in and eats inboard clearance. The short form: low ET, wheel out.
What happens if I fit a wider wheel at the same offset?
Both edges move out by half the extra width. An extra inch at the same ET puts 12.7 mm further into the arch and 12.7 mm further toward the fender, so you use up clearance at both ends at once. That is why wider wheels usually come with a higher offset if they are meant to tuck, or a lower one if they are meant to fill the arch — and why quoting a wheel as “9J ET20” tells you nothing until you compare it with what came off. The calculator above does that comparison.
Is backspacing ever negative?
Not on a road wheel. Backspacing is measured from the mounting face to the inboard rim edge and by convention it is always quoted as a positive number; it would only go negative if the mounting face sat outboard of the inboard flange, which would mean an offset larger than half the wheel’s width. Offset, on the other hand, is routinely negative — deep-dish and off-road wheels are often ET-12 or ET-25, which puts the mounting face inboard of the centreline and the wheel a long way out.
Can I use a spacer instead of a different offset?
Arithmetically, a spacer of thickness s reduces the effective offset by s and increases the backspacing by nothing — the wheel and everything on it simply moves outboard by s. So you can model one on this page by setting the second wheel’s ET to your ET minus the spacer thickness. Whether it is a good idea is a different question that this page cannot answer: it depends on stud length and thread engagement, on whether the spacer is hub-centric, on the extra bending load on the bearing and stud, and on what your local roadworthiness rules say. Ask someone who can see the car.
Will this wheel fit my car?
This page cannot tell you, and no page working from offset and width alone can. Inboard clearance depends on the strut or spring body, the brake caliper and the steering knuckle, and on how much the suspension compresses and steers; outboard clearance depends on the arch lip, the liner and the tyre’s own bulge, which itself depends on the rim width it is mounted on. Two cars sharing a stud pattern can accept wheels two inches apart. Use the numbers here to compare a candidate with what came off, then test fit one wheel with the suspension at full compression and full lock before you buy four. For the tyre side of the arithmetic see the tyre size converter.
Related calculators
References
- Mickey Thompson Tires, tech bulletin Backspacing and Offset (read 27 September 2026), for the definitions and the formula: backspacing is “the distance from the back of the mounting pad to the back lip of the wheel”, offset is “the distance from the mounting surface to the centerline of the wheel, expressed in millimeters”, and “(Wheel Width + 1)/2 + (offset × .03937)” gives the one from the other, with the inverse “Backspacing − (Wheel Width + 1)/2 × 25.4 = offset”.
- Wheel-size.com, Wheel Offset and Backspacing (read 27 September 2026), the same formula verbatim from a second publisher — “Backspacing = ((Wheel Width + 1) / 2) + (Offset / 25.4)” and “Offset = ((Backspacing − ((Wheel Width + 1) / 2)) * 25.4)” — and the note that backspacing “is always indicated as a positive number”.
- Element Wheels, Custom Wheels Offset (read 27 September 2026), a third publisher, which says out loud why the allowance exists: “Be sure not to use the published width of the wheel found stamped on the wheel. The actual width of the wheel is normally 1 inch more than the published width.” Three sources, one allowance, half an inch a side; nobody found here omits it.
- Wikipedia, Wheel sizing (read 27 September 2026), for the reason the stated width is not the outside width — “The width is the inside distance between the bead seat faces” — and for the offset sign convention: ET is from the German Einpresstiefe, “literally press-in depth”, and “A positive offset means the hub-mounting surface is closer to the outside edge of the wheel, while a negative offset means the hub-mounting surface is closer to the inside edge.”