Bicycle Fractional vs Decimal Tire Sizes: Are They Interchangeable?

Fractional and decimal tire sizes are generally not interchangeable, even if the numbers look similar. A tire marked 26 x 1 3/8 (fractional) fits a different rim than a tire marked 26 x 1.375 (decimal) because they have different bead seat diameters. The fractional size is designed for a rim with a bead seat diameter of 590 mm, while the decimal size fits a 559 mm rim. This means you cannot safely mount one on the rim designed for the other. To confirm compatibility, always check the ISO/ETRTO size marking (e.g., 37-590 vs 37-559) on the tire sidewall: the second number is the critical bead seat diameter.

What Do the Numbers on a Bicycle Tire Sidewall Mean?

Bicycle tire sidewalls typically show two sets of numbers: a traditional inch-based marking (fractional or decimal) and an ISO/ETRTO marking. The inch marking often looks like 26 x 1 3/8 or 26 x 1.375, where the first number is the approximate outside diameter of the tire in inches, and the second number is the tire width in inches (fractional or decimal). The ISO marking, such as 37-590 or 37-559, gives the tire width in millimeters first (37 mm), followed by the bead seat diameter in millimeters (590 mm or 559 mm). The bead seat diameter is the diameter of the rim where the tire bead sits, and it is the key dimension for determining whether a tire fits a rim.

Fractional Inch Sizing vs Decimal Inch Sizing

Fractional and decimal inch sizing are two different traditional systems that predate the unified ISO/ETRTO standard. They use different bead seat diameters for the same nominal tire diameter, which creates confusion.

Fractional Inch Sizing

Fractional sizes express the tire width as a common fraction, such as 26 x 1 3/8 or 27 x 1 1/4. These sizes originated with older British and European bicycles. For example, a 26 x 1 3/8 tire (ISO 37-590) has a bead seat diameter of 590 mm and is found on many classic English three-speeds and older utility bikes. A 27 x 1 1/4 tire (ISO 32-630) has a bead seat diameter of 630 mm, common on older road bikes.

Decimal Inch Sizing

Decimal sizes use a decimal point for the width, such as 26 x 1.375 or 26 x 1.75. These are typically American in origin and correspond to different bead seat diameters. For example, a 26 x 1.375 tire (ISO 37-559) fits a 559 mm rim, which is the standard mountain bike and cruiser size. A 26 x 1.75 tire (ISO 47-559) also fits a 559 mm rim but is wider.

The Crucial Difference: Bead Seat Diameter

The bead seat diameter (BSD) is the diameter of the rim shelf where the tire's bead sits. It is the most important dimension for tire-rim compatibility. Tires with the same BSD can be mounted on the same rim, provided the width is within a reasonable range. Tires with different BSDs are completely incompatible, even if their nominal outside diameters are similar.

In the example above, 26 x 1 3/8 has a BSD of 590 mm, while 26 x 1.375 has a BSD of 559 mm. The difference of 31 mm (about 1.22 inches) means the fractional tire will be too loose on a decimal rim and too tight on a fractional rim. Attempting to mount the wrong tire can damage the tire, rim, or cause a blowout.

How to Read ISO/ETRTO Size Markings

The ISO/ETRTO system uses a two-number format: [tire width in mm] - [bead seat diameter in mm]. For example:

• 37-622: 37 mm wide, fits a 622 mm rim (often called 700c or 28 inch).
• 32-630: 32 mm wide, fits a 630 mm rim (27 x 1 1/4).
• 47-559: 47 mm wide, fits a 559 mm rim (26 x 1.75 decimal).
• 37-590: 37 mm wide, fits a 590 mm rim (26 x 1 3/8 fractional).

Always look for the ISO marking on the tire sidewall. It is the most reliable way to identify compatibility.

Nominal Tire Diameter vs Actual Diameter

The first number in the inch marking (e.g., 26) is a nominal tire diameter, not an exact measurement. The actual outside diameter of a mounted and inflated tire depends on the rim width and tire design. For example, a 26 x 1.375 tire on a standard rim typically measures about 26 inches in diameter, but a 26 x 2.2 tire may measure over 27 inches. The nominal diameter is a class name, not a precise dimension.

How Width, Aspect Ratio, and Rim Diameter Affect Geometry

For bicycle tires, the aspect ratio (sidewall height to width) is not usually specified, but the width and rim diameter determine the overall diameter and circumference.

Calculating Approximate Overall Diameter

The overall diameter of a tire can be approximated as:

Overall Diameter = Bead Seat Diameter + 2 × (Tire Width × Aspect Ratio)

For most bicycle tires, the sidewall height is roughly equal to the tire width (aspect ratio ~1). So, a simpler approximation is:

Overall Diameter ≈ Bead Seat Diameter + 2 × Tire Width

For example, a 37-622 tire (37 mm wide, 622 mm BSD):

Overall Diameter ≈ 622 mm + 2 × 37 mm = 696 mm (about 27.4 inches).

For a 47-559 tire (47 mm wide, 559 mm BSD):

Overall Diameter ≈ 559 mm + 2 × 47 mm = 653 mm (about 25.7 inches).

These are estimates; actual dimensions vary with rim width and tire design.

Circumference and Speed Calculation

Circumference = π × Overall Diameter. Using the above approximations:

• 37-622: circumference ≈ 3.1416 × 696 mm = 2186 mm (2.186 m).
• 47-559: circumference ≈ 3.1416 × 653 mm = 2051 mm (2.051 m).

A difference in circumference affects speedometer calibration and gear calculations, but for tire interchangeability, only the bead seat diameter matters.

Tire and Rim Compatibility: What to Check

To determine if a tire fits a rim, follow these steps:

1. Check the bead seat diameter (BSD). The tire's BSD must exactly match the rim's BSD. Look for the ISO marking on both tire and rim, or measure the rim's bead seat if unmarked.
2. Check the tire width range. Rims have a recommended tire width range based on internal rim width. Using a tire too narrow or too wide can cause poor handling or tire damage. Consult a tire/rim compatibility chart.
3. Ignore the traditional inch marking for fitting. Never assume that 26 x 1 3/8 will fit a 26 x 1.375 rim. Always confirm the BSD.

Comparison Table: Fractional vs Decimal Sizes

As shown, the bead seat diameter varies significantly between fractional and decimal sizes, even when the nominal diameter is the same.

Conclusion

Fractional and decimal bicycle tire sizes are not interchangeable because they often correspond to different bead seat diameters. Always rely on the ISO/ETRTO marking (width-BSD) to ensure a correct fit. Ignoring this can lead to unsafe riding conditions. When in doubt, consult a bicycle mechanic or use a tire/rim compatibility chart.