Bicycle Tire Size Conversion and Comparison Chart

The numbers on a bicycle tire sidewall indicate its size in one of several measurement systems. The most reliable is the ETRTO (European Tyre and Rim Technical Organisation) size, which lists the tire width and the bead seat diameter in millimetres, such as 37-622. The traditional inch system (e.g., 26×1.95) and the French system (e.g., 700×35C) are still common but less precise. Converting between these systems requires knowing that the bead seat diameter (BSD) is the critical dimension for fit, while the nominal diameter (like 26″ or 700C) is an approximate overall wheel size.

What Do the Numbers on a Bicycle Tire Mean?

Tire sidewall markings are a combination of width, construction, and rim diameter. For example, a tire marked 700×25C means a nominal outer diameter of 700 mm, a width of 25 mm, and a rim code C (which corresponds to a 622 mm bead seat diameter). The ETRTO marking for the same tire would be 25-622. Similarly, a tire marked 26×1.95 uses the inch system: the wheel is nominally 26 inches in diameter, and the tire is 1.95 inches wide. Its ETRTO size is typically 47-559 (width 47 mm, BSD 559 mm).

ETRTO Size Markings (ISO Sizing)

The ETRTO system is the international standard and is unambiguous. It consists of two numbers separated by a dash: the first is the tire width in millimetres, and the second is the bead seat diameter (BSD) in millimetres. The BSD is the diameter of the rim shelf where the tire bead sits. This dimension must match exactly for the tire to fit the rim. For instance, a tire marked 32-622 is 32 mm wide and fits a rim with a 622 mm BSD – the same rim used by 700C and 29″ tires.

French Tire Sizes

French sizes are expressed as an outer diameter in millimetres, a width, and a letter code for the rim diameter. Common examples are 700C, 650B, and 650C. The letter historically indicated the rim width but now corresponds to a specific BSD: 700C = 622 mm, 650B = 584 mm, 650C = 571 mm. So, a tire marked 700×38C is equivalent to ETRTO 38-622. Note that 700C does not mean the wheel is 700 mm in diameter; that is the nominal size with a tire fitted, and actual diameter varies with tire width.

Inch Tire Sizes

Inch sizes are common on mountain bikes, cruisers, and older bikes. They are written as wheel diameter × tire width, such as 26×2.1 or 27.5×2.2. The wheel diameter is nominal, not exact, and the width is usually decimal (2.1 inches). The actual BSD for a 26″ tire can be 559 mm (most MTB), 571 mm (650C), or 597 mm (older Schwinn), which is why the ETRTO number is essential. A 26×1.95 tire typically has a BSD of 559 mm, equivalent to ETRTO 47-559.

Bead Seat Diameter

The bead seat diameter (BSD) is the only dimension that must match between tire and rim. It is the diameter of the rim at the point where the tire’s bead sits, measured in millimetres. Because different nominal systems can share the same BSD (e.g., 700C, 29″, and 28″ all use 622 mm), checking the ETRTO number is the safest way to ensure compatibility.

How to Read Bicycle Tire Size Markings

1. Find the size on the sidewall. It is usually printed near the tire name and may include multiple systems.
2. Identify the measuring system. Look for patterns: a two-number dash (e.g., 37-622) is ETRTO; a number with “×” and a letter (700×35C) is French; a number with “×” and a decimal or fraction (26×1.95) is inch.
3. Note the width. Width is given in mm (ETRTO, French) or inches (inch system). It affects clearance and ride comfort.
4. Note the bead seat diameter. In ETRTO, it is the second number. In French or inch markings, you may need to convert to BSD using a chart.
5. Check for additional codes. Some tires include load index, speed rating, or tubeless compatibility markings, but these are less common on bicycle tires.

Bicycle Tire Size Conversion Chart

The following table lists common tire sizes across the three systems. The ETRTO size is the definitive reference.

Note: This chart is a simplification; always verify the ETRTO number on your existing tire or rim.

Bicycle Tire Size Comparison: Width, Diameter, and Circumference

Two tires with the same bead seat diameter can have different widths, which changes the overall diameter and circumference. The nominal wheel diameter (like 26″ or 700C) is based on a standard tire width, so actual dimensions vary.

How Width Affects Sidewall Height and Overall Diameter

The tire’s sidewall height is roughly equal to its width for most bicycle tires (the aspect ratio is approximately 100%). Therefore, a wider tire is also taller. The overall diameter is the bead seat diameter plus twice the sidewall height.

Example: For a 32-622 tire, the BSD is 622 mm and the width (and approximate sidewall height) is 32 mm. The overall diameter is approximately 622 + 2×32 = 686 mm. For a 50-622 tire, the overall diameter is about 622 + 2×50 = 722 mm. That’s a 36 mm difference, which affects frame clearance and speedometer accuracy.

Calculating Tire Circumference

Circumference = π × overall diameter. Using the approximate overall diameter from above:

• 32-622: diameter ≈ 686 mm, circumference ≈ 2155 mm.
• 50-622: diameter ≈ 722 mm, circumference ≈ 2268 mm.

The difference is about 113 mm per wheel revolution, which is significant for bike computers that use wheel circumference to calculate speed and distance.

Bicycle Tire Width Conversion Chart

Widths are often given in both millimetres and inches. Here are common conversions:

Bicycle Wheel Size Comparison: What Fits Your Bike?

Wheel size is determined by the bead seat diameter, not the nominal size. Common BSD values and their associated names:

• 622 mm – Known as 700C (road, gravel, hybrid), 29″ (mountain), and 28″ (European city bikes). All tires with this BSD will fit the same rim, but frame clearance may limit width.
• 584 mm – Known as 650B or 27.5″. Used for smaller road bikes, gravel, and some mountain bikes.
• 559 mm – Known as 26″ (decimal, e.g., 26×2.0). Classic MTB size, still common.
• 571 mm – Known as 650C, used for smaller road bikes and some triathlon bikes.
• 590 mm – Known as 26×1⅜ (fractional, e.g., 26×1⅜), used on older city bikes.
• 507 mm – Known as 24″ (decimal), used for kids’ mountain bikes.
• 406 mm – Known as 20″ (decimal, e.g., 20×1.75), used for BMX, folders, recumbents.
• 451 mm – Known as 20″ (fractional, e.g., 20×1⅛), used for some BMX and recumbents.

Because two different nominal sizes can share the same BSD (e.g., 700C and 29″ both use 622 mm), a tire labeled 29×2.2 will fit the same rim as a 700×55C tire (if the frame has clearance). However, a 26×1.95 tire will not fit a 700C rim because its BSD is 559 mm vs 622 mm.

Nominal Wheel Diameter vs. Actual

The nominal wheel diameter is a rounded, traditional size, not a true measurement. For example, a 26″ mountain bike wheel has a BSD of 559 mm (about 22.0 inches), but with a 2.0″ tire, the overall diameter is about 26 inches. A 27.5″ wheel has a BSD of 584 mm (23.0 inches), which with a 2.0″ tire gives about 27.5 inches overall. The actual diameter depends on tire width and tread.

Compatibility and Fitment Considerations

When choosing a new tire, check three things:

1. Bead seat diameter must match exactly. The ETRTO second number must be identical to your rim’s BSD.
2. Width must be within the rim’s acceptable range. Very narrow tires on wide rims or vice versa can be unsafe. Rim manufacturers specify a range of compatible widths.
3. Frame and fork clearance. A wider/taller tire may rub against the frame, fork, or brakes. Check the maximum tire size stated by the bike manufacturer.

Performance factors: Wider tires generally provide more comfort and grip but have more rolling resistance (though modern tire design has narrowed this gap). Narrower tires are lighter and more aerodynamic, preferred for racing. Always ensure the tire is inflated to the recommended pressure range printed on the sidewall.