Wheel Offset Calculator

What offset measures

Offset is the distance between two lines on a wheel: the centerline of the barrel and the flat face that presses against the hub. It is measured in millimeters, and the sign tells you which side of the centerline the mounting face sits on. Every wheel sold in the United States carries that number, usually stamped on the back of a spoke as ET followed by the value.

The centerline is exactly what it sounds like. On a 9 inch wheel it sits 4.5 inches from either bead, and the mounting face is somewhere near it. Push the face outward by 35 mm and the wheel has to sit further inboard to keep the face on the hub, which is why positive offset tucks the rim toward the suspension. Pull the face inward and the rim swings out toward the fender.

That is the whole idea, and it is worth holding onto because the numbers look backwards at first. The offset does not describe where the wheel sits. It describes where the hub sits inside the wheel, and the wheel then hangs off that point in whichever direction the offset leaves room.

Offset and backspacing are one fact in two units

Backspacing is the distance from the mounting face to the inner lip of the rim, measured in inches with a straight edge and a tape. It answers the question a mechanic asks: how much room is there behind the wheel before something rubs. Offset answers the same question from the other end, and the two convert cleanly.

Neither number is more correct. Offset is what the manufacturer publishes and what the aftermarket sells, so it is the number you will see on a spec sheet. Backspacing is what you can measure in a driveway with the car on jack stands, so it is the number that tells you whether the new wheel will clear the strut. If you know one, you know the other.

The conversion is worth memorizing in both directions. One inch is 25.4 millimeters, so 25 mm is 0.98 inches and 12 mm is 0.47 inches. A wheel with 6.38 inches of backspacing and a 9 inch width has an offset of +35 mm, and no other combination produces that pair.

The formula, and why you add an inch

backspacing = (wheel width + 1) / 2 + offset / 25.4

The width in that formula is the advertised width, measured between the tire beads. The physical rim is wider, because each flange adds roughly half an inch, so the +1 converts bead width into overall width. Skip it and every answer comes out half an inch short, which is enough to turn a wheel that fits into one that rubs.

Run a 9 inch wheel at +35 mm through it. The width plus one is 10, half of that is 5, and the offset in inches is 35 / 25.4 = 1.378. Add them and the backspacing is 6.38 inches. The front spacing is the mirror image: 5 - 1.378 = 3.62 inches, and the two always add up to the 10 inch overall width.

Front spacing is not a term most spec sheets use, but it is the number that decides whether the wheel pokes past the fender. Measure from the hub face to the outer lip, compare it with the stock wheel, and you have the poke change before you buy anything.

Positive offset pulls the wheel in

Most factory wheels run positive offset, often between +35 and +55 mm, because the engineers want the tire inside the fender and the steering axis close to the center of the contact patch. A higher positive number moves the mounting face outward, which moves the barrel inward, so the tire tucks under the car.

That tuck has a cost. Every millimeter of extra positive offset takes a millimeter of clearance away from the strut, the control arm and the inner fender liner. On a car with a MacPherson strut and a wide tire, the inner sidewall is usually the first thing to rub, and it rubs on the spring perch rather than on anything visible from outside.

Positive offset also changes the scrub radius, which is the distance between the steering axis and the center of the tire contact patch. Push it too far and the steering feels heavier, the car tramlines on grooved pavement, and the front tires wear unevenly. A 10 mm change is barely noticeable. A 25 mm change is not.

Negative offset pushes the wheel out

Negative offset is the look most trucks and off road builds chase. The mounting face sits inboard of the centerline, so the barrel hangs outward and the tire sits wide of the fender. A 9 inch wheel at -12 mm has 4.53 inches of backspacing and 5.47 inches of front spacing, which is a stance you can see from across a parking lot.

The trade is clearance in the other direction. Wide offset pushes the tire into the fender lip on compression, throws road spray down the side of the truck, and loads the wheel bearings and ball joints further out from the hub. It also makes the steering kick back over bumps, because the contact patch is no longer near the steering axis.

Zero offset is the neutral case: the mounting face sits on the centerline, backspacing equals front spacing, and the wheel is symmetrical about the hub. A 9 inch wheel at 0 mm has 5.00 inches on both sides. It is a clean reference point, and it is rare on a factory car.

Worked example: 8 inch +45 to 9 inch +35

This is the most common swap in the aftermarket: one inch wider, ten millimeters less offset, and a hope that it fits.

• Stock backspacing: (8 + 1) / 2 = 4.5, and 45 / 25.4 = 1.77, so 6.27 inches
• Stock front spacing: 4.5 - 1.77 = 2.73 inches
• New backspacing: (9 + 1) / 2 = 5.0, and 35 / 25.4 = 1.38, so 6.38 inches
• New front spacing: 5.0 - 1.38 = 3.62 inches
• Outer edge: 3.62 - 2.73 = +0.89 inches further out
• Inner edge: 6.38 - 6.27 = +0.11 inches closer to the suspension

Both edges moved outward, which is what happens whenever the width grows faster than the offset comes down. The outer edge gains 0.89 inches and the inner edge gains 0.11 inches, so the wheel grows by an inch in total and almost all of it shows on the outside. If the fender has 0.75 inches of room, this wheel rubs.

Now reverse the offset change. Going from +45 to +55 mm on the same 9 inch wheel gives 7.17 inches of backspacing and 2.83 inches of front spacing, so against the stock wheel the outer edge moves out only 0.10 inches while the inner edge takes 0.90 inches more. Same width, different fit, which is the entire argument for shopping by offset rather than by width alone.

Width and tire size move together

Offset decides where the wheel sits, and the tire decides how much space that position uses. A 245 millimeter tire on an 8 inch wheel has a relaxed sidewall that sits close to the rim width. The same 245 on a 9 inch wheel stretches, the section width grows by roughly 0.2 inches, and the sidewall gets shorter, so the tire can rub where the wheel alone would clear.

Overall diameter matters for a different reason. A 245/40R18 is about 25.7 inches tall, and a 255/40R18 is about 26.0 inches. Changing the diameter changes the speedometer, the final drive ratio and the clearance at the top of the wheel arch, none of which the offset number can tell you. Check both before you order a set.

The tire placard on the driver door jamb lists the stock size and the recommended pressures, and NHTSA's tire safety information explains how the load rating works. A wider wheel with a lower offset usually wants a wider tire to protect the rim, and that wider tire is what touches the fender.

What to check before you buy

Start with the stock numbers, because they are the reference every aftermarket seller uses. Width and offset are stamped on the back of the wheel or printed on the door placard, and the tire size is on the sidewall. Write all three down before you open a single product page.

Then measure the car, not the wheel. Turn the front wheels to full lock and look at the gap between the inner sidewall and the strut, then look at the gap between the tire and the fender lip. Those two gaps are your budget, and the calculator tells you how much of each the new wheel will use.

Finally, plan for load and geometry. A wider track loads the wheel bearings further out, and a large offset change alters the scrub radius enough to feel in the steering. Tire Rack's guide to wheel offset walks through the same arithmetic from the fitment side, and it is a useful second opinion before you commit to a set of four.

Measurements are a guide, not a guarantee. Ride height, camber, brake caliper size and tire brand all change what fits. Test fit one wheel on the front before you mount all four.