Wheel Offset Calculator
A new wheel can look right in a listing and still rub the strut or stick past the fender. This tool compares your current wheel with a proposed one, width, offset and spacer included, and reports exactly how far each edge moves.
Wheel Offset Calculator
Compare a current wheel with a proposed wheel, spacer included.
Current wheel
New wheel
Estimates only. Always dry test-fit the wheel on the car and check clearance at full steering lock and through suspension travel before mounting tires. Actual fitment depends on the vehicle, fender shape, brakes, and tire size.
Quick Answer
Enter both wheels and any spacers, and the tool returns the offset difference, backspacing and frontspacing for each wheel, plus the inner and outer edge shifts. In the worked example, moving from a 9 inch wheel at ET+25 to a 10 inch wheel at ET+15 moves the inner edge 2.7 mm closer to the suspension and the outer lip 22.7 mm further out, an aggressive poke that needs a fender check.
- Quick Answer
- How to use this calculator
- What wheel offset (ET) actually means
- The four numbers that decide fitment
- The formula, worked by hand
- Worked example, checked against this exact tool
- How spacers change the effective offset
- Offset and handling, not just looks
- What competing calculators miss
- Frequently asked questions
How to use this calculator
Start with the numbers stamped on the wheels. Width is in inches and offset is in millimetres, so do not mix the units. Enter the current wheel first, then the proposed wheel, and add a spacer thickness for either setup if one is fitted or planned. Press Compare wheels and read the two shifts first: the inner shift tells you about strut and brake clearance, and the outer shift tells you about fender clearance. The diagram under the results draws both wheels against the same hub face, so a tuck or a poke is visible at a glance.
Measure the clearance you actually have before you trust any number. On the current setup, check the gap between the tire or wheel inner edge and the strut and caliper, and the gap between the outer edge and the fender lip at full steering lock. If the tool says the inner edge moves 12 mm closer and you only have 8 mm of gap, the new wheel will not clear, however good it looks in a photo.
What wheel offset (ET) actually means
Offset, usually stamped ET followed by a number on the back of the wheel, is the distance in millimetres between the wheel centerline and its hub mounting face. Positive offset (ET+35) puts the mounting face toward the outboard side, tucking the wheel inward, and this is standard on most modern cars. Zero offset (ET0) puts the mounting face exactly on the centerline. Negative offset (ET-20) pushes the wheel outward for an aggressive stance, common on trucks and off-road builds. Lowering the offset number moves the whole wheel outward. Raising it tucks the wheel inward.
The four numbers that decide fitment
The offset difference alone does not tell you whether a wheel fits, because width matters too. Backspacing is the distance from the mounting face to the inner lip, and it tells you how much room the wheel needs on the suspension side. Frontspacing is the distance from the mounting face to the outer lip, and it tells you how far the wheel reaches toward the fender. From those, the tool derives the two numbers that answer whether it fits: the inner edge shift, where positive means closer to the strut or brake caliper, and the outer edge shift, where positive means more poke toward the fender. Compare those shifts against the clearance you can measure on your current setup.
| Number | What it measures | What to check |
|---|---|---|
| Backspacing | Mounting face to inner lip | Strut, spring and caliper clearance |
| Frontspacing | Mounting face to outer lip | Fender lip and liner clearance |
| Inner edge shift | Change in backspacing, new minus current | Positive eats suspension-side room |
| Outer edge shift | Change in frontspacing, new minus current | Positive means more poke |
The formula, worked by hand
Every number in the tool comes from three short formulas, with 25.4 mm per inch doing the unit work. Effective ET equals stamped ET minus spacer thickness, because a spacer pushes the whole wheel outward. Backspacing equals half the width in millimetres plus effective ET. Frontspacing equals half the width in millimetres minus effective ET. The shifts are simple subtractions: new backspacing minus current backspacing, and new frontspacing minus current frontspacing.
Check the worked example by hand. Current wheel: half of 9 inches is 4.5 inches, which is 114.3 mm, so backspacing is 114.3 plus 25, or 139.3 mm, and frontspacing is 114.3 minus 25, or 89.3 mm. New wheel: half of 10 inches is 127.0 mm, so backspacing is 127.0 plus 15, or 142.0 mm, and frontspacing is 127.0 minus 15, or 112.0 mm. Inner shift is 142.0 minus 139.3, or +2.7 mm. Outer shift is 112.0 minus 89.3, or +22.7 mm. Offset difference is 15 minus 25, or -10.0 mm.
Worked example, checked against this exact tool
Take a truck with stock 20 by 9 inch wheels at ET+25, upgraded to 20 by 10 inch wheels at ET+15, with no spacers. Enter 9, 25 and 0 for the current wheel and 10, 15 and 0 for the new wheel. The tool reports backspacing of 139.3 mm current against 142.0 mm new, and frontspacing of 89.3 mm against 112.0 mm. Inner edge shift is +2.7 mm closer to the suspension. Outer edge shift is +22.7 mm more poke. Offset difference is -10.0 mm. The tool flags this as aggressive poke and warns you to check fender clearance at full lock and under load, which is the right warning: 22.7 mm is past the 20 mm threshold where fender work becomes likely.
Takeaway: a 1 inch wider wheel at 10 mm lower offset does not move both edges by 10 mm. The inner edge barely moves, up 2.7 mm, while the outer lip jumps 22.7 mm. Width and offset must be read together, never one alone.
How spacers change the effective offset
A wheel spacer sits between the hub and the wheel, pushing the whole wheel outward by its thickness. A 10 mm spacer on an ET+35 wheel behaves like an ET+25 wheel: effective ET equals stamped ET minus spacer thickness. Enter the spacer for each setup and the tool folds it into every number automatically, so a spacered current setup compares fairly against a new wheel. One caution belongs here: a spacer also changes the load path through the studs, so use hub-centric spacers, check stud engagement, and torque to the vehicle specification. The geometry tool cannot judge stud safety, and a correct ET number is not a safety approval.
Offset and handling, not just looks
Offset changes the scrub radius, the distance between the tire contact patch center and the steering axis. Big offset changes can alter steering feel, increase tramlining, and load wheel bearings differently. Staying within about 5 mm of the factory offset keeps geometry close to what the car was designed around. Larger changes deserve an alignment check and a careful test drive, and they deserve a second look at bearing life if the car is driven hard. For a daily driver, the safest upgrade is usually the one that keeps both edge shifts small, even if a wider stance would photograph better.
What competing calculators miss
The best-known offset tools fall into two camps, and both leave work for the reader. Some show a slick diagram but require you to measure fender and strut clearances yourself before the numbers mean anything. Others show only the offset difference and ignore how width changes move both edges, which is exactly the trap in the worked example, where a -10 mm offset difference hides a +22.7 mm poke. This page gives the complete picture in one pass: offset difference, backspacing and frontspacing for both wheels, the inner and outer edge shifts in millimetres, a cross-section diagram, spacer-aware effective offset, and a plain-language verdict on poke and clearance. What it cannot do, and no geometry tool can, is measure your car. Pair the numbers with a tape measure and a dry test-fit.
Related reading on the same site: the companion wheel rate calculator covers suspension stiffness at the wheel, while the MPG calculator and the RPM speed calculator cover the running costs and gearing that change when tire and wheel sizes change.
Frequently asked questions
Offset is the distance in millimetres from the wheel centerline to the hub mounting face. Positive offset moves the mounting face toward the outside of the wheel, tucking it in. Negative offset moves it inward, pushing the wheel out. Zero offset puts the mounting face exactly on the centerline.
It is usually stamped on the back of the wheel or on a spoke as ET followed by a number, like ET35. If the stamping is unreadable, measure from the centerline to the mounting face, or look up the wheel model in the manufacturer’s published specs.
Yes, when width stays the same. Dropping from ET+35 to ET+25 moves the wheel 10 mm outward. Width changes move the edges too, which is why this tool reports the outer edge shift rather than just the offset difference.
Offset is measured from the centerline. Backspacing is measured from the inner lip to the mounting face. Backspacing equals half the width in millimetres plus the offset. Two wheels with the same offset but different widths have different backspacing.
Compare the inner and outer edge shifts against the clearance your current setup has. More than about 20 mm of additional poke deserves a careful fender check, and any inner shift eats directly into strut and caliper clearance. Only a dry test-fit gives a certain answer.
Effectively, yes. A spacer pushes the wheel outward by its thickness, so effective ET equals stamped ET minus spacer thickness. A 15 mm spacer on an ET+40 wheel behaves like an ET+25 wheel.
Only if the new wheel width falls within the tire approved rim-width range. A tire printed width is measured on one specific design rim, and mounting it wider or narrower changes its real section width slightly.
Yes. Offset sets the scrub radius, so large changes can alter steering feel, increase tramlining on rutted roads, and change bearing loads. Small changes within about 5 mm of stock are usually unnoticeable.
It pushes the wheels outward for a wider look, but it also increases fender-rub risk, throws more road debris at the paint, and can accelerate bearing wear. Fitment is a trade-off, not a contest.
No. It computes exact geometry from your inputs, but real clearance depends on the car, fender shape, suspension travel, brakes, and tires. Always dry test-fit before mounting tires.
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References
- Driftworks. What is wheel offset? ET explained (ET definition, positive, zero and negative offset, where the stamping sits). Accessed October 24, 2026.
- Custom Wheels and Tires. Wheel Sizing Guide: Understanding Diameter, Width, Offset and Fitment (offset and backspacing definitions, backspacing conversion formulas). Accessed October 24, 2026.
- Tire Outlet. Wheel Offset and Backspacing Explained (measuring backspacing and centerline, deriving offset from those measurements). Accessed October 24, 2026.
- Dually Wheels. What Is Wheel Offset? Types, Measurement and Fitment (ET markings, higher positive offset means more backspacing and a tucked wheel). Accessed October 24, 2026.
- Wheel-Size.com. Wheel Offset (offset comparison by width and spacer, current and new wheel inputs). Accessed October 24, 2026.