What Size is a 31 Inch Tire? A Comprehensive Guide

A 31-inch tire is typically categorized as a “29-inch” tire in the bicycle industry due to the standardized measurement system used. The actual diameter of a 31-inch tire is approximately 78.74 centimeters (31 inches). However, bicycle tires are often measured using the ISO 5775 standard, which classifies tires based on their bead seat diameter (BSD). The BSD for a 31-inch tire is 622 millimeters, which falls under the 29-inch category in the ISO 5775 system.

Understanding the Technical Specifications of a 31-Inch Tire

To provide a comprehensive guide on the technical specifications of a 31-inch tire, it is essential to understand the relevant measurements and terms:

Tire Diameter

The overall diameter of the tire, measured from one bead to the other, including the tread and sidewalls. For a 31-inch tire, this measurement is approximately 78.74 centimeters (31 inches).

Bead Seat Diameter (BSD)

The internal diameter of the tire, where it fits onto the rim. For a 31-inch tire, the BSD is 622 millimeters, which is the basis for its classification under the 29-inch category in the ISO 5775 system.

Tire Width

The width of the tire, measured from one outer edge of the tread to the other. This measurement varies depending on the specific tire model and design. Common tire widths for 31-inch tires (29-inch category) range from 1.9 inches (48 mm) to 2.6 inches (66 mm).

Tread Pattern

The design and pattern of the tire’s tread, which can significantly impact the tire’s performance, traction, and durability. 31-inch tires (29-inch category) are available with a variety of tread patterns, including knobby, semi-slick, and smooth, depending on the intended use (e.g., off-road, road, or mixed terrain).

Tire Pressure

The recommended air pressure for the tire, measured in pounds per square inch (PSI) or bars. Proper tire pressure is crucial for ensuring optimal performance, safety, and tire longevity. For 31-inch tires (29-inch category), the recommended pressure typically ranges from 30 PSI (2.1 bar) to 60 PSI (4.1 bar), depending on the tire’s construction, intended use, and rider weight.

Rim Compatibility

The required rim diameter and width for the tire. For a 31-inch tire with a BSD of 622 millimeters, the rim diameter should be 622 millimeters (29 inches), and the rim width should fall within the manufacturer’s recommended range, typically between 19 mm and 35 mm.

It is important to note that the 31-inch measurement is not a standard size in the bicycle industry, and using it to determine tire compatibility or specifications can lead to confusion. Instead, it is recommended to rely on the ISO 5775 standard and the tire’s BSD to ensure proper fitment and compatibility with rims and other components.

Factors to Consider When Choosing a 31-Inch Tire (29-Inch Category)

what size is a 31 inch tire a comprehensive guide

When selecting a 31-inch tire (29-inch category), there are several factors to consider:

  1. Intended Use: Determine the primary terrain and conditions in which the tire will be used, such as off-road, road, or mixed. This will help you choose the appropriate tread pattern and tire width.

  2. Rider Weight: Consider the rider’s weight, as it will affect the recommended tire pressure and the tire’s load-bearing capacity. Heavier riders may require higher tire pressures or a tire with a higher load rating.

  3. Rim Compatibility: Ensure that the tire’s BSD of 622 millimeters (29 inches) is compatible with the rim diameter and width of your bicycle.

  4. Tire Durability: Look for tires with a robust construction and high-quality materials to ensure long-lasting performance and resistance to punctures or wear.

  5. Tubeless Compatibility: If you prefer the benefits of tubeless tires, such as improved puncture resistance and lower rolling resistance, choose a tire that is specifically designed for tubeless use.

  6. Aesthetic Considerations: While not a technical specification, the tire’s appearance and color may also be a factor in your decision, especially if you want to match the overall aesthetic of your bicycle.

By considering these factors, you can select the most suitable 31-inch tire (29-inch category) for your specific needs and preferences, ensuring a comfortable and reliable riding experience.


In summary, a 31-inch tire is typically classified as a “29-inch” tire in the bicycle industry due to the standardized measurement system used. Understanding the technical specifications, such as tire diameter, bead seat diameter, tire width, tread pattern, tire pressure, and rim compatibility, is crucial when selecting the right 31-inch tire (29-inch category) for your bicycle. By considering the intended use, rider weight, and other factors, you can make an informed decision and ensure a safe and enjoyable riding experience.