Tire size explained: how to read the numbers
By FitMyTires · Published · Updated
A size such as 225/45R17 describes nominal tire width, sidewall proportion, construction, and rim diameter. The three numbers use different units. The 45 is a percentage, and the 17 is the rim diameter in inches.
- 225
- Section width: 225 mm
- 45
- Sidewall: 45% of width
- R17
- Radial tire, 17-inch rim
These are nominal size markings, not measurements of a particular mounted tire. A load index and speed symbol, such as 94W, add requirements beyond those dimensions. Reading the size does not validate those ratings. Michelin explains sidewall markings and service descriptions.
How tall is a 225/45R17 tire?
Multiply the width by the aspect ratio: 225 × 0.45 = 101.25 mm of nominal sidewall height. Convert the rim to millimeters: 17 × 25.4 = 431.8 mm. Add two sidewalls to the rim: 431.8 + 202.5 = 634.3 mm, or 24.97 inches overall.
This explains why profile numbers cannot be compared in isolation. A 45-profile tire at a different width has a different sidewall height. Likewise, an 18-inch rim does not mean an 18-inch-tall tire; the tire sidewalls add to that diameter.
Compare 225/45R17 with 245/40R18
Where should you find the starting size?
The sidewall identifies the installed tire. The vehicle placard and owner documentation identify the vehicle specification; the installed tire may already differ from it. Check the full required specification before choosing a replacement. The calculator lets you compare dimensions, but it cannot determine whether the existing setup was correct.
What about LT, ZR, and half-inch rims?
FitMyTires accepts supported metric prefixes and service-description formats for entry convenience. It also accepts the 16.5, 17.5, 19.5 and 22.5-inch LT and commercial rims. This only tells the calculator which dimensions to use. It does not make a passenger tire interchangeable with a light-truck tire or establish that a different construction meets your vehicle's requirements.
If your tire uses a marking such as 35x12.50R17, use the metric and flotation conversion guide. For a full list of reference pages, browse tire dimensions by rim size.