A 245/45R18 tire is 245 millimeters wide, with a sidewall 45% as tall as the width, radial construction, and an 18-inch wheel.
For the full breakdown, see our best 245/45R18 Tires guide.
That code molded into your tire’s sidewall isn’t random — it’s the tire’s complete geometry compressed into five characters. Once you know what 245/45R18 means, you can read any tire size on sight: width, sidewall height, construction type, and wheel diameter.
It’s the first thing to check before you buy, because those numbers decide whether the tire will even mount on your wheels. A 245/45R18 won’t fit a 17-inch rim, and it won’t fit every 18-inch rim either — the width and the vehicle’s own specs have a say.
The Numbers And Letters In 245/45R18, One By One
The five characters carry four separate specs: 245, 45, R, and 18. Each one controls a different part of the tire’s shape.
- 245 — section width in millimeters. This is the tire’s widest point, measured across the outer sidewalls, about 9.6 inches for this size. It is not the tread width, which is almost always narrower.
- 45 — the aspect ratio. The sidewall height equals 45% of the width: 0.45 x 245 mm gives roughly 110 mm, or 4.3 inches. Drop to a 40 and the sidewall gets shorter and the ride stiffer; go to a 50 and it gets taller and softer.
- R — radial construction. The internal cords run at 90 degrees to the direction of travel, the standard for essentially every passenger tire sold today.
- 18 — wheel diameter in inches. The wheel’s bead-seat diameter must be exactly 18 inches. On this number there’s no wiggle room.
Read together, the four values describe the tire’s footprint and height. That’s why two tires can both fit an 18-inch wheel yet look completely different on the car — the width and aspect ratio change everything else. It’s also why sticking with the original equipment size matters: the car’s suspension geometry, ABS, and traction control are calibrated around the factory tire’s diameter.
How Big Is A 245/45R18 Tire Actually?
Run the numbers and a 245/45R18 stands about 26.7 inches tall, roughly 9.6 inches wide at its widest point, and turns about 756 revolutions per mile. Here’s the full set of derived dimensions.
| Dimension | Value | What It Means |
|---|---|---|
| Section width | 245 mm (9.6 in) | Widest point of the tire, sidewall to sidewall |
| Sidewall height | 110 mm (4.3 in) | 45% of 245 mm |
| Overall diameter | 26.7 in | Height of the mounted, inflated tire |
| Circumference | 83.8 in |
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