The formula
How it works
Decode a tire size like 225/45R17 into real dimensions. Enter the section width, aspect ratio and rim diameter, and the calculator gives the sidewall height, overall diameter, circumference and revolutions per mile.
FAQ
What do the numbers on a tire mean?
In 225/45R17, 225 is the section width in millimetres, 45 is the aspect ratio — the sidewall height as a percentage of the width — and 17 is the rim diameter in inches. The R means radial construction. Enter those three numbers and the calculator does the rest.
Why does overall diameter matter?
It sets your ride height and, more importantly, your speedometer accuracy. Fitting a tire with a different diameter changes how far the car travels per wheel revolution, so the speedometer and odometer read high or low. Keeping the overall diameter close to the original avoids that.
How much of a diameter difference is acceptable?
Most tire professionals suggest staying within about 3% of the original overall diameter. Beyond that, speedometer error, gearing and clearance problems become more noticeable, and on some vehicles it can trigger warning lights from the traction or ABS systems.
What does “plus sizing” mean?
It means fitting a larger-diameter wheel with a lower-profile tire so the overall diameter stays about the same. A “plus one” upgrade might go from a 205/65R15 to a 215/60R16, keeping ride height and speedometer accuracy close to stock while changing the look and handling.
Can I mix different tire sizes on one car?
It is generally not recommended except where the manufacturer specifically allows staggered fitments, since mismatched diameters can confuse all-wheel-drive systems and stability control, and cause uneven handling. Always match the size on both ends of an axle.
What is the load index and speed rating for?
They are separate codes, usually printed after the size, that state the maximum weight a tire can carry and the maximum speed it is rated for. This calculator focuses on the size dimensions; the load index and speed rating should still match or exceed the vehicle manufacturer’s original specification.
Why do two tires with the same size read slightly different diameters?
Manufacturers have some tolerance in how they build a given size, so actual diameters can vary by a small percentage between brands and models even with an identical size code. This calculator gives the nominal, calculated diameter, which is a close estimate rather than an exact measurement.
About the tire size calculator
This calculator translates a tire’s coded size — such as 225/45R17 — into the physical measurements that actually matter: sidewall height, overall diameter, circumference and how many times the tire turns in a mile. The code mixes millimetres, a percentage and inches, so working out the real diameter by hand is awkward. The calculator combines them into figures you can compare between tires.
How to use it
Enter the three numbers from the tire’s sidewall: the section width in millimetres, the aspect ratio as a percentage, and the rim diameter in inches. The calculator returns the sidewall height and the overall diameter in both millimetres and inches, plus the circumference and revolutions per mile. For a 225/45R17, that is a sidewall of 101.25 mm and an overall diameter of about 24.97 inches.
The formula
The sidewall height is the width times the aspect ratio, , and the overall diameter is the rim plus twice the sidewall, , after converting units to match. Circumference is , and revolutions per mile is one mile divided by the circumference — the figure that drives speedometer readings.
Where it is used
Drivers use it when choosing replacement or upgrade tires, to check that a new size keeps the overall diameter close to the original and the speedometer honest. Enthusiasts use it to compare plus-sizing options, and mechanics use it to confirm fitment. Because a small change in aspect ratio or rim size can shift the diameter noticeably, seeing the numbers side by side prevents a costly mismatch.