Tire Size Calculator

By Steven Wooding
Last updated: Aug 15, 2021

Using our tire size calculator, you can find the tire measurements of your car tire, such as its diameter, sidewall height, and circumference. It's also a tire size comparison calculator, allowing you to compare your current tire with the one you are thinking of buying. It tells you whether the new tire will be a good fit, and how it might affect your speed dial.

The text below also explains how tire sizes work, and what the tire code you see on your tire means. So let the tremendous tire comparison begin!

A wall of car tires
Photo by Imthaz Ahamed on Unsplash

How do tire sizes work?

You've probably seen these strange set of numbers and letters on your car's tires and wondered what exactly they mean. It's called a tire code, and it specifies the size and type of tire. Let's go through each part of the code and make you a tire code expert.

Tire code explained diagram
By F l a n k e r - Own work, CC BY 3.0, wikimedia.org. Modified version, focusing on the tire size code.

The first letter is optional, so you might not always see it. It gives information about the intended use or the class of vehicle the tire fits. Possible values are:

  • P: Passenger car
  • LT: Light Truck
  • ST: Special Trailer
  • T: Temporary (e.g., "space-saver" spare wheels)

Up next is a 3-digit number that indicates the width of the tire in millimeters, measured at the widest point of the sidewall (technically called the "nominal section width"). Expect the width of the tire actually in contact with the road to be smaller than this value.

Next is a forward-slash for character separation, then there's a 2- or 3-digit number representing the profile of the tire. This number is the ratio of sidewall height to the tire's width as a percentage (also called the aspect ratio). So, for example, if the value was 50%, the sidewall height would be half of the width. If it is not present, you can assume a profile ratio of 82%. If the number is more than 200, then it represents the tire's diameter in millimeters.

Following the profile ratio, you may see an optional letter indicating the type of tire construction. Radial (R) is now the most common and the standard design for car tires today, with over a 98% market share. Here is a summary of radial and some other constructions you may see:

  • R: radial - cords that make up the structure of the tire are at right angles to the direction of travel
  • D: diagonal (or cross-ply) - cords are at an angle diagonal to the direction of travel
  • B: bias belt - same as diagonal, but with an extra layer of reinforcing belts under the tread
  • if no letter is present, a diagonal tire construction is assumed

The wheel diameter is indicated by the final 1- or 2- digit number, in inches. You might also see the wheel (or rim) diameter given to an accuracy of half an inch. In some rare cases, the number is three digits long. In that case, it represents the wheel diameter in millimeters.

So that's the tire code and tire size explained.

How to calculate tire size?

Let's learn how to find tire sizes. We've got the tire code and the data required for us to calculate various tire measurements, such as the tire diameter and sidewall height.

To calculate the sidewall height, we multiple the profile ratio by the tire width (dividing by 100 as the ratio is a percentage). The formula is, therefore:

  • sidewall height = profile ratio * width / 100

So, for the tire code shown in the figure above, P215/65R15, you would calculate the sidewall height as:

  • sidewall height = 65 * 215 / 100 = 139.75 mm

Converting to inches (divide by 2.54 * 10), that's 5.5 inches. We know the diameter of the wheel is 15 inches, so the diameter of the tire is given by:

  • tire diameter = wheel diameter + (2 * sidewall height)

So, for our example, that's:

  • tire diameter = 15 + (2 * 5.5) = 26 inches

To calculate the circumference, we use the classic circle equation:

  • circumference = π * diameter

So, for our exemplary tire, the tire circumference is:

  • circumference = 3.1416 * 26 = 81.7 inches

Our tire size calculator can also tell you how many revolutions a tire completes in a mile (or kilometer). This value is found by converting a mile into inches and dividing the result by the tire's circumference:

  • revolutions number = 63360 / 81.7 = 775

How to use the tire size calculator?

Using our tire size calculator is easy:

  1. Enter the values for the tire's width, profile ratio, and wheel size diameter obtained from either the tire code or from measuring into the first three fields of the calculator. You may also specify the construction type of the tire, though it will not affect any tire's size dimensions.
  2. You will then see confirmation of the tire code.
  3. As a result, you will see the tire diameter, sidewall height, circumference, and the number of revolutions completed per mile.
  4. Don't forget to click on the units to switch to your preferred unit for each input and output.

Want to compare one tire with another one? Then select "Yes" when asked "Compare with another tire?". The same inputs and outputs will appear for the second tire.

At the end of the tire size conversion calculator, you will see the absolute difference in the diameters of the two tires and the corresponding percentage change. Note that if you are looking to put this second tire on your vehicle, the difference in tire diameter should be no more than 2.5% smaller or larger. If it is larger, the tire might foul the wheel arch of your car, or the tire may be too weighty for the car's suspension and shock absorbers. Too small a tire size may mean the vehicle's ground clearance is not practical (scrapping speed bumps) anymore.

Changing the tire diameter will also affect the speedometer reading when compared to the actual speed of the car. Fortunately, this is also a tire size speedometer calculator too! Enter a speed into the "Speedometer reading" row, and you see the actual speed displayed. This information could be important when keeping to speed limits. If it is a modern vehicle, you might be able to set the tire diameter size in the car's electronics system. Your garage should be able to help you out, depending on the make and model of your vehicle.

Example of tire size comparison

Let's take a more in-depth look at the tire comparison feature by comparing two tires with the tire codes 195/65R15 and 195/85R15. If you enter these values into the tire size calculator, we can check them out in more detail.

The diameter calculated for the first tire is 25 inches, and the second one is just over 28 inches in diameter. That's a difference of over 3 inches. You'll see that's a 12.3% difference - well over the recommended amount. The percentage change is calculated like this:

  • percentage change = 100 * absolute difference / first tire diameter

So, for this example, inputting the values gives:

  • percentage change = 100 * 3.07 / 25 = 12.3 %

This percentage change in tire diameter is also the difference between the speedometer reading and the actual speed of the car. For example, if the speedometer says 30 mph, the actual speed will be:

  • actual speed = 30 * (1 + (12.3 / 100)) = 33.69 mph
Steven Wooding
Profile ratio
Wheel diameter
Tire construction
Radial (R)
Tire Code
Tire diameter
Sidewall height
Tire comparison
Compare with another tire?
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