Tire Size Calculator
Calculate 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:
|Tire One||Tire Two||Difference|
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How to Read Tire Size
If you need new tires, the first thing you need to know is how to size them for your vehicle. But, tire sizes can be confusing, and it might not be immediately clear what the numbers in a tire size mean.
Today, most passenger vehicles use metric or P-metric tire size designations such as P225/65R17 98H. A P-metric size designation contains several key pieces of information about the tire.
The first letter in the tire designation is the vehicle type and denotes which type of designation the tire uses. A “P” indicates that the size is a P-metric size, but there are also euro metric and LT-metric types.
The first three numbers in the tire designation before the slash (“/”) denote the tire width in millimeters. This is the width from sidewall to sidewall and is also called the section width or tread width.
The two or three numbers immediately after the slash (“/”) indicate the aspect ratio of the tire. Similar to the aspect ratio for a TV, the aspect ratio is the ratio of the tire height to the width and is used to determine the height using a formula.
The letter following the aspect ratio indicates the tire construction. An “R” is the most common and denotes a radial construction, but some other types might be used, such as “D” for diagonal bias construction.
The next number denotes the rim diameter, or wheel size, in inches. For instance, a P225/65R17 98H tire requires a 17″ rim.
After the rim diameter, there will be a space then another number, which denotes the tire’s load index. The load index is a code that corresponds to a load rating for the tire.
The final letter in the tire designation is the speed rating and corresponds to the rating for the maximum speed the vehicle can travel when using the tires. Ensure the tires have a rating equal to or greater than the vehicle’s speed capability.
Of course, you can use a calculator to compare two tires, but keep reading to learn how to calculate the size of a tire.
How to Calculate Tire Size
When changing tires, it’s important to calculate tire size to ensure they will fit the vehicle and wheels, have the correct stance, and ensure the speedometer reading will be accurate.
The important components of a tire designation that indicate the tire’s size are the width, aspect ratio, and rim diameter. Using these three values, you can calculate a tire’s height (or diameter), circumference, sidewall, and revolutions per mile (or kilometer).
You can use some simple formulas to calculate tire size given the width, aspect ratio, and rim diameter.
How to Calculate Sidewall Height
The sidewall height is determined by the section width and the aspect ratio. The formula to find the sidewall is:
sidewall = section width × aspect ratio / 100
To find the sidewall, simply enter the section width and aspect ratio into the formula and solve. The result will be the sidewall measurement in millimeters; use a conversion calculator to convert to inches.
For example, let’s find the sidewall 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 using the sidewall dimension from the previous step. The formula to calculate diameter is:
diameter = (2 × sidewall) + rim diameter
Thus, the tire diameter is equal to the sidewall times two, plus the rim diameter.
For example, let’s find the diameter of a 245/40R18 tire. Recall that the sidewall for this tire is 98mm.
Start by converting the sidewall to 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 is required to calculate speedometer error. You can find the tire’s circumference using the diameter and a simple formula:
circumference = diameter × π
The circumference is equal to the diameter times pi. Pi is equal to roughly 3.14.
For example, let’s find the circumference of the 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 the tire circumference, it’s possible to find the revolutions per mile using this formula:
rev/mile = 63,360 ÷ circumference
The revolutions per mile are equal to the number of inches in a mile divided by the circumference in inches.
For example, let’s find the revolutions per mile of the 245/40R18 tire. Remember that the circumference is 80.8″.
rev/mile = 63,360 ÷ 80.8″
rev/mile = 784.16
Tire Size Chart
The tire size chart below shows common metric tire sizes and their height and width converted to inches.
|Tire Size||Section Width (Tread Width)||Tire Height (Diameter)|
While euro metric and P-metric sizing are the most common, you might also see sizing using a different standard when looking for high-flotation or light truck tires. For instance, you might see a tire size in the format 28.5x9R17LT 115R.
Flotation tire sizes are a little different in that they are measured in inches rather than millimeters and the numbers refer to different attributes of the tire. here’s what the parts of a flotation tire size 28.5x9R17LT 115R mean.
|Part of Tire Size||Meaning|
|28.5||tire height in inches|
|9||section width in inches|
|17||rim diameter in inches|
You can use a conversion calculator to find the equivalent P-metric and flotation tire sizes.
- General Tire, Tires 101: Making Some Sense of Metric and American Standard Tire Sizing, https://generaltire.com/our-world/latest-news/making-some-sense-of-metric-and-american-standard-tire-sizing
- Tires Plus, Tire Load Index Chart, https://www.tiresplus.com/tires/tire-buying-guide/tire-load-index-chart/
- 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/what-are-flotation-tires/