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;
What the tire code you see on your tire means; and
How to use our tire size calculator to compare tires.
So let the tremendous tire comparison begin!
How do tire sizes work?
You've probably seen this strange set of numbers and letters on your car's tires and wondered what they mean. It's called a tire code, and it specifies the size and type of tire. Getting properly sized parts for your vehicle is crucial, so make sure to visit the wheel offset calculator to get that right, too. Let's go through each part of the code and make you a tire code expert.
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; and
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.
If you found this calculator useful our fuel cost calculator may also be of interest to you.
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 multiply 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:
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:
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.
You will then see confirmation of the tire code.
As a result, you will see the tire diameter, sidewall height, circumference, and the number of revolutions completed per mile.
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.
✅ Now that you know your tire size, you can use it in the speedometer gear calculator to find the right gear for your vehicle, too!
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:
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:
When factory tires wear out or you just want to put on other wheels, the question arises: what tire sizes will fit my car? The fact is that each car is designed for a certain wheel diameter and tread width. Usually, this information is contained on the back of the gas tank cap or in the operating documents. If you deviate from these sizes by more than 2-3 percent, then gas mileage will increase, the speedometer will begin to lie, and in case of a large difference, driving can become simply dangerous.
But how to choose the right size tires if there are some incomprehensible numbers written on the profile? Do not measure them with a ruler, by God. It is for these purposes that this tire calculator was created. It allows you to determine the difference between tires in centimeters, inches and percentages. In particular, with the tire calculator you can calculate and compare tire diameter, tread width, profile height and circumference. Additionally, the calculator detects potential differences in speedometer readings, ground clearance changes, and differences in revolutions per kilometer (or mile).
Calculator displays visual differences in tire diameter, profile, clearance and width. On the right side, a dynamic wheel pattern is generated, with a dotted pattern and parameters. At the top is a visual representation of the old tire (original size), and at the bottom is your potential new tire. The picture is displayed in two projections: lateral and frontal. Both can be downloaded to a computer in png format. To do this, right-click on the image and select "Save As...".
How to use the online tire calculator?
Using the virtual tire calculator is very easy. There are drop-down boxes in the upper left corner of the application. In the top row, you need to select the size of your original factory tire (or just the tires that are on your car at the moment). You can simply look at these indicators on the tire profile (side surface). The first field is the tire width in millimeters. The second field is the ratio of the profile height to the tire width in percent. The third field is the disc diameter in inches.
In the second row, you need to enter the size of the new tires, i. e. those tires that you are going to buy or have already bought. After that, click on the green "Calculate" button. The tire calculator will instantly calculate the differences between tires and display them in a table. Namely: diameter, width, circumference and height of the tire profile, the number of revolutions per kilometer and changes in ground clearance. The first two columns of the table will display the parameters of old and new tires, and the third column will show the nominal and percentage difference between them. Our recommendation will be displayed at the very bottom of the table. If the difference in diameters exceeds 3%, then we strongly do not recommend installing such tires, as this can be dangerous.
At the very bottom, you can see two speedometers that show the difference between the displayed and actual speed when changing tires. You can enter other values in the left speedometer using the arrows or directly from the keyboard. Changes will instantly be displayed on the right. By default, the difference is calculated at a speed of 60 kilometers per hour.
If you need to calculate in inches , then simply click on the inscription "Inches" in the switch, which is located under the green button.
Tire diameter - what is it, how is it indicated, how to calculate
Car tires have a number of important parameters. The tire diameter is divided into landing and outer. Let's figure out what the parameters affect, how they are calculated and where you can see.
What is the diameter and radius of a tire
Most drivers confuse the radius and diameter of the tire. In practice, the radius does not apply, it is the distance from the center of the tire to its edge. Measuring the radius is difficult and necessary. The diameter is the distance between the opposite edges of the tire, measured along the line going through the center.
In practice, only the diameter is used, the parameter is easy to measure. It shows well the possibilities for installing a tire on a specific car.
Bore diameter - what does
mean
Most often, drivers encounter the inner diameter. Usually it is called - the landing diameter of the tire. It shows the permissible parameters of the rim on which the tire fits.
The parameter is indicated in the size on the sidewall of the tire. Let's take 205/65 R15 as an example. The diameter here comes immediately after the letter R, it is indicated in inches. Specifically, here it is 15 inches or 381 millimeters.
When choosing a tire, you should take into account that the bore diameter must exactly match the rim. If there is more, it will not work to mount the wheel. If the tire size is smaller than the disk, the tire is likely to be damaged during installation.
Tire outer diameter
Rubber also has an outer diameter. This is the distance between opposite points on the outer perimeter of the circle, in our case the wheel. Calculates the parameter as follows, take a tire with a size of 205/65 R15.
Calculate the profile height. We consider 65% of 205 mm, it turns out 133 mm.
Multiply height by 2, tire on both sides of rim, total height 266 mm.
Next, we consider the inner diameter, convert inches to millimeters, it turns out 381 millimeters.
We add the inner diameter and height, we get 647 millimeters.
Please note that the increased diameter will affect the speedometer reading. The odometer is calculated for a certain distance traveled by the wheel in one revolution, if it changes, the speed is measured incorrectly. Let's count on an example.
Let's take the same size and compare it with 205/80 R15. We need a diameter - 701 millimeters. First, we calculate the length of the circle. The formula used is P=π*d, where d is the diameter. We consider.
8X4.1R15
9R15
3x8.9R15
5R15
5X13.5R15
9x8.9R16
9x8.1R16
1x10R16
5R16
5
5R17
3X5.7R17
8x9.3R17
5R17
4X5.3R18
6x9.6R18
5x10.4R18
5R18
3x10.4R19
2x9.3R19
2x12.4R20
3X9.3R20
5R20
5R20
7X9.6R21
3x10.4R22
5R22
5x10.8R24
8x11.6R26
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.
0, wikimedia.org. Modified version, focusing on the tire size code.
