How to Read a Tire Size
Updated 8 November 2026. Every figure on this page comes from the same formulas as our calculators.
The code on the sidewall of a tire tells you its size, how it is built and how much load and speed it is rated for. Using 225/45R17 94W as the example, this is what each part means and how to turn it into real dimensions.
The parts of the size
| Part | Meaning | In 225/45R17 94W |
|---|---|---|
| 225 | Section width: the width of the tire from sidewall to sidewall, in millimetres | 225 mm (8.86 in) |
| 45 | Aspect ratio: sidewall height as a percentage of the width | 45 % of 225 mm = 101.3 mm (3.99 in) |
| R | Construction: R is radial, which almost every modern car tire is | Radial |
| 17 | Rim diameter, in inches | Fits a 17-inch wheel |
| 94 | Load index: the maximum load per tire, looked up in a table | 670 kg (1,477 lb) per tire |
| W | Speed rating: the maximum sustained speed the tire is rated for | 270 km/h (168 mph) |
The size mixes millimetres and inches: the width is metric, the rim is in inches, and the aspect ratio is a percentage rather than a length. That is why two numbers that look close can describe very different tires.
Working out the real dimensions
The overall diameter is the rim plus two sidewalls, one above the hub and one below:
- Sidewall height = 225 × 0.45 = 101.3 mm
- Rim diameter = 17 × 25.4 = 431.8 mm
- Overall diameter = 431.8 + 2 × 101.3 = 634.3 mm (24.97 in)
- Circumference = diameter × π = 1,993 mm (78.45 in), which gives about 807.6 revolutions per mile (501.8 per km)
These are the nominal dimensions from the size code. A real tire on a loaded car is slightly squashed, and different brands measure a little differently, so the manufacturer's published revolutions per mile can differ by a fraction of a percent. For comparing two sizes, the nominal figures are what matter, because the same error applies to both.
The tire size calculator does this for any size, and the comparison tool puts two sizes side by side.
Prefixes: P, LT and none
- P225/45R17 is a US "P-metric" passenger tire.
- 225/45R17 without a prefix is a European (ETRTO) metric size. The dimensions are the same as the P-metric size.
- LT265/70R17 121/118S is a light-truck tire. The two load indices are for single and dual rear wheels, and LT tires reach their rated load at higher pressures than passenger tires.
Because load ratings depend on pressure and tire type, never swap between passenger and LT tires, or drop to a lower load index, without checking the vehicle's placard (usually on the driver's door frame) or asking a tire professional.
Inch sizes: 33x12.50R15
Off-road and truck tires are often sold in "flotation" sizes written entirely in inches: 33x12.50R15 is about 33 inches tall, 12.5 inches wide, for a 15-inch rim. A metric size can be written the same way: 285/70R17 works out to 32.7x11.2R17, which is why it is often called a "33-inch" tire. The metric to inch converter goes both ways.
Other markings you will see
- XL or Reinforced: a higher load index than the standard version of the same size, reached at a higher pressure.
- M+S: mud and snow. A manufacturer's marking, not a winter test.
- Three-peak mountain snowflake: the tire passed a standard snow traction test.
- DOT date code: the last four digits are the week and year of manufacture. 2324 means week 23 of 2024.