How to Read a Tire Size
Every metric tire is stamped with a three-part size code such as 225/45R17. This code is a shorthand for three numbers that define the tire's physical dimensions:
- 225 - Section width in millimeters, measured tread face to tread face.
- 45 - Aspect ratio: sidewall height as a percentage of section width. Here, 45% of 225mm = 101.25mm.
- R - Radial construction. A "B" would mean bias-ply and is very rare in modern tires.
- 17 - Rim diameter in inches.
From just those three numbers, every other dimension - overall diameter, circumference, revolutions per mile - can be calculated precisely.
Quick Reference
225/45R17: 17 + 2 x 101.25 / 25.4 = 24.97 inches. Memorize this and you can estimate any tire size in your head.The Formulas
The calculator uses the standard tire dimension formulas defined by the ETRTO and Tire and Rim Association. Every output is derived from width (mm), aspect ratio (%), and rim diameter (inches).
The number 63,360 is the number of inches in one mile (5,280 feet x 12 inches). For metric users, revs per kilometer uses 1,000,000 millimeters in the numerator.
Section Width vs Tread Width
Common Sizes Reference
These are some of the most common factory tire sizes on modern passenger cars and light trucks:
| Tire Size | Diameter (in) | Sidewall (in) | Sidewall (mm) | Circumference (mm) | Revs/mi |
|---|---|---|---|---|---|
195/65R15 | 24.98" | 5.00" | 126.92mm | 804.78mm | 788 |
205/55R16 | 24.88" | 4.44" | 112.79mm | 781.37mm | 791 |
225/45R17 | 24.97" | 3.99" | 101.25mm | 784.07mm | 788 |
235/40R18 | 25.41" | 3.70" | 94.00mm | 798.04mm | 775 |
245/40R19 | 26.72" | 3.86" | 98.00mm | 839.28mm | 736 |
265/70R17 | 31.61" | 7.30" | 185.50mm | 992.52mm | 623 |
Why These Dimensions Matter
Diameter is the single most important number. Your vehicle's speedometer, odometer, transmission shift points, ABS calibration, and traction control are all calibrated to a specific overall diameter. Changing it by more than 3% can cause noticeable errors; more than 5% can trigger ABS/ESC warning lights.
Section width determines contact patch area. Wider tires grip better on dry pavement but perform worse in snow and slush because they have less weight per square inch of contact.
Sidewall height controls ride comfort and steering response. Tall sidewalls absorb bumps; short sidewalls transmit road feedback directly to the driver.
Diameter Tolerance
Best Practice
Frequently Asked Questions
The number 225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percentage of width, so 45% of 225 = 101.25mm), R stands for radial construction, and 17 is the rim diameter in inches. Combined, these three numbers define every physical dimension of the tire.
Diameter in inches = rim size + 2 x (width x aspect / 100) / 25.4. For 225/45R17: 17 + 2 x (225 x 0.45) / 25.4 = 17 + 7.97 = 24.97 inches (or 634.3mm). The 2x factor is because there are two sidewalls, one on top and one on bottom.
Revolutions per mile (rev/mi) is how many times your tire spins in one mile of travel. It is calculated as 63,360 inches (one mile) divided by the circumference in inches. Your speedometer, odometer, and transmission shift logic all assume a specific rev/mile figure, so changing tire size changes how those systems behave.
No. Aspect ratio is a percentage, not a dimension. Sidewall height is the actual measurement. For a 225/45 tire, the sidewall is 45% of 225mm = 101.25mm tall. A lower aspect ratio means a shorter sidewall relative to width, which typically improves handling response but reduces ride comfort.
Historical reasons. When metric tire sizing was standardized in the 1970s, rims were already produced in inch increments (13, 14, 15, etc.) and the industry kept that convention. The result is a hybrid format where the first two values are metric and the last is imperial. Light truck (LT) tires in the USA sometimes use an all-inch format like 33x12.50R17.
Generally yes. A tire with a higher aspect ratio has a taller sidewall, which absorbs more road shock and vibration. The trade-off is less precise steering and more sidewall flex under hard cornering. Most manufacturers pair smaller rims with higher-profile tires on comfort-focused trims and larger rims with low-profile tires on sport trims.
The size code is a nominal design specification, not a perfect physical measurement. Real mounted tires vary by +/-2-3% depending on rim width, inflation pressure, and tread wear. For the most precise diameter, measure the mounted tire directly, or use this calculator as a starting point and verify with the manufacturer data sheet.