Quick Answer
- Bead seat diameter (BSD) is the diameter of the rim where the tire bead sits. For a car or motorcycle rim it is the rim size in inches times 25.4: a 17 inch rim is 431.8 mm.
- Bicycle tires use ETRTO: in a size such as 25-622 the second number is the bead seat diameter in mm. Michelin says a 700c road tire and a 29 inch mountain bike tire share the same 622 mm bead seat diameter (about 24.49 in).
- The tire and rim must share the same bead seat diameter. A tire marked 17 does not fit a 622 mm rim.
Bead Seat Diameter
431.8 mm
A tire and wheel must share this exact number: the "17" in a size like 225/45R17 refers to this same diameter.
Get a plain-English explanation of your results and what they mean for your vehicle.
Automotive Rim Sizes: Inches to BSD (mm)
| Rim (in) | BSD (mm) |
|---|---|
| 13" | 330.2 |
| 14" | 355.6 |
| 15" | 381.0 |
| 16" | 406.4 |
| 17" | 431.8 |
| 18" | 457.2 |
| 19" | 482.6 |
| 20" | 508.0 |
| 21" | 533.4 |
| 22" | 558.8 |
| 24" | 609.6 |
Bicycle ETRTO Bead Seat Diameters
Per Michelin, the second number of an ETRTO bicycle tire size (for example 25-622) is the rim's bead seat diameter in mm, and a tire and rim must share it. Michelin notes that 29 inches is the overall diameter including the tire, not the rim seat. Source: Michelin bike tire sizes chart.
| Wheel Size | ETRTO BSD (mm) |
|---|---|
| 20" (ETRTO 406) | 406 |
| 24" (ETRTO 507) | 507 |
| 26" (ETRTO 559) | 559 |
| 27.5" / 650B (ETRTO 584) | 584 |
| 28" / 29" / 700c (ETRTO 622) | 622 |
Background on why it matters: bead seat diameter guide.
Bicycle Bead Seat Diameter Chart (ETRTO)
Bead seat diameters from the Michelin USA bike tire size chart. Inches are calculated from mm and are not the wheel's marketing name.
| Wheel name | Bead seat diameter mm | In inches (calc.) | Example ETRTO size |
|---|---|---|---|
| 12 inch | 203 | 7.99 | 47-203 (12 x 1.75) |
| 14 inch | 254 | 10.00 | 47-254 (14 x 1.75) |
| 16 inch | 305 | 12.01 | 47-305 (16 x 1.75) |
| 18 inch | 355 | 13.98 | 47-355 (18 x 1.75) |
| 20 inch | 406 | 15.98 | 50-406 (20 x 2.00) |
| 24 inch | 507 | 19.96 | 50-507 (24 x 2.00) |
| 26 inch (MTB) | 559 | 22.01 | 50-559 (26 x 2.00) |
| 27.5 inch / 650B | 584 | 22.99 | 54-584 (27.5 x 2.00) |
| 27 inch (old road) | 630 | 24.80 | 28-630 (27 x 1 1/8) |
| 28 inch / 700c (road) | 622 | 24.49 | 25-622 (700 x 25C) |
| 29 inch (MTB) | 622 | 24.49 | 54-622 (29 x 2.10) |
Car and Motorcycle Rim Size to Bead Seat Diameter
Rim diameter in inches times 25.4. Calculated.
| Rim diameter in | Bead seat diameter mm |
|---|---|
| 10 | 254.0 |
| 12 | 304.8 |
| 13 | 330.2 |
| 14 | 355.6 |
| 15 | 381.0 |
| 16 | 406.4 |
| 17 | 431.8 |
| 18 | 457.2 |
| 19 | 482.6 |
| 20 | 508.0 |
| 21 | 533.4 |
| 22 | 558.8 |
| 24 | 609.6 |
Next check width: rim and tire compatibility calculator; for motorcycle sizes see the dirt bike tire size calculator.
What Michelin and USTMA Say, and How to Use This Calculator
| Source | Statement (paraphrased) |
|---|---|
| Michelin USA, Bike Tire Sizes Explained | In an ETRTO bicycle tire size such as 25-622 the first number is the tire width in mm and the second is the rim's bead seat diameter in mm. The term 29 inches is often used for the rim seat, but 29 inches is the overall diameter including the tire. |
| Michelin USA, Bike Tire Sizes Explained | Michelin says a tire and rim must share the same bead seat diameter, and that 700c tires and rims all have a 622 mm bead seat diameter whatever their width. 650b is 584 mm and the older 26 inch size is 559 mm. |
| USTMA, Care and Service of Passenger and Light Truck Tires | Check that the diameter designation of the tire and the rim are the same, for example a P235/75R16 tire goes on a 16 inch rim. |
- 1Enter the rim diameter in inches (the number after R in a car tire size, for example 17).
- 2Read the bead seat diameter in mm.
- 3For a bicycle, find the second number of the ETRTO size (for example 622 in 25-622) in the bicycle chart below.
- 4Check the tire width against the rim width with the rim and tire compatibility calculator.