Look at Your Sidewall
Everything starts with a string like 225/45R18, printed right on the tire.
Crouch down next to any tire on your car and look at the sidewall — the part that faces outward. You'll see a mix of numbers and letters. The one you want looks like 225/45R18.
That's your tire size. And the last number in it — the 18 — is the diameter of the wheel the tire is mounted on. That's the single most useful number when you're shopping for a replacement wheel.
Check more than one tire. If a previous owner swapped wheels at some point, the spare or a mismatched corner could throw you off. When in doubt, check the back of the wheel itself (next section).
What the Numbers Mean
Four parts. Once you know them, you'll read any tire like a menu.
225 — Section Width
How wide the tire is, in millimeters, measured sidewall to sidewall. Bigger number, wider tire.
45 — Aspect Ratio
How tall the sidewall is, as a percentage of the width. Here the sidewall is 45% of 225 mm. Lower numbers mean a thinner sidewall — the "low profile" look.
R — Construction
Stands for radial, the standard construction on virtually every passenger car tire. You can ignore this one — just know it's not a speed rating or a size.
18 — Wheel Diameter
The number you're after. This tire fits an 18-inch wheel. When you hear someone say they have "18s," this is what they mean.
Double-Check the Wheel Itself
The tire tells you the diameter. The wheel tells you the rest.
Tire sizes only reveal the wheel's diameter (the 18). To get the full wheel size, look at the wheel itself. Most manufacturers stamp or cast the size — something like 18×8 — into the back of a spoke, the barrel, or near one of the lug holes.
The first number is the diameter in inches; the second is the width in inches. So 18×8 is an 18-inch wheel that's 8 inches wide.
No stamp visible? A quick phone photo of the wheel's inner barrel (through the spokes) often catches it. Or snap a picture of your tire sidewall — that at least locks in the diameter.
Wheel Size Isn't the Whole Story
Two wheels can both be 18×8 and still not be interchangeable.
Here's the catch that trips up most first-time buyers: wheel size is just one of several specs that decide fitment. A wheel that's the right diameter and width can still be wrong for your car if the bolt pattern, offset, or center bore doesn't match.
- Bolt pattern — 5×114.3 and 5×120 are both "5-lug," but they're not interchangeable
- Offset — changes how far the wheel sits in or out of the wheel well
- Center bore — the center hole has to clear your hub
We walk through all of them, with pictures, in How to Check Wheel Fitment. If you're only going to read one more thing before ordering, make it that.
Enter your Year, Make & Model in the search bar at the top of this page. We'll show you wheels that match your vehicle — size, bolt pattern, offset, all of it.
Know Your Size? Now Find Your Wheels.
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