Tire Size Decoder Tool
Decode any tire size like 225/45R17 into sidewall height, overall diameter, circumference and revolutions per mile. See exactly what the numbers on your tire mean.
Updated 2026-06-14 · Free · No sign-up · Runs privately in your browser
Show the formula & steps
How the Tire Size Decoder Works
Every tire stamps its size on the sidewall in a code like 225/45R17. This decoder breaks that code into real-world dimensions you can actually compare: the sidewall height, the overall diameter, the circumference, and the number of revolutions per mile. Enter the three numbers from your tire and the tool does the unit conversions for you.
What the Numbers Mean
For a size written as W/A R D:
- W — section width in millimeters (e.g. 225).
- A — aspect ratio: the sidewall height as a percentage of the width (e.g. 45).
- R — radial construction.
- D — wheel rim diameter in inches (e.g. 17).
The Formula
Sidewall height (mm) = width × aspect ÷ 100 Overall diameter (mm) = rim × 25.4 + 2 × sidewall Circumference (mm) = π × overall diameter Revolutions per mile = 1,609,344 ÷ circumference (mm)
One inch equals 25.4 mm, and one mile equals 1,609,344 mm. The rim diameter is the distance across the wheel, so the sidewall counts twice — once at the top and once at the bottom — to get the full tire diameter.
Worked Example
For a 225/45R17 tire:
- Sidewall = 225 × 45 ÷ 100 = 101.25 mm (about 3.99 in)
- Overall diameter = 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm (about 24.97 in)
- Circumference = π × 634.3 = 1,992.7 mm (about 78.45 in)
- Revolutions per mile = 1,609,344 ÷ 1,992.7 = about 808
Comparing Tire Sizes
When upsizing or downsizing, keep the overall diameter within about 3% so your speedometer stays accurate and the ABS and traction systems read correct wheel speeds.
| Tire size | Overall diameter | Revs/mile |
|---|---|---|
| 205/55R16 | 632.0 mm (24.88 in) | 811 |
| 225/45R17 | 634.3 mm (24.97 in) | 808 |
| 245/40R18 | 653.2 mm (25.72 in) | 784 |
Frequently asked questions
What does 225/45R17 mean?+
225 is the section width in millimeters, 45 is the aspect ratio (sidewall height is 45% of the width), R means radial construction, and 17 is the wheel rim diameter in inches. Together they define a tire that fits a 17-inch rim and is 225 mm wide with a 101.25 mm sidewall.
How do I calculate tire diameter from the size?+
Convert the sidewall height to millimeters (width × aspect ÷ 100), then add it twice to the rim diameter in millimeters (rim inches × 25.4). For 225/45R17: 225 × 0.45 = 101.25 mm sidewall, plus 17 × 25.4 = 431.8 mm rim, giving 431.8 + 2 × 101.25 = 634.3 mm, about 24.97 inches.
What is the aspect ratio on a tire?+
The aspect ratio is the sidewall height expressed as a percentage of the section width. A lower number means a shorter, sportier sidewall, while a higher number means a taller sidewall that gives a softer ride and more cushioning over bumps.
Why does revolutions per mile matter?+
Revolutions per mile is how many times a tire turns to travel one mile. Your speedometer and odometer are calibrated to the original tire's value, so changing to a tire with a different diameter changes the readings. Matching revolutions per mile keeps the gauges accurate.
Can I fit a different tire size on my car?+
You can fit a plus-size or minus-size tire as long as the overall diameter stays within roughly 3% of the original, the load and speed ratings match or exceed the originals, and it physically clears the bodywork. Use this decoder to compare overall diameters before buying.