Tire Size Calculator
Tire Size Calculator - instant results with formula, steps, and examples. No sign-up required.
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About Tire Size Calculator
Decode Tire Sizes and Compare Fitments
That string of numbers on your tire sidewall - something like 225/45R17 - packs a lot of information into a compact code. Our Tire Size Calculator decodes it all, showing you the tire's width, aspect ratio, diameter, sidewall height, circumference, and revolutions per mile. It's an essential tool for anyone buying new tires, upsizing wheels, or comparing alternative fitments.
Understanding Tire Size Notation
A tire size like 225/45R17 breaks down as follows. The first number (225) is the section width in millimeters - the distance across the widest point of the tire when mounted and inflated. The second number (45) is the aspect ratio - the sidewall height expressed as a percentage of the width. So 45% of 225 mm gives a sidewall height of 101.25 mm. The "R" indicates radial construction (virtually all modern tires). The last number (17) is the wheel diameter in inches.
From these three numbers, our tire size calculator derives everything else: overall diameter, circumference, revolutions per mile, and speedometer correction factors when comparing two different tire sizes.
Why Tire Size Matters Beyond Fit
Choosing the right tire size affects far more than whether the tire physically fits your car. Speedometer accuracy depends on tire circumference - a tire that's 3% larger in diameter will make your speedometer read 3% low. At an indicated 60 mph, you'd actually be traveling 61.8 mph. Our calculator shows you exactly how much a size change will affect your speedometer reading.
Ride quality and handling change with aspect ratio. Lower-profile tires (smaller aspect ratio) provide sharper steering response and better cornering grip but transmit more road imperfections to the cabin. Higher-profile tires absorb bumps better but feel less precise. Understanding these tradeoffs starts with understanding the numbers.
Fuel economy is affected by tire width and rolling resistance. Wider tires have more contact patch area, which generally increases grip but also increases rolling resistance. For daily drivers prioritizing fuel economy, staying close to the OEM tire size is usually the best approach.
Plus-Sizing: Bigger Wheels, Lower Profile
One of the most common reasons people use a tire size calculator is to explore plus-sizing options. Plus-sizing means going to a larger wheel diameter while reducing the tire's sidewall height to maintain approximately the same overall diameter. For example, moving from 205/55R16 to 225/45R17 increases wheel size by one inch while keeping the overall tire diameter nearly identical.
Our calculator lets you compare two tire sizes side by side, showing the difference in overall diameter, circumference, and speedometer impact. This is invaluable when shopping for wheels and tires together, ensuring your new setup will fit your fenders, clear your brakes, and maintain accurate instrumentation.
Metric, Imperial, and Everything in Between
Tire measurements uniquely blend metric and imperial units - width in millimeters, diameter in inches. Our Tire Size Calculator converts everything into both systems so you can work in whichever you prefer. All results are displayed clearly with both millimeter/centimeter and inch values.
The tool is free, runs in your browser, and requires no account. Whether you're a car enthusiast planning a wheel upgrade, a mechanic advising a customer, or a driver simply trying to understand what's on their sidewall, this calculator puts all the answers in one place.