Use the following calculator to find the dimensions of a tire based on its code. It can also provide a comparison of alternative tires that are compatible with a given wheel size.
Use this calculator to compare the sizes of two tires both numerically and visually.
The tire size calculator turns the alphanumeric code on the sidewall of a tire — something like 265/70R17 — into the real, physical dimensions you actually care about: the tire's overall diameter, its section width, the sidewall height, the circumference, and how many revolutions the tire makes per mile and per kilometer. It also does two more jobs. The comparison calculator puts two tire sizes side by side and shows exactly how much they differ, including the effect on your speedometer. And the alternative-size feature suggests a list of tires that fit a different wheel diameter while keeping the overall diameter close to the original, which is invaluable when you are shopping for new wheels. Every number is calculated with the same formulas used by tire manufacturers, so the results match what you would measure on a real tire.
Whether you are buying replacement tires, considering a "plus-size" wheel upgrade, checking whether a set of used tires will fit, or simply curious about what that string of numbers means, this calculator gives fast, exact answers in both imperial (inches, miles) and metric (millimeters, kilometers) units.
Car tires are described by a standardized alphanumeric code molded into the sidewall. Being able to read this code is important whenever you consider new wheels or tires, because it is what you use to calculate tire size and to make sure a replacement will fit. A typical code looks like P215/65R15 95H. Each part describes a characteristic of the tire:
This calculator uses the three dimensional parts of the code — section width (mm), aspect ratio (%), and wheel diameter (inches) — because those are what determine the tire's physical size.
Once you know the three parts of the code, every other dimension follows from simple geometry. The calculator works internally in millimeters and converts to inches at 25.4 mm per inch.
Sidewall height (mm) = width × (aspect ratio / 100)
Wheel diameter (mm) = wheel size (in) × 25.4
Tire diameter (mm) = wheel diameter + 2 × sidewall height
Circumference (mm) = π × tire diameter
Revolutions / mile = 1,609,344 / circumference (mm)
Revolutions / km = 1,000,000 / circumference (mm)
Take the default tire, 265/70R17. The section width is 265 mm. The sidewall height is 265 × 0.70 = 185.5 mm. The wheel is 17 inches = 431.8 mm. So the overall tire diameter is 431.8 + 2 × 185.5 = 802.8 mm, which is about 31.6 inches. The circumference is π × 802.8 ≈ 2,522 mm (99.3 inches), and because one mile is 63,360 inches, the tire turns 63,360 / 99.3 ≈ 638 revolutions per mile. These are exactly the numbers a manufacturer's spec sheet would list.
Of all the dimensions, the overall diameter (and the closely related circumference and revolutions per mile) is the one that most affects how your vehicle behaves. Because your speedometer and odometer are calibrated for the diameter of the tires that came on the car, changing to a tire with a noticeably different diameter throws both readings off. A larger-diameter tire covers more ground per revolution, so the speedometer will read slower than you are actually going; a smaller-diameter tire makes it read faster. Overall diameter also affects ground clearance, gearing feel, the accuracy of ABS and traction-control systems, and whether the tire physically rubs against the fender or suspension. For all of these reasons, the general rule when changing tire or wheel sizes is to keep the overall diameter within about 3% of the original.
"Plus-sizing" is the popular practice of fitting a larger-diameter wheel while choosing a tire with a shorter sidewall so that the overall tire diameter stays roughly the same. Going up an inch in wheel size is called a "plus one," two inches a "plus two," and so on. The payoff is usually sharper handling and a more aggressive look; the trade-offs are a firmer ride, a higher chance of wheel damage from potholes, and often a higher price. The key to doing it right is keeping the overall diameter close to stock.
That is exactly what the alternative-size feature does. Select "Change to a new wheel size," enter the wheel diameter you want, and the calculator lists tire sizes for that wheel whose overall diameter lands within about 3% of your original tire. For each suggestion it shows the new diameter, width, and sidewall height, along with the percentage difference from your starting tire, so you can quickly spot the closest match. Keep in mind, as the tool notes, that a size appearing on the list only means the diameter is compatible — the tire may not meet your vehicle's load requirements, may not be available, and may not be the most cost-effective option, so always confirm with a tire professional.
The Tire Size Comparison Calculator is built for exactly the moment you are deciding between two sizes, or checking how a used set differs from your factory tires. Enter two complete tire codes and it lays out every dimension — diameter, wheel size, sidewall height, width, circumference, and revolutions per mile and kilometer — for both tires side by side, plus the percentage difference between them. A pair of scaled diagrams shows the two tires at their true relative sizes so the difference is easy to see at a glance.
The comparison also includes a speedometer difference table, which is often the most practical output of all. It assumes your speedometer is calibrated for Tire 1 and shows what your actual speed would be on Tire 2 at 20, 40, 60, 80, 100, and 120 units. For example, swapping the default 265/70R17 (31.6″) for a 265/40R23 (31.3″) means that when the speedometer reads 60, you are really doing about 59.5 — a small but real difference that grows with speed and with the size gap. If you ever fit tires of a very different diameter, this table tells you how far off your speedometer and odometer will be.
Suppose your vehicle came with 265/70R17 tires (31.6″ overall) and you want to fit sharper-looking 19-inch wheels. Enter 265/70R17, choose "Change to a new wheel size," and type 19. The calculator returns a list of 19-inch tire sizes whose diameters stay within 3% of 31.6″. Near the middle of that list you will find sizes such as 265/60R19, which comes out to an almost identical overall diameter — the same rolling distance and speedometer reading, just with a two-inch-larger wheel and a correspondingly shorter, sportier sidewall. That is plus-sizing done correctly.
The standard P-metric and Euro-metric codes this calculator uses (width in mm, aspect ratio as a percentage, wheel diameter in inches) cover the vast majority of passenger and light-truck tires. You may also encounter LT "flotation" sizes written entirely in inches, such as 35×12.50R17, where the first number is the overall diameter in inches, the second is the width in inches, and the last is the wheel diameter. These are common on off-road trucks. While the input format differs, the underlying geometry — diameter, circumference, and revolutions per mile — is calculated the same way, and you can compare a flotation tire to a metric one by converting its dimensions to millimeters.
It is a tire code. 265 is the section width in millimeters, 70 is the aspect ratio (the sidewall height is 70% of the width), R means radial construction, and 17 is the wheel diameter in inches. Together they describe a tire about 31.6 inches in overall diameter.
Multiply the width by the aspect ratio (as a decimal) to get the sidewall height in millimeters, double it, and add the wheel diameter converted to millimeters (wheel inches × 25.4). For 265/70R17: 265 × 0.70 = 185.5 mm sidewall; 17 × 25.4 = 431.8 mm wheel; diameter = 431.8 + 2 × 185.5 = 802.8 mm ≈ 31.6 inches.
Your speedometer reads correctly only for the diameter it was calibrated for. A tire with a larger overall diameter makes the speedometer read slower than your true speed, and a smaller one makes it read faster. The percentage error equals the percentage difference in diameter, which is why keeping within about 3% is recommended. The comparison calculator's speedometer table shows the exact effect.
Plus-sizing means fitting a larger-diameter wheel with a lower-profile tire so the overall tire diameter stays about the same. It typically improves handling and appearance at the cost of ride comfort. Use the "Change to a new wheel size" option to find compatible sizes for the wheel you want.
Sometimes. The tire must match the wheel's diameter (the "R" number) and be within the wheel's approved width range. You can change the section width and aspect ratio somewhat, but the overall diameter should stay within roughly 3% of stock and the tire must clear the vehicle. Always verify fitment for your specific car.
A higher aspect ratio (say 70) means a taller sidewall, which generally gives a softer ride and more protection against potholes. A lower aspect ratio (say 40) means a shorter, stiffer sidewall, which sharpens steering response and handling but transmits more road harshness.
This Tire Size Calculator is provided for educational and general informational purposes. It uses standard tire-code geometry and the results closely match manufacturer specifications, but actual tire dimensions vary slightly between brands and models and with inflation, load, and wear. Alternative sizes shown are compatible in diameter only and may not meet your vehicle's safety, load, or clearance requirements. Always confirm tire and wheel fitment with a qualified tire professional before purchasing.