Wheel Offset Calculator
Wheel offset calculator with visual fitment guide. See poke, tuck, inner clearance, and spacer size when changing wheel width or ET.
A wheel offset calculator works out how the outer and inner edges of a new wheel will sit compared to stock when you change wheel width or offset (ET). Enter your stock and new wheel specs and the tool shows how much the wheel will poke past the fender or tuck under it, how much closer the inner edge moves to the suspension, and what size spacer would restore stock outer alignment.
About Wheel Offset Calculator
What Is Wheel Offset?
Wheel offset is the distance in millimetres from the wheel's centerline to the hub mounting face, marked as ET on the back of the wheel. ET stands for the German word Einpresstiefe (insertion depth), the standard adopted by ETRTO across Europe and now used worldwide on cast and forged wheels.
Three offset cases matter:
- Positive offset (e.g. ET+45): mounting face sits toward the front (outer) of the wheel. The wheel sits more inward toward the suspension. Standard on most front-wheel-drive cars and modern crossovers.
- Zero offset (ET 0): mounting face is exactly on the wheel's centerline.
- Negative offset (e.g. ET-25): mounting face sits toward the back (inner) of the wheel. The wheel pushes outward, common on trucks, 4x4s, and aggressive street builds.
The other measurement you will see is backspacing, which is the distance from the back edge of the wheel to the hub mounting face, measured in inches. Backspacing and offset describe the same thing from different reference points. The Tire Rack conversion is approximately: backspacing (in) = (wheel width / 2) + (offset mm / 25.4) + 0.5. The 0.5 inch (12.7 mm) allowance is the standard flange thickness used across the industry.
If you also need to check how a different overall tyre diameter affects speedometer readings, the tire size comparison tool covers the rotational maths. To decode a stamped sidewall like 225/40R18, the tyre size decoder breaks down width, aspect ratio, load index, and speed rating.
How the Calculator Works
The calculator uses two formulas to position each wheel relative to the hub face. Outer edge distance from hub = (wheel width / 2) - offset. Inner edge distance from hub = (wheel width / 2) + offset. The signs work because positive offset moves the mounting face outward, which mathematically reduces the distance to the outer edge and increases the distance to the inner edge.
Worked example. Stock 8.0J ET+45 to new 9.5J ET+25:
- Stock width = 8.0 in x 25.4 = 203.2 mm. Outer-from-hub = 203.2/2 - 45 = 56.6 mm. Inner-from-hub = 101.6 + 45 = 146.6 mm.
- New width = 9.5 in x 25.4 = 241.3 mm. Outer-from-hub = 241.3/2 - 25 = 95.65 mm. Inner-from-hub = 120.65 + 25 = 145.65 mm.
- Outer change = 95.65 - 56.6 = +39.05 mm. The new wheel pokes 39 mm further than stock.
- Inner change = 145.65 - 146.6 = -0.95 mm. Inner edge is essentially unchanged.
This is the classic muscle-car-style stance change. The wider wheel with a much lower offset shifts almost all the extra width outward toward the fender. The new wheel already sits 39 mm further outboard than stock, so a spacer would only make the poke worse - spacers can only push a wheel out, never recess it. To pull the outer edge back you would need a higher-offset wheel (e.g. ET+45 instead of ET+25) or a narrower rim.
Common Stock Offset Ranges by Vehicle Type
Stock offsets cluster by drivetrain because each layout has different suspension geometry. Use this as a sanity check before entering values.
| Vehicle Type | Typical Stock Offset Range | Common Width |
|---|---|---|
| FWD hatchback / sedan | ET+38 to ET+50 | 6.5J to 7.5J |
| RWD sports coupe (E46, 350Z, MX-5) | ET+30 to ET+45 | 7.0J to 8.0J |
| Modern performance saloon (M3, Giulia QV) | ET+25 to ET+40 | 8.5J to 10.0J |
| Subaru WRX / STI | ET+48 to ET+55 | 7.5J to 8.5J |
| Mustang / Camaro / Challenger | ET+24 to ET+38 | 8.5J to 9.5J |
| Pickup truck (F-150, Silverado) | ET+18 to ET+44 | 8.0J to 9.5J |
| Off-road 4x4 (Wrangler, Bronco) | ET-12 to ET+18 | 8.0J to 9.0J |
| European luxury SUV (X5, Q7) | ET+30 to ET+45 | 8.5J to 10.0J |
FWD vehicles run higher offsets because the suspension and drivetrain are packaged tightly inside the wheel arch, so the wheel has to sit further inboard. Body-on-frame trucks and live-axle 4x4s have wider chassis rails and use lower or negative offsets to clear the frame and put the tyre out where the suspension travel allows.
Poke and Tuck Terminology
The fitment community uses specific terms for how far the wheel sits relative to the fender. The calculator reports outer edge change directly in millimetres so you can map to whichever term applies.
| Term | Outer edge change | What it looks like |
|---|---|---|
| Deep tuck | -20 mm or more | Wheel sits clearly recessed inside the arch |
| Moderate tuck | -10 to -20 mm | Visibly inset from fender |
| Slight tuck | -1 to -10 mm | Just inside fender line |
| Flush | Within +/- 1 mm | Outer edge aligns with fender |
| Slight poke | +1 to +10 mm | Tyre sidewall noticeably extends past fender |
| Moderate poke | +10 to +20 mm | "Hellaflush" territory, often needs fender rolling |
| Significant poke | +20 mm or more | Visible from a distance, almost always requires fender modification and may be illegal on public roads |
Different regions have different rules on poke. In the UK, MOT guidance under the Road Vehicles (Construction and Use) Regulations 1986 requires that no part of the tyre extends beyond the bodywork on a horizontal plane. Germany's TUV enforces similar rules, and most poke fitments need a fender lip extension to be certified. In most US states, the rules are looser but a few (notably California under Vehicle Code 27600) require the tyre to be covered by the fender or a mudflap.
Do I Need Spacers?
Spacers go between the hub and the wheel to push the wheel outward by their thickness, effectively reducing offset by that amount. The calculator's spacer recommendation tells you exactly how thick a spacer would need to be for the new wheel's outer edge to match stock alignment - useful if the new wheel's offset alone leaves it tucked too far in.
There are two spacer types. Slip-on (hub-centric) spacers sit on the existing hub and use longer wheel studs or bolts. They're cheaper but the original studs must extend through the spacer plus the wheel - check that you have enough thread. Hub-centric bolt-on spacers have their own studs pressed into them and bolt onto the original hub like a hub adapter. These are stronger and usually preferred for spacers over 15 mm.
Spacer thickness guidance commonly cited by Tire Rack and reputable specialists:
- Under 5 mm: hub-centric slip-on spacer with original studs is fine.
- 5-12 mm: slip-on or extended studs - thread engagement becomes the limit.
- 15 mm and over: bolt-on hub-centric spacer with its own studs is the safer choice. Some manufacturers (e.g. H&R, Eibach) test their spacers to TUV standards.
One subtle point: a spacer also moves the inner edge of the wheel by exactly the same amount. If your stock wheel's inner edge already sits close to a strut or control arm, a spacer wide enough to fix poke might also make the inner edge unfit. The calculator reports inner edge change separately so you can sanity check both sides of the equation.
Inner Clearance and Rubbing Risk
Lower offset (or wider wheels with the same offset) moves the inner edge closer to the suspension and brake calipers. The most common rubbing points are the front strut housing, the upper control arm on double-wishbone setups, and the inner fender liner at full steering lock or full suspension compression.
A general rule of thumb published by Tire Rack and echoed by most reputable fitment guides is to keep at least 5-10 mm clearance between the inner edge of the tyre (not the wheel) and any suspension component at full lock and full compression. Remember that the tyre sidewall bulges out beyond the wheel's inner edge by a few millimetres, especially when loaded into a turn. Tyre stretch (mounting a narrower tyre on a wider wheel) reduces this bulge but adds a different set of risks like bead unseating under hard cornering.
Big brake kits (BBKs) make this worse. A larger caliper combined with a wheel that has its inner edge close to stock can foul on the caliper body itself. Most aftermarket BBKs publish a minimum wheel diameter and a clearance template - check both against the wheel you're considering before ordering. Some kits also need a small spacer (3-5 mm) just to clear the caliper, even though the rest of the geometry would fit.
Heavily loaded vehicles (vans, towing setups) need more inner clearance because the suspension compresses more under load. Lowered cars also lose suspension travel headroom and can rub at points stock cars never would. Always test new fitment with a slow drive over a speed bump and a full-lock turn before committing to longer drives - if you also want to size up the performance impact of wider rolling resistance, check the quarter mile calculator after you settle the fitment.
Common Mistakes
- Comparing wheels by offset alone. A 9.5J ET+25 wheel sits 19 mm further out than an 8.0J ET+25 because the wider wheel adds 38 mm of width split between inner and outer. Always include width in the comparison - the calculator handles this for you.
- Forgetting the tyre profile. A wider wheel usually requires a wider tyre, which can add 5-15 mm of stretch or bulge beyond the rim edge. The calculator handles rim geometry only - tyre sidewall bulge is separate and adds to the apparent poke.
- Mixing units. Offset is always in millimetres globally, even on cars sold in imperial markets. Width is sometimes given in inches (8.0J = 8 inches wide) and sometimes in millimetres in OEM documentation. Use the unit toggle to keep the two consistent.
- Trusting the stamped offset on used wheels. Refurbished wheels sometimes have machined-down hub faces that change the actual offset by a few millimetres from what's stamped. Measure if you're unsure - lay the wheel face-down, place a straight edge across the rear lip, and measure the gap from the hub face to the straight edge with a steel rule.
- Ignoring centre bore. A wheel that fits the offset but has a larger centre bore than the hub will rely on lug-centric mounting, which can cause vibration. Hub rings restore centring without changing offset. Most aftermarket wheels come with a centre bore that fits a wide range of hubs and rely on rings for an exact match.
- Setting up a staggered fitment without checking both axles. Many rear-wheel-drive cars run wider wheels on the rear than the front. The front and rear stock offsets are often different too. Run the calculator twice - once per axle - to make sure both fitments work.
Sources
Frequently Asked Questions
What is wheel offset (ET)?
Offset is the distance in millimetres from the wheel's centerline to the hub mounting face. Positive offset (e.g. ET+45) means the mounting face sits closer to the outside of the wheel, which pulls the wheel inward toward the suspension. Negative offset (e.g. ET-25) puts the mounting face toward the inside, pushing the wheel outward. ET stands for Einpresstiefe, German for insertion depth.
What does poke and tuck mean?
Poke is when the outer edge of the wheel sticks out past the fender. Tuck is when the wheel sits further under the fender than stock. The calculator measures the outer edge change in millimetres - positive values mean poke, negative values mean tuck. Flush fitment means the outer edge sits within about 1 mm of the fender.
How do I know what offset my wheels are?
Most wheels have the offset stamped on the back of one of the spokes or near the centre bore, marked as ET followed by a number (e.g. ET45). Your vehicle's stock offset is in the owner's manual or on a label inside the door jamb. Many manufacturers also publish factory wheel specs on their support sites.
How much offset change is safe without spacers?
Most enthusiasts treat a 5-7 mm offset change as the safe limit for the same wheel width before clearance becomes an issue. Larger changes usually need a wider track, fender rolling, camber adjustment, or smaller tyres to avoid rubbing. The calculator's inner-edge change shows how much closer the wheel moves to the suspension - check this against your strut and control arm clearance.
Does the calculator account for wheel width changes?
Yes. Both width and offset together determine where the inner and outer edges sit relative to the hub. A wider wheel with a higher offset can still poke more than a narrower wheel with a lower offset, depending on the maths. The tool calculates outer and inner edge positions from both inputs and shows the net change.
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