Skip to Content

How Dirt Bike Tire Sizes Work: The Quick Rundown on Reading Tire Size

How Dirt Bike Tire Sizes Work: The Quick Rundown on Reading Tire Size

Dirt bicycle tires look the same except for the seemingly random numbers and letters etched into them. These numbers define the size of the tire, but they are often as confusing as a computer programming language. Manufacturers used to express tire sizes simply in inches, but today you need a guide to translate them for you.

Generally, modern dirt bike tires come as a sequence of three numbers. These numbers represent the diameter of the rim and the widest distance between the sidewalls along with the aspect ratio of the sidewall. They are usually expressed in millimeters.

Getting the right dirt bike tire can be the difference between a great ride and a few broken bones. As such, by reading further, you will learn how to decide which tire size would work best for your style of riding and your budget.

A Complete Guide to Dirt Bike Tire Sizes

Standardization is a sign of the times and for go reasons. Standards make everything easier to build and buy. You no longer need to understand multiple conflicting formulas when buying tires for our dirt bikes. However, that only works if you know and understand the standards.

Here’s a quick video explanation with more info and charts below:

Until recently, bike tire sizes were just the width and outer diameter in inches. Under this system, you could guess the tire size you needed based on a simple formula. For instance, a 500cc dirt bike required a 5-inch by 18-inch tire.

However, things changed when the industry switched to the current metric-based system. While still as simple to understand, it can often challenge even the experts.

Generally, bike tire sizes come as combinations of three semi-related numbers. They are supposed to reflect the same diameter and width values but in millimeters. However, the introduction of specialized rims that required specific wall thicknesses, rendered direct measurements useless.

Did you know you can get Dirt Bike Tires Here from Amazon now. Good tires that won’t brake the bank too.

What Do the Dirt Bike Tire Numbers Mean?

The current industry standard rates dirt bike tires by their width, aspect ratio, and rim diameter which must be easily readable along the side edge. As a result, you will find them listed consecutively, separated by a slash {/} and a dash {-}, on every tire you buy. Some examples include “120/80-19” and “110/90-19”.

Tire Width

The first number represents the width or the widest distance between the sidewalls, and it is usually expressed in millimeters (mm). For example, the widths for the examples above at 120mm and 1100 respectively. This value is a direct measure of the width. So, you can simply measure your old tires to get a good estimate for your new ones.

Tire Aspect Ratio

Separated from the Width by a slash, the second number, or tire aspect ratio, details the height of the sidewall. However, it is not a direct measurement. Instead, it is rated as a percentage of the width. For instance, the heights of the examples above are 80 percent of 120mm (or 96mm) and 90 percent 110 (or 99mm). As a result, larger aspect ratios usually represent taller tires.

Tire Rim Diameter

The third size number is the rim diameter in inches. Most dirt bike tire rims range from 18 to 21 inches with the numbers inversely proportional to the aspect ratio. For instance, high aspect ration tires often have 18-inch rims.

Other Tire Size Numbers

The above three numbers represent a tire’s true size, but they may not always be the only numbers listed on the sidewall. There are no standards for these extra numbers, which means they can vary widely between brands. Although, you will typically see the following common tire characteristics:

  • Date code
  • Speed rating
  • Tire model number
  • The direction of rotation will find that

Did you know you can get Dirt Bike Tires Here from Amazon now. Good tires that won’t brake the bank too.

What Size are my Dirt Bike Tires?

Knowing the size of your dirt bike’s tires is always a good thing. You need the info when buying new tires or repairing the old ones. Luckily, you will find determining the sizes of your bike tires simple and easy. Though, the age and condition of your tires will determine how easy it is.

Reading the Size of the Tire

Reading the size code is the easiest way to know the size of your dirt bike tires. The code should be etched or painted onto the sidewall of the tire arranged as mentioned above. You should be able to easily spot the width, aspect ratio, and rim diameter from the slash and dash that separate them.

From there, you can convert the values into inches as needed. Please note that your front and rear tires may have slightly different sizes.

Manually Measure the Tire Size

If you cannot read the size code for any reason, you can manually measure the size of your bike tires. You also only need a ruler and a calculator. Once you have them, you can start measuring your tire’s size by:

  1. Measure the edge width of the tire in millimeters. This is your tire’s width.
  2. Measure the length of a sidewall in millimeters
  3. Divide the edge width by the sidewall length.
  4. Multiply the result by 100 and round to the nearest whole number. This is the tire’s aspect ratio.
  5. Measure the inner diameter of the tire in inches. This is the rim diameter.

How to Choose the Right Dirt Bike Tire Size

You only need to know the size of your dirt bike tires when you are replacing them. This is because you always want the best-sized tires for your riding style. Tire size is directly related to bike performance, and anything else will give you a very comfortable ride.

Luckily, there are some simple tips you can follow when buying new bike tires that will ensure you get the right ones.

  • Check your bike’s user manual for recommended tire sizes
  • Ask the bike’s manufacturers for additional advice and tire size suggestions
  • Consider the optimum dirt bike tire compound available to you
  • Research the popular tire brand, model, and size used by local off-road racing groups


Dirt bike tires are easy to understand if you know what the numbers mean. Fortunately, they are direct measurements of the tires or closely related to a measurement. Thus, you can usually take the manufacturer’s word as is or you can measure the tires manually if you must know for certain.

Sharing is caring!