Tire Size Calculator
Enter a metric tire size like 265/70R17 to get the tire's overall diameter, sidewall height, width, circumference and revolutions per mile, in inches and millimeters.
How to read a tire size
The size is molded into the sidewall and listed on the door-jamb placard. Take P265/70R17 as an example.
- P means a passenger car tire. LT means a light truck tire.
- 265 is the section width in millimeters, measured from sidewall to sidewall.
- 70 is the aspect ratio: the sidewall is 70% as tall as the tire is wide.
- R means radial construction.
- 17 is the wheel diameter in inches.
The numbers and letter after the size, such as 112S, are the load index and speed rating. They don't affect the tire's dimensions.
Many off-road and light truck tires use a flotation size instead, like 35x12.50R17. Those numbers are already in inches: 35 is the overall diameter, 12.50 is the section width and 17 is the wheel diameter. Flotation sizes give the diameter directly, so the calculator is set up for metric sizes.
How to calculate tire diameter
Sidewall height (mm) = width × aspect ratio ÷ 100
Overall diameter (in) = (2 × sidewall height + wheel diameter × 25.4) ÷ 25.4
Revolutions per mile = 63,360 ÷ (π × overall diameter)
Width is in millimeters and wheel diameter is in inches, so the wheel diameter is multiplied by 25.4 to put everything in millimeters before converting back. The tire has a sidewall above and below the wheel, which is why the sidewall height is doubled. 63,360 is the number of inches in a mile.
Example: 265/70R17
- Sidewall height is 265 × 70 ÷ 100 = 185.5 mm, or 7.30 inches.
- Overall diameter is (2 × 185.5 + 17 × 25.4) ÷ 25.4 = 31.61 inches.
- Circumference is π × 31.61 = 99.29 inches.
- Revolutions per mile are 63,360 ÷ 99.29 = 638.
Common tire sizes in inches
| Tire size | Diameter (in) | Width (in) | Sidewall (in) | Revs per mile |
|---|---|---|---|---|
| 225/45R17 | 25.0 | 8.9 | 3.99 | 808 |
| 245/40R18 | 25.7 | 9.6 | 3.86 | 784 |
| 225/65R17 | 28.5 | 8.9 | 5.76 | 707 |
| 235/75R15 | 28.9 | 9.3 | 6.94 | 698 |
| 245/75R16 | 30.5 | 9.6 | 7.23 | 662 |
| 265/70R17 | 31.6 | 10.4 | 7.30 | 638 |
| 275/65R18 | 32.1 | 10.8 | 7.04 | 629 |
| 285/70R17 | 32.7 | 11.2 | 7.85 | 617 |
| 285/75R16 | 32.8 | 11.2 | 8.42 | 614 |
| 275/60R20 | 33.0 | 10.8 | 6.50 | 611 |
| 295/70R18 | 34.3 | 11.6 | 8.13 | 589 |
| 315/70R17 | 34.4 | 12.4 | 8.68 | 587 |
Calculated from the nominal size with the formula above. Actual tires vary: BFGoodrich notes that sizes are average design values and a tire may differ by up to 3% of section height, which changes the overall diameter. Size markings follow Michelin's and Goodyear's tire size guides.
What bigger tires do to your speedometer and gearing
A taller tire covers more ground per turn. A speedometer set up for the original size reads low by the same percentage the diameter grew. Going from 265/70R17 (31.61 inches) to 285/70R17 (32.71 inches) is 3.5% taller, so when the speedometer shows 60 mph you're doing about 62.1.
The taller tire also acts like a numerically lower axle ratio, so the engine turns fewer RPM at a given speed and the truck feels slower off the line. Use the gear ratio calculator to find a ratio that brings the RPM back, and the vehicle speed calculator to check your real speed at a given RPM.
Frequently Asked Questions
What does 265/70R17 mean?
The tire is 265 millimeters wide, its sidewall is 70% as tall as it is wide, R means radial construction, and it fits a 17-inch wheel.
How do you calculate tire diameter?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall height in millimeters. Double it, add the wheel diameter times 25.4, and divide by 25.4 for inches. A 265/70R17 works out to 31.6 inches.
How do you convert a metric tire size to inches?
Calculate the overall diameter as above, then divide the width in millimeters by 25.4. A 285/70R17 is about 32.7 inches tall and 11.2 inches wide.
What do the numbers on a flotation tire like 35x12.50R17 mean?
They are in inches: 35 is the overall diameter, 12.50 is the section width and 17 is the wheel diameter.
How many revolutions per mile does a tire make?
Divide 63,360 by the tire's circumference in inches. A 265/70R17 calculates to about 638. A specific tire can measure slightly different, because actual tires vary a little from the nominal size.
How much do bigger tires throw off the speedometer?
By the same percentage the diameter grows. Going from 265/70R17 to 285/70R17 is 3.5% taller, so the speedometer shows 60 mph at about 62.1 mph.
