Measure Wheel Offset

If you're looking at used wheels or trying to verify specs on a wheel you already own, knowing how to measure offset is essential. You don't need special equipment—just a straight edge, a tape measure, and about ten minutes. Backspacing is actually easier to measure than offset, and once you have backspacing, the math to get offset is straightforward.

Tools You'll Need

• Tape measure (25-foot minimum, steel is best)
• Straight edge (a level, aluminum rule, or straight board works)
• Calculator (or your phone)
• Wheel specifications (width and diameter)

That's it. You probably have everything already. If you don't have a straight edge, any long, straight piece of metal or wood works—it just needs to span across the entire wheel.

Method 1: Measuring Backspacing (Easiest)

Backspacing is the distance from the mounting face (where the wheel bolts to the hub) to the innermost edge of the wheel. It's the easiest dimension to measure because you can measure it directly with a tape measure.

Steps:

1. Lay the wheel flat on the ground with the tire facing up.
2. Place your straight edge (level or rule) across the back of the wheel, resting on the innermost edge.
3. Measure from the straight edge to the mounting face (the part that attaches to the hub). Make sure you're measuring perpendicular to the wheel, not at an angle.
4. That measurement is your backspacing.

For a 9-inch-wide wheel with a mounting face at the back, backspacing might be 5.5 inches. For the same wheel with the mounting face more forward, it might be 5.0 inches.

Common backspacing values: 4.0–6.0 inches for most passenger cars, 5.0–7.0 inches for SUVs and trucks.

Method 2: Calculating Offset from Backspacing

Once you have backspacing, calculating offset is simple math. The formula is:

Offset (mm) = (Backspacing × 25.4) − (Wheel Width (inches) × 25.4 ÷ 2)

Or more simply:

Offset (mm) = (Backspacing − (Width ÷ 2)) × 25.4

Example:

• Wheel width: 9 inches
• Backspacing: 5.5 inches
• Offset = (5.5 − (9 ÷ 2)) × 25.4 = (5.5 − 4.5) × 25.4 = 1.0 × 25.4 = +25.4mm

So that wheel has approximately +25mm offset. Easy.

Another example:

• Wheel width: 9.5 inches
• Backspacing: 5.0 inches
• Offset = (5.0 − (9.5 ÷ 2)) × 25.4 = (5.0 − 4.75) × 25.4 = 0.25 × 25.4 = +6.4mm

The formula works whether the result is positive or negative. If your math gives you a negative number, the offset is negative (wheel pokes out further).

Method 3: Measuring Offset Directly (More Complex)

You can measure offset more directly, but it requires a bit more setup. The idea is to find the distance from the mounting face to the wheel's centerline.

Steps:

1. Place the wheel face-up on a flat surface (or between two straight edges to keep it level).
2. Use a straight edge to find the center of the wheel by measuring the width and marking the midpoint.
3. Measure from the mounting face to that centerline mark.
4. Subtract that measurement from half the wheel width. The result is your offset.

This method is less accurate because it's harder to find the exact centerline without special tools, but it works for a rough check.

Verifying Your Measurements

The best way to verify is to compare your measurement to spec. Most wheel manufacturers stamp the offset on the back of the wheel. Look for markings like "ET+45" or "OFFSET +50"—that "ET" stands for "Einpress Tiefe," which is German for offset, and it's the standard marking worldwide.

If you find the mark and your calculation is within 2–3mm of the stamped offset, you've measured correctly. Small variations happen because wheels aren't perfectly uniform and your measurement might be slightly off one direction or the other.

Why This Matters

Measuring offset before buying wheels prevents expensive mistakes. You can measure a wheel for sale before committing to a purchase. You can verify that wheels you already own will fit when you change suspension or tire size. And you can compare used wheels to your factory spec to see if they'll work on your car.

A few important notes:

• Different tires (different brands or sizes) can make a wheel look like it has different fitment, but the offset doesn't change. The offset is a property of the wheel, not the tire.
• Wheel width and diameter should be stamped on the wheel. If they're not visible, look up the wheel model online or ask the seller.
• Offset affects how far the wheel sits relative to the fender. Negative offset makes it poke out; positive offset tucks it in. There's no "good" offset—only the right offset for your specific car and suspension.

A Practical Example: Comparing Used Wheels

You're looking at a set of used wheels advertised as "18x9.5 +45 offset." You want to verify before buying.

Measure:

1. Backspacing comes out to 5.75 inches
2. Width is marked as 9.5 inches
3. Calculate offset: (5.75 − (9.5 ÷ 2)) × 25.4 = (5.75 − 4.75) × 25.4 = 1.0 × 25.4 = +25.4mm

Your calculation gives +25mm, but the wheels are advertised as +45mm. That's a 20mm difference—big enough that something's wrong. Either the wheels are different than advertised, or the backspacing measurement wasn't accurate. Double-check by looking for the ET mark on the wheel. If you find "ET+25," the seller made an error. If you find "ET+45," your measurement was slightly off (measure again more carefully, or accept that the wheels are what they're supposed to be).

Quick Reference: Offset vs. Backspacing

Offset: Distance from the mounting face to the wheel centerline, measured in millimeters (positive or negative). It's what wheel shops and manufacturers list.

Backspacing: Distance from the mounting face to the inside edge, measured in inches. It's easier to measure by hand but less commonly listed.

Relationship: Once you know backspacing and width, offset is just math. Backspacing and offset describe the same thing—where the mounting face sits relative to the wheel.

With these methods, you can verify any wheel's offset before buying. And that confidence is worth the ten minutes it takes to measure.