Metric Tire Size Decoder
Read a code like 225/45R17. Each part is explained. Use the calculator below. Results update as you type.
How the Metric Tire Size Decoder Works
This decoder works by splitting a sidewall marking into its fields and translating each field into a plain measurement. It reads 9 fields: service type, width, aspect ratio, speed bracket, construction, rim, load index, speed rating and load type.
- Type the marking as printed, for example P225/45ZR17 94W XL.
- Let the decoder separate each letter and number group.
- Read the meaning of each field with metric and imperial units.
- Copy the decoded width, aspect and rim into any other calculator on the site.
The decoder accepts slashes, spaces and missing letters. It flags fields it cannot read instead of guessing.
The Formula
The formula has no single equation; it applies 3 conversion rules to the numeric fields.
Width (in) = Width (mm) ÷ 25.4 Sidewall (mm) = Width (mm) × Aspect Ratio ÷ 100 Rim (mm) = Rim (in) × 25.4Letter fields use lookup tables. The load index maps to a load in kilograms (kg) from a fixed chart, the speed rating maps to a maximum speed, and the service type maps to a load standard.
Worked example for 225/45R17: width is 8.86 in, sidewall is 101.25 mm (3.99 in), and rim is 431.8 mm.
Conversion Table
The conversion table decodes all 9 fields of the marking P225/45ZR17 94W XL. Each row shows the field name, the printed value, its meaning and its unit conversion.
Read the table top to bottom in the same order the fields appear on the tire. The last 3 rows (load index, speed rating, load type) form the service description, which sits a few centimeters after the size on the sidewall.
| Field | Value | Meaning | Unit / Conversion |
|---|---|---|---|
| Service type | P | Passenger car (P-metric) tire | No letter = Euro-metric |
| Section width | 225 | Sidewall-to-sidewall width | 225 mm = 8.86 in |
| Aspect ratio | 45 | Sidewall height is 45% of width | 101.3 mm = 3.99 in |
| Speed bracket | Z | Older marking for speed above 149 mph | Over 240 km/h |
| Construction | R | Radial ply casing | B = bias belted, D = diagonal |
| Rim diameter | 17 | Wheel bead seat diameter | 17 in = 431.8 mm |
| Load index | 94 | Maximum load per tire | 670 kg = 1,477 lb |
| Speed rating | W | Maximum sustained speed | 270 km/h = 168 mph |
| Load type | XL | Extra Load (reinforced) casing | Rated at 42 psi (2.9 bar) |
What Is the Metric Tire Size Decoder?
The Metric Tire Size Decoder is a tool that translates a metric tire marking into width, profile, construction, rim size, load capacity and speed limit. Metric sizing is the system used on almost every passenger car, SUV and light truck sold since the late 1970s.
A metric marking has 2 parts. The size (225/45R17) gives the physical dimensions. The service description (94W) gives the load and speed rating. Prefix letters (P, LT, ST, T) state the load standard, and suffix letters (XL, RF, C) add load or construction details.
Common Uses
This decoder has 5 uses: reading sidewalls, checking replacements, confirming load capacity, learning the code and validating listings.
- Read sidewalls: Turn any metric marking into plain measurements.
- Check replacements: Compare a new tire's marking with the door placard.
- Confirm load capacity: See the kilograms and pounds behind a load index.
- Learn the code: See what each letter and number means.
- Validate listings: Spot errors in online tire listings before ordering.