Tire Size Calculator
Calculate tire size or compare a tire to an alternate tire size by entering the parts of the tire code below. See the tire’s dimensions, such as diameter, width, sidewall, circumference, and revolutions per mile as a result.
Tire Dimensions and Specs:
On this page:
- Tire Size Calculator
- How to Calculate Tire Size
- How to Calculate Sidewall Height
- How to Calculate Tire Height (Diameter)
- How to Calculate Tire Circumference
- How to Calculate Revolutions Per Mile
- How to Read Tire Size
- Where to Find the Tire Size
- What Do the Tire Size Numbers Mean?
- Tire Size Chart
- Flotation Tires
How to Calculate Tire Size
You can find lots of good reasons to make a change in your tires. There are some drivers who prefer to switch for reasons of lower cost. Then, there are others who would like to improve the performance of their vehicles by upgrading to tires that give them an extra level of cornering and acceleration.
Whatever your reason, when you change tires on your vehicle to a new size, it is very important to calculate the size of your tires, to be sure that they will fit the vehicle and your wheels. You also need to know whether your new tires will cause any speedometer error so that you can correct it if necessary.
The key components of a tire’s identification code that indicate the tire’s size are the section width, the aspect ratio, and the rim or wheel diameter. Using these three values, you can use some simple formulas to calculate a tire’s sidewall height, its overall height (or diameter), its circumference, and the number of revolutions it will turn per mile (or kilometer).
How to Calculate Sidewall Height
The sidewall height is determined by the section width and the aspect ratio. The formula to find the height of the sidewall is:
sidewall = section width × aspect ratio / 100
To find the sidewall height, simply enter the tire’s section width and aspect ratio into the formula and solve. The result will be the sidewall measurement in millimeters; you can use a conversion calculator to convert this measurement to inches.
For example, let’s find the sidewall height of a 245/40R18 tire.
sidewall = 245mm × 40 / 100
sidewall = 245mm × .4
sidewall = 98mm
How to Calculate Tire Height (Diameter)
You can find the tire height/diameter using the sidewall height dimension from the previous step. The formula to calculate tire height/diameter is:
diameter = (2 × sidewall) + rim diameter
Thus, the tire height/diameter is equal to the sidewall height times two, added to the rim diameter.
For example, let’s find the tire height/diameter of the same 245/40R18 tire. Recall that the sidewall height for this tire from the previous sidewall height calculation is 98mm.
Start by converting the sidewall to inches (because the rim diameter is measured in inches). 98mm is equal to 3.86 inches.
diameter = (2 × 3.86″) + 18″
diameter = 7.72″ + 18″
diameter = 25.72″
How to Calculate Tire Circumference
The circumference of a tire, when compared to the circumference of the original tire, is required to calculate speedometer error. You can find the tire’s circumference using the diameter and a simple formula:
circumference = diameter × π
The tire circumference is equal to the diameter times pi. Pi is equal to roughly 3.14.
For example, let’s find the circumference of the same 245/40R18 tire using the 25.72″ diameter that we calculated above.
circumference = 25.72″ × 3.14
circumference = 80.8″
How to Calculate Revolutions Per Mile
Once you have calculated the tire circumference, you can find the tire’s revolutions per mile by using this formula:
rev/mile = 63,360 ÷ circumference
The number of tire revolutions per mile is equal to the number of inches in a mile (63,360) divided by the circumference of the tire in inches.
For example, let’s find the revolutions per mile of the same 245/40R18 tire. Remember that the tire circumference from the previous calculation is 80.8″.
rev/mile = 63,360 ÷ 80.8″
rev/mile = 784.16
How to Read Tire Size
If you need new tires, the first thing you need to know is the correct size tires for your vehicle. Tire size information can be confusing, so it may not be immediately clear what the numbers in a tire size mean, or where you can find the correct tire size for your vehicle.
Where to Find the Tire Size
At this point, you may be wondering where you can find the correct tire size for your vehicle. You can find it in two places:
1: Each tire has the size code printed or embossed in the sidewall of the tire. Note that while this is the actual tire installed on the vehicle, it could have been replaced in the past and may not be the suggested size tire for the vehicle.
2: A yellow or white “Tire and loading information” sticker should be visible on the inside of the driver’s door jamb. This will list the tire size(s) and pressures suggested by the manufacturer for the vehicle, but for the reason stated above, it may not match the size of the tires that are currently installed.
What Do the Tire Size Numbers Mean?
Every tire designed for a passenger vehicle comes with a standardized alphanumeric code that provides you with key information about that tire. Here’s what the letters and numbers in the tire size code mean:
This first letter in the code tells you what type of vehicle the tire is intended to be used on. P means passenger car, LT means light truck, ST means special trailer, and T stands for temporary (as in a temporary spare tire that many vehicles use).
The section width is the first numerical value. It is located before the slash (“/”) and is measured in millimeters. This is the width of the tire from its inner sidewall to its outer sidewall, not including any protective ribs, raised letters, or other decorative elements.
When changing the tire size on a vehicle, it’s also important to ensure that the inner suspension clearance and outer fender clearance are adequate with the new tires. Try our wheel offset calculator to find the changes in inner and outer clearance with your new set of tires.
The aspect ratio is the second numerical value and is located after the slash (“/”). Similar to the aspect ratio for a TV, this is the ratio between the section width of the tire (as described above) and the height of the tire itself. For example, if the tire has an aspect ratio of 65, it means that the height of the tire is 65% of its width.
This letter explains how the tire is constructed. R means radial ply (most passenger car tires are radials), D means diagonal ply, B means bias-belted, and the absence of a letter means that the tire is of cross-ply construction.
The rim or wheel diameter is the third numerical value. It is located after the R and is usually measured in inches. This is the size of the wheel that will fit the tire, and it is also the inside diameter of the tire. For instance, a P225/65R17 98H tire will fit a wheel with a 17″ diameter.
This is the last numerical value in the code and is expressed in either 2 or 3 digits. It normally ranges from 70 to 126. The load index number, when looked up in its table, lets you know the maximum weight that each tire can carry at a given inflation pressure.
This final letter in the tire sizing code tells you the highest sustained speed that the tire is capable of being run at, assuming that the tire is not damaged, is not overloaded, and is properly inflated. Here is a list of speed ratings for various types of tires. Be sure that the tires you select have a speed rating that is equal to or greater than the vehicle’s maximum speed capability.
Of course, you can use a calculator to compare two tires, but it’s a lot easier to use our handy tire size chart below.
Tire Size Chart
The tire size chart below shows common metric tire sizes, with their section widths and tire heights converted to inches.
|Section Width (Tread Width)
|Tire Height (Diameter)
While euro-metric and P-metric sizing are the most common, you may also see sizing using a different standard when looking for high-flotation or light truck tires for your vehicle. For instance, you might see a tire size in the format like this: 28.5x9R17LT 115R.
Flotation tire sizes are different because they are measured in inches rather than millimeters, with the numbers referring to different attributes of the tire. Here is what the parts of a flotation tire size 28.5x9R17LT 115R mean.
|Part of Tire Size
|Tire height/diameter in inches
|Section width in inches
|Rim diameter in inches
|LT vehicle type
You can use a conversion calculator to find the equivalent P-metric and flotation tire sizes.
- Tire Rack, What Is Load Index?, https://www.tirerack.com/upgrade-garage/what-is-load-index
- BFGoodrich Tires, Tire Speed Rating Chart, https://www.bfgoodrichtires.com/auto/learn/tires-101/tire-speed-rating-and-tread-life
- OK4WD, What are Flotation Tires?, https://ok4wd.com/blog/?p=what-are-flotation-tires