🛞 Wheel Offset Calculator

How to Use This Calculator

Enter the wheel width and backspacing in inches. Hit Calculate Offset. The headline is the ET offset in millimeters with its sign explained (positive tucks in, negative pushes out). Optionally enter a second wheel to compare: the details show both offsets and exactly how many millimeters further in or out the new wheel sits. Measure backspacing with a straightedge across the back of the rim and a ruler to the mounting face.

The Method: Backspacing to Centerline

Backspacing measures from the mounting face to the back edge; subtracting half the width moves the reference to the centerline; multiplying by 25.4 converts inches to millimeters. The worked example: an 8-inch wheel with 5 inches of backspacing has its mounting face 1 inch outboard of center: (5 - 4) x 25.4 = +25.4 mm, or ET25. Positive means the face sits outboard of center, tucking the wheel inward under the fender. Compare: a 9-inch wheel with 5.5-inch backspacing gives (5.5 - 4.5) x 25.4 = +25.4 mm too: same offset, but the wider wheel extends 12.7 mm further in both directions.

Worked Example

Current: 8-inch wide, 5-inch backspacing = (5 - 4) x 25.4 = +25.4 mm (ET25). New: 9-inch wide, 4.5-inch backspacing = (4.5 - 4.5) x 25.4 = 0 mm (ET0). The offset drops 25.4 mm and the extra inch of width adds 12.7 mm per side; combined, the outer face moves 38.1 mm outward (verify: outer face = width/2 - offset from the mounting plane: 101.6 - 25.4 = 76.2 mm vs 114.3 - 0 = 114.3 mm). Enter 8/5 and 9/4.5 above and the calculator's "Outer face moves" line reports the same +38.1 mm: that is the number your fender clearance must absorb.

Clearance Checking

Measure current gaps: outer tire-to-fender and inner wheel-to-suspension, at full steering lock both ways, with the suspension loaded. The new wheel must preserve a few millimeters everywhere. Lower offset moves the outer face out (fender risk) and the inner face out too (more suspension room); higher offset does the reverse. Taller or wider tires compound the change: compute the tire's section width, not just the wheel. When in doubt, test-fit one wheel before buying four.

Beyond Fitment

Offset changes scrub radius: pushing wheels outward increases it, making steering heavier and more reactive to bumps and torque steer. Bearing loads rise with the lever arm, so extreme offsets shorten hub bearing life. Keep changes modest (within ~10mm of stock) unless the build calls for more. Also verify hub bore (use hub-centric rings if larger), bolt pattern, and load rating: fitment is geometry plus hardware, and the calculator covers the geometry half.

Plus-Sizing: Width, Diameter, and Offset Together

Plus-sizing (wider wheels, lower-profile tires, same overall diameter) is where offset math earns its keep. The goal is keeping the tire's outer diameter constant so gearing and speedometer stay honest, while the wheel gets wider for grip and looks. But wider wheels change where the tire sits: every added inch of width splits half an inch inboard and half outboard unless offset compensates. To keep the outer face flush where it was, increase offset by half the added width in millimeters: going from 8 to 9 inches wide needs about 12.7mm more positive offset to hold the outer position. Most plus-size packages do not do this exactly, which is why they poke slightly. That poke is fine within fender clearance, but know it is happening. Also remember the tire, not the wheel, is what rubs: a 9-inch wheel with a 255 tire versus a 245 tire moves the sidewall out 5mm per side regardless of offset. Compute the full stack: wheel position from this calculator plus tire section width.

Hardware: Hub Bore, Lugs, and Load Ratings

Offset is only one of five fitment numbers. Hub bore must match or be adapted with hub-centric rings; a wheel centered only by lug nuts (lug-centric) vibrates at highway speed no matter how perfect the offset. Bolt pattern must match exactly: close is not close enough, and wobble bolts are a compromise, not a solution. Lug seat type matters too: conical, ball, and flat seats are not interchangeable, and the wrong seat loosens over time. Load rating must meet or exceed the factory wheel's rating, especially on trucks and SUVs; a pretty wheel with a low rating is a failure waiting for a pothole. Finally, brake clearance: big brake kits need spoke clearance that offset alone does not guarantee, so check the barrel-to-caliper gap physically. Get all five right and the wheels disappear into the driving experience, which is the whole point.

Offset vs. Backspacing: The Same Truth, Two Languages

Offset measures from the wheel's centerline in millimeters; backspacing measures from the inner lip in inches. They describe the same geometry, and the conversion is simple: backspacing equals (width / 2) plus (offset / 25.4). A 9-inch-wide wheel with +35 mm offset has backspacing of 4.5 + 1.38 = 5.88 inches. Positive offset tucks the wheel inward, negative pushes it outward. The aftermarket speaks offset, old-school shops speak backspacing, and mixing them up is how wheels end up rubbing fenders. Convert before you buy, not after the test fit fails.

Worked Example: Plus-Sizing Without Rubbing

Stock: 17x7.5 wheels, +45 mm offset, 215/55R17 tires. You want 18x8 with 235/45R18, keeping overall diameter close: stock diameter is about 26.3 inches, new is 26.3 inches, perfect. Now the offset: the wheel is 0.5 inches wider, so to keep the inner clearance identical, add half the width increase (0.25 in = 6.35 mm) to the offset: target about +51 mm. Available wheels come in +50, so the inner edge sits 1 mm further out than stock, safe, and the outer edge moves out by the full half inch plus the 5 mm offset difference, about 18 mm wider stance. Check fender clearance at full lock and full compression; the numbers say it fits, but the fender liner is the final judge. This is the calculation behind every clean plus-size fitment.

Fitment Mistakes

Mistake one is buying for looks with no math: aggressive negative offset on a daily driver that now throws road grime down the car's flanks and fails inspection in some states. Mistake two is ignoring load rating. A wheel rated for 1,500 pounds on a 5,000-pound SUV is a failure waiting for a pothole; match or exceed the stock rating. Mistake three is the hub bore. Wheels with a larger bore than the hub need hub-centric rings, a $15 part that prevents vibration; without them the lugs carry shear loads they were never designed for. Rings are not optional. They are the difference between a wheel that centers and one that hopes.