Tire Size Calculator
Enter a tire code such as 205/55R16 to see its sidewall height, overall diameter, and circumference in both metric and imperial units.
Example
For a 205/55R16 tire: sidewall height = 205 × 55 / 100 = 112.75 mm. Overall diameter = (16 × 25.4) + 2 × 112.75 = 631.9 mm = 24.88". Circumference = π × 631.9 ≈ 1985 mm (78.16"), giving about 811 revolutions per mile.
How it works
Sidewall height = width (mm) × aspect ratio / 100. Overall diameter = rim diameter (in × 25.4 mm) + 2 × sidewall height. Circumference = π × overall diameter. Inches are mm ÷ 25.4.
Good to know
This Tire Size Calculator turns a standard P-metric tire code like 205/55R16 into the dimensions that actually matter when you compare tires: sidewall height, overall diameter in both millimeters and inches, rolling circumference, and revolutions per mile. It's aimed at anyone weighing a tire swap, a wheel upgrade, or a plus-sizing project, as well as DIY mechanics who want to sanity-check a replacement before ordering.
The most common reason to reach for it is to check whether a different tire will throw off your speedometer and odometer. Both instruments are calibrated to the original tire's diameter, so a taller or shorter tire changes how many times the wheel turns per mile. The revolutions-per-mile figure and the overall diameter are the two numbers to watch when you're cross-shopping sizes.
To read the result, run your current size first and note the overall diameter, then run the candidate size and compare. A larger diameter raises ground clearance and effective gearing but makes the speedometer read lower than your true speed; a smaller diameter does the opposite. The sidewall height value also tells you about ride feel, since a taller sidewall cushions bumps while a shorter one sharpens steering response.
One practical caveat: these are nominal calculations from the printed size, not measured values. Real mounted diameter varies slightly by brand, tread depth, inflation, and load, and the math here ignores fitment factors like rim width range, load index, and speed rating. Use the output to narrow choices and protect your speedometer accuracy, then confirm clearance and load ratings against your vehicle's specs before buying.
Frequently asked questions
What do the numbers in a tire size like 205/55R16 mean?
205 is the tread width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of the width), the R means radial construction, and 16 is the rim diameter in inches.
How much can I change tire size without affecting my speedometer?
Keep the overall diameter within about 3% of the original. A larger diameter makes the speedometer read low (you go faster than shown); a smaller one reads high. Compare the diameter values for both sizes to check.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
What does 205/55R16 convert to in inches?
A 205/55R16 tire has an overall diameter of about 24.88 inches, a sidewall height of roughly 4.44 inches (112.75 mm), and a rolling circumference near 78.16 inches. The 16-inch figure is the rim diameter, not the whole tire.
How do I calculate tire diameter from the size code?
Multiply the width in millimeters by the aspect ratio and divide by 100 to get sidewall height, then add twice that to the rim diameter converted to millimeters (rim inches times 25.4). Divide the total by 25.4 to express the overall diameter in inches.
What is a tire's aspect ratio?
The aspect ratio is the middle number in the size code and represents the sidewall height as a percentage of the tread width. In 205/55R16, the 55 means the sidewall is 55 percent of 205 mm, or about 112.75 mm tall.
Will bigger tires make my odometer inaccurate?
Yes, because the odometer counts wheel revolutions assuming the original tire diameter. A larger diameter covers more distance per revolution, so the odometer and speedometer will both read lower than your actual distance and speed.
How many revolutions per mile does a tire make?
Divide one mile in millimeters (1,609,344) by the tire's circumference in millimeters. A 205/55R16 turns about 811 times per mile; larger-diameter tires turn fewer times and smaller ones turn more.
What does the R in a tire size mean?
The R stands for radial, the construction type used by nearly all modern passenger tires, where the body ply cords run radially across the tire. Other letters like D (diagonal/bias) or B (belted bias) are rare on today's vehicles.
Can I put a different aspect ratio tire on the same rim?
Often yes, as long as the overall diameter stays close to stock and the tire's width fits the rim's recommended range. Changing only the aspect ratio while keeping the same rim shifts the sidewall height, which alters overall diameter and ride characteristics.
How much does tire diameter affect speedometer reading?
The percentage difference in overall diameter roughly equals the percentage error in the speedometer. A tire 3 percent larger means the speedometer shows about 3 percent slower than your true speed, which is why staying within about 3 percent of stock is a common guideline.
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