What This Calculator Does

The Tire Size Calculator interprets standard tire codes like “P215/65R15” and turns them into meaningful measurements. It extracts important information such as section width, aspect ratio, internal construction type, and wheel diameter to compute the overall tire diameter and sidewall height. Additionally, you can compare two tire sizes side-by-side to evaluate the impact of switching tire dimensions, ensuring your speedometer and odometer stay accurate.

Our calculator follows the industry-standard formula:

• Section Height = (Aspect Ratio × Tire Width)

• Tire Diameter = Wheel Diameter + 2 × Section Height

Whether you’re a driver, a mechanic, a car enthusiast, or a student studying automotive technology, this tool offers clear and practical insight into tire specifications.

Decoding Your Tires: Understanding Tire Size and Codes

Cracking the Tire Code: What Do the Numbers and Letters Mean?

Car tires come with an alphanumeric tire code, kind of like a secret language that tells you a lot about the tire. Understanding this tire code is really helpful, especially when you’re thinking about getting new wheels or tires, as it’s essential for figuring out things like tire size. Each part of the code describes a specific characteristic of the tire. Let’s decode it:

Intended Use: Identifying the Vehicle Type

The very first part of the tire code is one or two letters (P, LT, ST, T) that tell you what kind of vehicle the tire is designed for. There are four main categories:

• P: Stands for passenger car. Tires with this “P” specification are made for cars, SUVs, crossovers, minivans, and smaller pickup trucks.
• LT: Stands for light truck. Tires marked “LT” are built to handle heavier loads and are capable of towing trailers.
• ST: Stands for special trailer. These tires are specifically for trailers like boat trailers, utility trailers, and travel trailers.
• T: Stands for temporary. Tires with a “T” are usually spare tires and aren’t meant for regular, everyday driving.

Nominal Section Width: Measuring Tire Breadth

The second part of the tire code is a three-digit number. This number tells you the nominal section width of the tire in millimeters (mm). Think of this as the tire’s widest point, measured from one sidewall to the other. For example, if you see “215,” it means the tire is 215 mm wide.

Aspect Ratio: Understanding Tire Profile

Next up, after the forward slash ( / ), is a two- or three-digit number. This is the aspect ratio, which shows the relationship between the tire’s section height and its width. The section height is how tall the tire is from the rim to the outer edge, and the section width is, as we just learned, the width from sidewall to sidewall. A tire code like “xxx/65” means the tire’s section height is 65% of its width. So, for a tire with the code 200/65, the section height would be 200 mm × 65% = 130 mm.

Internal Construction: Inside the Tire

The next part of the code is a single letter (B, D, R) that tells you about the tire’s internal construction. Inside every tire are layers of cord called the tire’s carcass. This internal structure is a big part of the tire’s strength and how long it lasts. This letter is sometimes left out of the code. If it’s not there, it’s generally assumed the tire is a cross-ply tire.

• R: Stands for radial. Most modern passenger cars use radial tires. In this design, the cords run across the tire from one side to the other, perpendicular to the direction the tire rolls (or its axis of rotation).
• D: Stands for diagonal. In this type of construction, the cords also run across the tire, but at a 30-40° angle from one bead (the part that touches the wheel rim) to the other, with the layers crisscrossing.
• B: Indicates a bias-belted tire. These tires are similar to diagonal tires but have an extra layer above the main plies. This layer consists of stabilizer belts, often made of steel or other strong materials, applied at different angles to the main plies. This adds support and makes the tire stiffer.

Cross-ply tires have an internal structure made of nylon cords placed diagonally across each other, usually at about a 55° angle.

Diameter of the Wheel: Fitting the Rim

The next part of the code is a one- or two-digit number that tells you the diameter of the wheel (rim), in inches, that the tire is designed to fit. There are many different tire sizes, and different tire sizes can fit the same wheel as long as the center of the tire is the correct size for the rim.

Load Index: How Much Weight Can It Carry?

Following the wheel diameter is a two- or three-digit number called the load index. This number indicates the maximum weight a single tire can safely carry when it’s inflated to a specific pressure. For example, a load index of 95 means the tire can carry 1,520 pounds (690 kg) at a tire pressure of 42 psi.

Speed Rating: How Fast Can You Go?

The very last part of the tire code is a one- or two-digit/letter that represents the speed rating of the tire. This rating is the maximum speed at which the tire can carry a load corresponding to its load index. For instance, a tire with a load index of 95 and a speed rating of “H” can carry 1,520 pounds (690 kg) at 42 psi and a top speed of 130 mph (210 km/h).

Keep in mind that tires often have other markings and codes, such as the DOT code.

DOT Code: Tracking Tire Production

The DOT code is a required marking in the United States that identifies the tire company, factory, batch, mold, and date of production. The production date is particularly important and is shown with two digits for the week of the year and two digits for the year. For example, a code like “1324” means the tire was made in the 13th week of the year 2024. When buying new tires, it’s a good idea to check this date. Due to the aging of the rubber and other materials, tires made more than 10 years ago shouldn’t be used, no matter how good they look. Some manufacturers even recommend replacing tires every six years, regardless of mileage.

How to Calculate Tire Size

Our calculator uses these simple formulas to figure out tire size:

• Section Height = Aspect Ratio × Tire Width
• Tire Diameter = Wheel Diameter + 2 × Section Height

The aspect ratio and tire width are taken directly from the tire code. The tire width is the three-digit number before the slash ( / ) and is in millimeters. The aspect ratio is the two- or three-digit number right after the slash. Multiplying these gives you the section height.

Once you have the section height, you can calculate the tire diameter using the section height and the wheel diameter. The wheel diameter is the one- or two-digit number that comes after the letter indicating the tire’s internal construction. If the internal construction letter isn’t there, the wheel diameter follows the aspect ratio. Remember that wheel diameter is given in inches. The tire diameter is the wheel diameter plus twice the section height.

Things to Think About When Changing Tire or Wheel Size

People change their tire and wheel size for various reasons, often for looks. However, when you change the size, it’s important to consider a few things. If you don’t, it can lead to problems like increased wear on your vehicle’s suspension and brake parts (especially anti-lock brakes), which can affect handling or cause other mechanical issues with the car’s transmission.

While different tire sizes can be used on the same rim as long as the center of the tire fits correctly, a common change is to get bigger rims. When you increase the size of your car’s rims, you usually need to use thinner tires because the overall tire needs to be small enough to fit inside the wheel well. Generally, it’s recommended to choose tires with a diameter that’s within 3% of the factory-recommended tire size.

Besides changing tires for appearance, people also replace them due to blowouts or wear. When replacing tires, it’s best to change all four at the same time. If that’s not possible, at least change the front two or the back two together, depending on which tire needs replacing. If you’re only changing one or some tires, it’s highly recommended that the new tire(s) be the same type and brand as the others. Differences in tires can cause problems like loss of control or spinouts while driving, inaccurate speedometer readings, or even damage to the vehicle’s stability control system.

Changing tires also affects your car’s speedometer and odometer because they are calibrated based on the original tire size. So, while it’s usually safe to use different-sized rims and tires, it will change how your car behaves. Larger rims and tires increase the total circumference of the wheel, meaning the tire travels a greater distance with each revolution. This results in a speedometer that reads lower than your actual speed and an odometer that records a lower distance than you’ve actually traveled. Conversely, smaller tires will make your speedometer read higher than your actual speed and your odometer record a higher distance.

Make Tire Decisions with Confidence

With so many variables involved in tire selection, it’s easy to make mistakes. The Tire Size Calculator at BestCalculator.io simplifies the process. It gives you clarity on dimensions, safety thresholds, and fitment compatibility — so you can avoid costly errors, reduce wear and tear, and maintain your vehicle’s performance.

Whether you’re upgrading your ride, replacing a damaged tire, or just learning more about tire codes for a school project, this calculator is the smart, accurate, and free solution you’ve been looking for.

Start calculating now and drive smarter!