Tire Size Calculator

Wheel Size
Diameter. (mm)
Tire Size
Diameter (MM)
Estimated Wheel Circumference/Computer Setting
Millimeter (MM)
Centimeter (CM)
Inches to MM Converter

About Tire Size Calculator

What is tire size measured in?

Your car's entire performance and safety depend on the installation of the proper tyres. Your choice of tyres should be informed by the road conditions you encounter as well as your preferences for the handling and reaction of your car. But when it comes time to purchase tyres, you must be able to read tyre size in order to make the best choice. Fortunately, figuring out which tyre size is best for your car is not too difficult. Either in your owner's manual or on the vehicle itself, you can get all the information you require right away.

Tire Type 

The service or kind of vehicle for which a tyre was manufactured is referred to as the tyre type. Passenger tyres, sometimes known as "P" tyres, are the most popular kind of tyre. You may also encounter the notation "LT" (light trucks that carry high loads) when referring to a particular tyre type. Other tyre classifications include "C," "T," and "ST," which stand for trailer tyres (commercial). If there isn't a letter, the tyre was likely made in Europe for cars made in that continent. These tyres are known as metric or Euro-metric tyres and resemble passenger tyres in many aspects.

Tire Width 

Is the tire's breadth, measured from sidewall to sidewall in millimetres. The tyre width is indicated by the first three digits of the tyre size. For instance, the width of a tyre with the size P215/65 R15 is 215 millimetres.

Aspect Ratio

The two-digit aspect ratio is separated from the tyre width value by a forward slash. The sidewall, or "profile," of the tyre will be higher or taller the larger the aspect ratio. The aspect ratio is shown as a percentage on the tyre sidewall. It is the sidewall height represented as a percentage of tyre width, calculated from the wheel rim to the top of the tread. The aspect ratio in this illustration is 65, which means that the sidewall is 65 percent as tall as the tyre is wide. The sidewall height is determined by converting the 215 mm tyre width to inches (8.46). After that, double this by 65%. (.65). The result is 5.5, which is the sidewall height in inches.

Construction Type 

This is referring to the design of the tyre. Since radial tyres are the most popular form of tyre made these days, you'll typically only see "R" for "radial tyre." Radial tyres have a smoother ride, a longer tread life, and use less fuel since they are built by joining piles perpendicular to the direction they move.

Wheel Size or Rim Diameter 

It's critical to understand the tire's intended wheel size. The wheel or rim diameter is represented by the last digit in the series. This diagram illustrates the appropriate wheel size for the tyre. The rim's diameter code, expressed in inches, is the following number, which is where the tyre can be placed. A tyre with the dimensions P225/70R16 91S, for instance, would fit a rim with a 16-inch diameter. If you choose to alter the size of your rims, you will also need to purchase new tyres that accommodate the size adjustment. New 18-inch rims will not expand to suit tyres made to fit a 16-inch wheel diameter.

Load Index 

When a tyre is fully inflated, the load index is the number used to indicate the maximum weight that it can support. The more weight you can place on the tyres, the greater the load index rating of the tyre. However, merely by glancing at this figure, you cannot immediately determine how much weight it supports. To determine how your load index rating relates to the weight the tyre can support, you need a tyre load index rating chart. A load index of 81, for instance, indicates that the tyre can support up to 1,019 pounds. The normal vehicle has a load index rating of 75 to 100, although some tyres have ratings as high as 150, or 7,385 pounds.

Speed Rating 

Another number that requires referencing with the aid of a chart is the speed rating, also known as the maximum speed rating. This number pertains to the maximum speed at which the tyre can be used. Higher speed rated tyres offer better heat handling and improved control at higher speeds. Family sedan, SUV, and van tyres most frequently have speed classifications of S and T, meaning 112 or 118 mph. H (130 mph) is frequently used for sports cars, coupes, SUVs, and passenger automobiles. N, P, Q, or R are used by other light trucks (87, 93, 99 and 106 mph).

DOT Symbol

The DOT logo should be visible on the sidewall of every tyre used in the US. The Department of Transportation is referred to as the DOT. You can tell if a tyre conforms with federal motor vehicle safety standards by looking for this mark. If you are purchasing a top-tier brand, there is no need to be concerned because every dependable manufacturer employs the DOT mark.


Your tire's serial number functions similarly to your car's identification number. You can find out everything you need to know about a tyre by looking up its Tire Identification Number (TIN). The DOT letters are followed by this number. To identify the most crucial features of the tyre, the TIN might range from eight to thirteen letters or numbers. It will not only show the manufacturer's code and the date the tyre was built, but also where the tyre was made.


The UTQG, or Uniform Tire Quality Grading, is thus named. The Department of Transportation developed this grading system, which gives you the information you need to buy the best tyres based on temperature capacity, traction, and treadwear. The traction grade displays the tire's wet traction performance under controlled testing conditions. When driving on slick roads, the tyre with an AA rating offers the optimum traction.

A quick way to determine whether the tyre size you're thinking about will fit your car, SUV, sports car, light truck, or crossover is to use a tyre size calculator. But keep in mind that's just an estimate. It's crucial to adhere to your vehicle's size tolerances. Incorrectly sized tyres may tug on the steering wheel, rub against your car's body or suspension, restrict hill clearance, or make the ride stiffer or noisier.