Bicycle Inch Size Lookup
Sort out which 26-inch or 27-inch size you have. Use the calculator below. Results update as you type.
How the Bicycle Inch Size Lookup Works
This lookup works by listing every bead seat diameter that an inch label can mean and pointing to the most likely one from the marking style. Decimal and fraction widths point to different BSDs.
- Enter the inch label, for example 26 x 1-3/8.
- Read the possible BSDs: 559, 571, 584, 590, 597.
- Read the match for the fraction marking: 590 mm (650A).
- Confirm by reading the ISO number on the old tire.
The Formula
The formula applies 2 rules: decimal widths map to the mountain-bike BSD, and fraction widths map to a table.
Decimal (26 x 2.1) → BSD 559 Fraction (26 x 1-3/8) → BSD 590 Fit = BSD_tire = BSD_rimSheldon Brown's rule of tire sizing states that a decimal and a fraction size that are mathematically equal are not interchangeable. Worked example: 26 x 1.75 maps to 559 mm, and 26 x 1-3/4 maps to 571 mm.
Conversion Table
The conversion table lists 7 inch labels and every BSD each label can mean. The 26 in label covers 5 BSDs, the most of any label.
The last column explains which marking style points to which BSD.
| Inch Label | Possible BSD (mm) | Most Common BSD | Marking Style That Points to It |
|---|---|---|---|
| 20 in | 406, 451 | 406 | Decimal (20 x 1.95) = 406; fraction (20 x 1-1/8) = 451 |
| 24 in | 507, 520, 540, 547 | 507 | Decimal = 507 |
| 26 in | 559, 571, 584, 590, 597 | 559 | Decimal = 559; fractions vary |
| 27 in | 630 | 630 | 27 x 1-1/4 |
| 27.5 in | 584 | 584 | Decimal MTB sizes |
| 28 in | 622, 635 | 622 | 28 x 1-1/2 can be 635 |
| 29 in | 622 | 622 | Decimal MTB sizes |
What Is the Bicycle Inch Size Lookup?
The Bicycle Inch Size Lookup is a tool that resolves ambiguous inch bicycle tire sizes into exact bead seat diameters. Inch labels started as outer diameter estimates, so tires with different rims ended up sharing a label.
A wrong BSD means the tire will not mount or will blow off the rim. The lookup prevents that mistake.
Common Uses
This lookup has 4 uses: avoiding wrong purchases, kids' bikes, older bikes and cruisers.
- Avoid wrong purchases: Confirm BSD before ordering.
- Fit kids' bikes: Resolve 20 in and 24 in labels.
- Fit older bikes: Decode 26 in and 27 in fractions.
- Fit cruisers: Separate 26 x 1.75 from 26 x 1-3/4.