The tire code 245/45R18 describes a 245 mm wide radial tire with a 45 percent sidewall height made for an 18-inch wheel.
Every tire sidewall carries a size code that packs four separate measurements into one string, and decoding what 245/45R18 means takes about thirty seconds once you know the pattern. The code covers the four basics you need before matching a replacement tire to your car: width, sidewall profile, construction type, and wheel diameter.
Decoding 245/45R18: What Each Number Means
Each part of 245/45R18 describes one physical measurement, and together they define the tire’s width, height, construction, and the wheel size it fits.
- 245 — section width in millimeters. This is the tire’s widest point, measured from sidewall to sidewall, roughly 9.6 inches. It is not the tread width, which is usually a bit narrower.
- 45 — aspect ratio. The sidewall height equals 45 percent of the section width, or about 110 mm (4.3 inches). Lower numbers mean a shorter sidewall and a sportier, firmer ride.
- R — radial construction. The internal cord plies run radially, or at 90 degrees, to the direction of travel. This is the standard construction for modern car tires.
- 18 — wheel diameter in inches. The tire is built to mount on an 18-inch rim.
How Big Is A 245/45R18 Tire, Really?
A 245/45R18 tire stands about 26.7 inches tall with a circumference of about 83.8 inches, which is why this size shows up on midsize sedans and crossovers that need a balanced blend of ride comfort and handling. Tire calculators such as 1010Tires’ 245/45R18 size calculator list the full set of derived numbers.
| Measurement | In Real Terms | What It Affects |
|---|---|---|
| Section width | 245 mm (about 9.6 in) | Tread footprint and wheel-width fit |
| Sidewall height | About 4.3 in | Ride softness and curb protection |
| Overall diameter | About 26.7 in | Speedometer accuracy and clearance |
| Circumference | About 83.8 in | Distance rolled per revolution |
| Wheel diameter | 18 in | Must match the rim exactly |
| Approved rim width | 7.5–9.0 in | Safe mounting and bead seating |
| Construction | Radial (R) | Flexible sidewalls, stable tread |
The whole tire has to clear your fenders, brakes, and suspension, so the overall diameter matters as much as the rim size. Replacing a 245/45R18 with a size whose diameter differs by more than a fraction of an inch can throw off the speedometer and, on all-wheel-drive cars, put uneven strain on the drivetrain. Tire makers such as Nokian publish approved rim-width ranges for each size, and the 7.5 to 9.0-inch window above is typical for this one.
If you’re shopping for replacements, our hands-on list of the best 245/45R18 all-season tires covers sets that fit this exact size and clear these fitment requirements.
Common Mistakes With 245/45R18 Tires
Three mistakes cause most sizing errors with this tire size, and all three come from misreading what the code actually describes.
- Treating 245 mm as tread width. The 245 measures the tire at its widest bulge, sidewall to sidewall, not the rubber that contacts the road. The actual tread is narrower.
- Reading 45 as 45 mm. The 45 is a percentage. The sidewall is 45 percent of 245 mm, or