Tire Size Calculator (Free Online Comparison)

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How to Use the Tire Size Calculator?

This is a very simple tire size calculator that accepts section width, aspect ratio, and wheel/rim diameter (ex: 195/55 R16) and measures various tire dimensions and parameters such as the diameter of the tire, width of the sidewall, circumference of the tire, and number of revolutions per mile.

How to Read Tire Sidewall Numbers?

The average lifespan of car and light truck tires in the U.S. is 50,000 to 60,000 miles (approximately 80,000 to 95,000 km). Regardless of the mileage, it is recommended to replace car tires every 6 years, due to rubber degradation over time. When it’s time to buy new tires, the important thing you need to know is the correct size of the tire. “My car has 18” wheels. So, I can just go ahead and buy 18” tires. Right?”. Sadly, the answer isn’t that straightforward. The number “18-inch” in the previous scenario is just a part of the solution.

Where do we get the correct and proper information regarding tire size? There are a couple of places where you can get this data. The first place is the tire placard sticker on the driver-side doorjamb. If you have the owner’s manual, then look for tire specifications. You can find all the possible tire sizes, inflation pressure, and other relevant tires. There is a third and more intriguing way to get the tire size. It is the sidewall of your car’s tire.

Understanding Tire Code Markings

You might have never noticed but, every car tire will have some markings on its sidewall in the form of alphabets and numbers. These markings represent a coded-information corresponding to the type of the car, the width of the tire, height of the sidewall (not directly, but through aspect ratio), tire construction, wheel diameter (18” in the earlier scenario), and many more as recommended by the U.S. Department of Transport (DOT). Let us see an example of the markings on a car sidewall and decode their meaning.

Tire Class

The first letter (or sometimes two letters) of the code represents the class of the tire and its intended usage. As per the International Standards Organization (ISO), this part of the code can have the following 4 values.

• P (Passenger Car): Suitable for regular cars, SUVs, minivans, crossovers, and small (light-duty) pickup trucks.
• LT (Light Truck): Suitable for large SUVs, pickup trucks, and vans that carry heavy loads or tow trailers.
• ST (Special Trailer): Suitable for RVs, boat trailers, utility and travel trailers.
• T (Temporary): Suitable for spare tires with limited travel speed and distance.

If a passenger car tire begins with a “P” marking, then the standard they follow is P-Metric. This part of the code will not be present in most modern cars that follow the European Metric (Euro-Metric) standard.

For large trucks with “LT” tires, the standard is known as LT-Metric. The European counterpart of this is the Euro-Metric-Commercial (C-Type). Unlike LT markings that start at the beginning, the C-Type has a “C” marking after the wheel diameter (example: 235/65R16C).

Section Width

The next part of the codes consists of 3 numbers that indicate the nominal section width of the tire measured from sidewall to sidewall (measurement of widest point of the tire). The units of this measurement are millimeters (mm).

So, in the above example, the number “225” indicates the nominal section width of the tire is 225mm. This value usually depends on the wheel or rim on which we mount the tire (the wider the rim, the larger the section width). An interesting tidbit regarding the tire’s cross-section width is they have an increment of 10mm from the previous value. For instance, if one size is 245, then the next possible value is 255.

Aspect Ratio

After the section width, there will usually be a forward slash (/) but sometimes a hyphen (–). The next 2 (or very rarely 3) numbers following this represent the Aspect Ratio of the Tire. This is a very important parameter of a tire. It is the ratio of the height of the cross-section (from tread to bead) of the tire to the width of the cross-section (sidewall to sidewall). In other words, the aspect ratio of a tire indicates the height of the sidewall as a percentage of the section width.

For example, if the aspect ratio of the tire is 65, then it means the height of the sidewall is 65% of the section width. In the above example, we have the section width of 225mm and the aspect ratio is 55. So, the height of the sidewall measured from the wheel rim to the top of the tread is approximately 124mm.

The aspect ratio of a tire (or the height of the sidewall) determines the “profile” of the tire. A low-profile tire will have a smaller aspect ratio than a regular profile tire for the same width. This will have a huge impact on the performance of the tire as well as the drive.

Tires with higher aspect ratios will have taller profiles and they can provide a relatively soft ride. This is because such tires can deflect more when traveling over irregular surfaces or with heavy loads. Hence, large trucks and commercial vehicle tires will usually have a taller aspect ratio.

Performance tires usually have a low profile (lower aspect ratio). Such tires will have stiffer sidewalls and they eliminate sidewall flex at high speeds. This also improves the traction.

Sometimes tires will not have any aspect ratio numbers. In this case the number is 80.

Tire Construction

Next, we have a single letter that indicates the type of internal construction of the tire. This determines the stability of the tire. The possible values are:

• R – Radial Ply Construction
• D – Diagonal Bias Ply Construction (sometimes left blank)
• B – Belted Bias Ply Construction

A typical tire consists of tread (the exterior rubber portion), plies or cords (the reinforcement part of the tire), sidewalls (sides of the tire will slightly less reinforcement), inner liner (covering of casing), and bead strands (bands of steel wire).

Even though we have three types of tire constructions, the majority of the tires manufactured today (more than 98%) are of Radial type. Passenger cars, SUVs, four-wheel drive vehicles, large trucks, and almost all vehicles use radial tires.

In this type of tire, the plies of cords extend from bead to bead in a radial direction i.e., at a 90° angle to the axial direction of the wheel (the direction of rotation). Radial Tires have the advantage of minimal side deflection. In general, these tires are very durable and offer good traction, fuel economy, and braking.

Next, we have a slightly older form of tire construction; the Bias Ply Construction. In this type, the plies are laid alternatively in a crisscross fashion. The crossing angle of the plies varies from 30 to 38° (sometimes up to 45° or more). As a result, these tires are also known as diagonal, or cross-ply tires. This construction results in a strong sidewall and tread stability. Bias Ply or Diagonal Bias tires have sub-par traction and are often used as specialty off-road and rugged tires.

There is a third and less common type of tire construction known as the Belted Bias Ply. These types of tires have a very similar construction to that of the bias ply tires except that they have an additional layer of belt (sometimes two or more) under the tread that runs along the circumference of the tire. Depending on the type of material used for the belts, these tires are known as Belted Bias (with textile material) or Braced Tread Bias (with steel cords).

Wheel Diameter

The next part of the code is usually a 2-digit number (but also rarely a 1-digit number). This is the size of the wheel in inches or to be specific the diameter of the wheel for which the tire is designed. It is the longest distance between two points on the rim (if it is a perfect circle, then it is the diameter).

Whenever we say the term “16-inch” wheels or “18-inch” wheels, we usually mean the diameter of the wheel and not the tire. In the above example, the number after “R” is 19. This means that the tire is suitable for a wheel with a diameter of 19 inches.

Load Index

After the wheel diameter, the next 2 or 3 digits are the load ratings of the tire commonly known as the Load Index of the tire. This number indicates the maximum amount of weight or load that the tire can handle at the maximum rated tire pressure. The load index part of the tire markings is related to the weight of your car/truck and also its load-carrying capacity.

Load Index numbers start from 0 and go all the way to 150. In the case of passenger car tires, the load index is usually in the range of 75 to 100. The following table shows a complete set of possible load index codes and their corresponding load capacity.

In the above example, the number after the wheel diameter 19 is 99. If you look at the table, load index 99 means the load capacity of the tire is 1709 lbs. or approximately 775kg. In general, the higher the load index number, the greater its load-carrying capacity.

Speed Rating

The last crucial rating on a tire is the Speed Rating. This is usually a 1 or 2-letter code right after the load index. The speed rating of a tire is an indication of the maximum speed at which the tire can travel with the maximum possible load for prolonged periods.

Here’s a table with all the possible speed rating codes.

In the previous example, the speed rating of the tire is “H”. This means the tire can travel at a maximum speed of 210kmph while carrying a load of 775kg at the maximum rated air pressure.

Other Information

The markings that we discussed till now are more than sufficient to purchase a new tire for your car or truck. Apart from this data, tires might also have additional markings that provide even more information about the tire.

Among them, the important ones are the DOT (Department of Transport) Codes, especially for all tires that are sold in the U.S. These include the name of the company, manufacturing plant, tire size, manufacturer-specified characteristics, and date of production. Speaking of the production date, it is a 4-digit code where the first 2 digits indicate the week of the year while the next 2 digits correspond to the year of production.

For instance, if the tire has a code “1523”, then it means that the tire was manufactured in the 15^th week of the year 2023. Rubbers degrade over time and hence it is important to have a clear marking of the production date of the tire.

Some tires will have an “M&S” or “M+S” marking on the sidewall. This means the tire is suitable for mud and snow usage. In addition to this, tires might also have markings for tire wear, traction, and temperature.

Data You Need When Buying New Tires

If you are looking to replace your car tires, then you have to note the marking on the tire’s sidewall (or from the owner’s manual or car’s doorjamb) and give that information to the people at the tire shop (or use this information while making an online purchase). You need:

• Section Width (ex: 195)
• Aspect Ratio (ex: 55)
• Tire Construction (ex: R)
• Wheel Diameter (ex: 16)
• Load Index (ex: 88)
• Speed Rating (ex: V)

How to Calculate Tire Size and Dimensions?

Now that we have understood all the markings and their meanings of a tire, let us see how to use this information and calculate a tire’s size. From the tire’s sidewall, we have access to three parameters of a tire. They are:

• Section Width (SW) in mm
• Aspect Ratio (AR)
• Wheel Diameter (D)

We will take the previous example from the image: 225/55 R19

As the section width of the tire is in mm, you can divide this number by 25.4 to convert it into inches (1 inch = 25.4mm).

Now the first important measurement of a tire is Sidewall Height (SH) which defines its “profile”. We know that the Aspect Ratio of a tire defines how much percentage of the section width is its sidewall height. In this case, the aspect ratio is 55. This means the sidewall height is 55% of 225mm, which is 123.75mm (or 4.87 inches).

 Sidewall Height (SH) = SW * (AR/100) 

Next, we can calculate the Tire Diameter (TD) by adding the diameter of the wheel with two times the section height.

 Tire Diameter (TD) = D + (2×SH/25.4) 

In our example, TD = 19 + (2×123.75/25.4) = 28.74-inches (or 730mm)

With the diameter of the tire in our hands, we can easily calculate the circumference of the tire simply by multiplying the tire diameter with pi (π).

 Circumference of Tire (C) = π × TD 

This means the circumference of the tire in our example is π × 28.74 = 90.3 inches (or 229.3cm).

Last but not least, we can also calculate the number of revolutions the tire will take to go a kilometer or a mile. If we have the circumference (C) in meters, then the revolutions per kilometer are as follows:

 Revolutions Per Kilometer = 1,000/C 

In our example, we have a circumference of 229.3cm or 2.293m. So, the revolutions per kilometer is 1,000/2.293 ≈ 436.

We have provided an easy-to-use tire size calculator that takes the basic information such as the section width, aspect ratio, and wheel diameter and calculates all these tire dimensions for you.

Things to Consider When Changing Wheel or Tire

Changing a tire and changing a wheel are two completely different things. The wheel is the metal component (usually made from steel or aluminum alloy) that is connected to the axle of the car while the tire is the rubber part that fits around the wheel. It is possible to change both the wheel and tire of a car as long as you know what you are doing and have good reasons for going away from the default options.

Changing Tires

First of all, let us understand the concept of changing the tires of a car. Tires wear down over time, reducing their grip on the road. This wear compromises your ability to stop quickly and handle the car, especially in adverse weather conditions. Additionally, worn tires increase the risk of punctures and blowouts, which can lead to dangerous accidents. To avoid these risks, inspect your tires frequently and replace them when the tread wears thin or damage becomes evident.

If you are looking for a drop-in replacement for the tires that came with the car, then all you need to do is use the markings on the tire’s sidewall (section width, aspect ratio, wheel diameter, load index, and speed rating) and search for tires with similar ratings. If that particular tire size is popular, then you won’t have any problem finding replacements.

When changing tires, it is a good idea to change all tires at the same time to maintain balanced performance and safety. When you only replace one or two tires, the new tires will have deeper tread compared to the older ones. This discrepancy can cause uneven traction, especially in wet or slippery conditions. The vehicle may pull to one side, making it harder to control and increasing the risk of skidding. Vehicles with all-wheel drive (AWD) or four-wheel drive (4WD) rely on balanced tire wear to function correctly.

Moreover, different tire treads affect the vehicle’s suspension system. Uneven tire wear can place additional stress on the suspension, leading to premature wear and costly repairs. By changing all tires at once, you ensure even wear and consistent handling, which prolongs the life of your suspension components.

Changing Wheels

Coming to changing the wheels, the reasons can be as simple as aesthetics or to improve performance. The simplest reason to change your car wheels is to upgrade them from steel to alloy rims. Alloy wheels, made from aluminum or magnesium, are lighter and can improve performance. They also dissipate heat better than steel wheels, which helps during braking.

From an aesthetic point of view, you can change the wheel with a different diameter or with a different section width. If you want to upgrade the wheel diameter, you have to use low-profile tires so that the new setup fits inside the wheel arch.

You can also change the wheels with a wider section. Correspondingly, you have to fit tires with bigger section widths. The benefit of having wider tires is you can get better traction, handling, and braking while taking a toll on fuel efficiency.

Impact on Measurement and Safety

New tires can impact your vehicle’s speedometer and odometer readings. When you install new tires, especially if they differ in size from the originals, the diameter changes. A larger or smaller diameter alters the distance the tire covers per revolution, affecting the accuracy of speed and distance measurements. Consequently, your speedometer may show a different speed than your actual speed, and the odometer might record more or fewer miles than you travel.

Additionally, Anti-lock Braking System (ABS) and Electronic Stability Control (ESC) systems depend on precise tire data. These systems use sensors to detect wheel speed and wheel slippage and adjust braking and engine power to maintain control.

ABS sensors monitor the rotational speed of each wheel to prevent lockup during braking. If the tires have different diameters due to uneven wear or improper replacement, the sensors receive inconsistent data. This can impair the ABS function, compromising your ability to stop safely in emergencies.

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