Last updated: March 2026
Understanding Tire Size Conversions
Tire sizes use a hybrid measurement system that mixes millimeters, percentages, and inches in a single designation. A size like 225/65R17 tells you the width is 225 millimeters, the sidewall height is 65% of the width, and the wheel diameter is 17 inches. This converter translates everything into both metric and imperial measurements so you can understand the actual dimensions.
The most common conversion people need is from metric tire sizes to overall diameter in inches. Off-road enthusiasts often describe tires by their height in inches (e.g., "33-inch tires"), but the tire industry uses the metric format. This tool bridges that gap instantly, showing you the real-world height of any metric tire size.
The Metric Tire Sizing System Explained
The ISO metric tire sizing system was standardized to create a universal language for tire dimensions across all manufacturers worldwide. Before this system, tire sizes varied by manufacturer and region, making cross-compatibility difficult. The metric system encodes three essential dimensions — width, aspect ratio, and wheel diameter — in a compact, standardized format.
The width is always measured in millimeters because the system originated from European (primarily German) engineering standards. The aspect ratio is expressed as a percentage because it scales proportionally with the width — a 65-series tire always has a sidewall that is 65% of the width, regardless of how wide the tire is. The wheel diameter remains in inches because wheel sizing has used inches for over a century in virtually all markets.
Converting Inches to Metric Tire Sizes
If you know the overall diameter you want in inches, you can work backward to find equivalent metric sizes. Start with your target diameter, subtract the wheel diameter, and divide by 2 to get the required sidewall height in inches. Convert that to millimeters (multiply by 25.4), then divide by your chosen tire width to find the aspect ratio.
For example, to find a "33-inch" tire for 17-inch wheels: (33 - 17) / 2 = 8 inches of sidewall = 203mm. If you choose a 285mm-wide tire, the aspect ratio would be 203 / 285 = 0.71, or approximately a 285/70R17. Use the converter above to verify the exact measurements of any size you're considering.
Frequently Asked Questions
How do I convert a metric tire size to inches?
A metric tire size like 225/65R17 uses millimeters for width and a percentage for sidewall height. To convert: the width (225mm) divided by 25.4 equals 8.86 inches. The sidewall height is 225 x 0.65 = 146.25mm or 5.76 inches. The overall diameter is (5.76 x 2) + 17 = 29.52 inches. This calculator does all conversions automatically.
What is a 33-inch tire in metric?
A true 33-inch tire is approximately 285/75R16 (32.8" actual diameter), 305/70R16 (32.8"), or 285/70R17 (32.7"). No standard metric size produces exactly 33 inches, so enter different sizes to find the closest match for your needs.
Why are tire sizes in millimeters?
Tire sizing uses the ISO metric system adopted internationally. The width is in millimeters because it originated from European (primarily German) tire standards. The wheel diameter remains in inches because wheel sizes have historically been measured in inches in most markets. This hybrid system is now the global standard.
How do I convert inches to a metric tire size?
Start with the overall diameter in inches, subtract the wheel size, divide by 2 to get sidewall height in inches, convert to mm (multiply by 25.4), then choose a width and calculate the aspect ratio. For example, a 33" tire on a 17" wheel: (33-17)/2 = 8" sidewall = 203mm. With 285mm width: 203/285 = 71%, so approximately 285/70R17.
What does the R mean in tire sizes?
The 'R' stands for radial construction, meaning the tire's internal cord plies run radially (perpendicular to the direction of travel). Nearly all modern passenger and light truck tires are radial. You may occasionally see 'B' for bias-ply or 'D' for diagonal, but these are rare in modern vehicles.