Quick Answer
- Bead seat diameter (BSD) is the diameter of the rim at the ledge where the tire bead sits and seals. For a car, motorcycle or tractor tire it is the rim diameter code in the size: the 17 in 225/65R17 is 17 inches, or 431.8 mm.
- For bicycles it is the second number of the ETRTO size: 29 inch mountain bike tires and 700c road tires are 622 mm (about 24.49 in). The "29 inch" name is the overall diameter including the tire, not the rim.
- Bead seat diameter is not rim width. USTMA defines rim width as the measurement on the inside of the rim between the two flanges.
Bead Seat Diameter on Car, Bicycle, Motorcycle and Tractor Tires
Where to Read the Bead Seat Diameter
The marking that gives the bead seat diameter depends on the tire type. Millimetres and the bicycle inch equivalents are calculated at 25.4 mm per inch.
| Tire type | Example size | Where it is | Bead seat diameter | In mm |
|---|---|---|---|---|
| Car | 225/65R17 | 17 (after R) | 17 in | 431.8 |
| Motorcycle | 120/70ZR17 | 17 (after ZR) | 17 in | 431.8 |
| Tractor | 18.4-30 | 30 (after the dash) | 30 in | 762.0 |
| Bicycle (29 inch MTB, 700c road) | 57-622 (ETRTO) | 622 (second number) | 24.49 in | 622 |
| Bicycle (27.5 inch / 650B) | 54-584 (ETRTO) | 584 (second number) | 22.99 in | 584 |
| Bicycle (26 inch MTB) | 50-559 (ETRTO) | 559 (second number) | 22.01 in | 559 |
Bead Seat Diameter vs Rim Width
Bead seat diameter
The diameter across the rim circle at the bead seat, seen from the side. It must be the same on the tire and the rim: 17 inch tire, 17 inch rim.
Rim width
USTMA: the measurement on the inside of the rim between the two flanges, seen in cross-section. A tire has an approved range of rim widths, and tires should only be mounted on approved rim widths.
Check the width range for your size with the rim width range calculator and the rim and tire compatibility checker.
What USTMA and Michelin Say
| Source | Statement (paraphrased) |
|---|---|
| USTMA, Care and Service of Passenger and Light Truck Tires (2017) | Defines the rim diameter code as the nominal rim diameter in inches, rim width as the measurement on the inside of the rim between the two flanges, and the tire bead as bundles of wire that secure the tire to the wheel. Tire and rim diameter designations must be the same, for example a P235/75R16 tire on a 16 inch rim. |
| Michelin USA, Bike Tire Sizes Explained | In an ETRTO bicycle tire size such as 25-622 the second number is the rim's bead seat diameter in mm and a tire and rim must share it. The "29 inches" of a mountain bike tire is its overall diameter including the tire, not the rim seat. |
Frequently Asked Questions
Convert a Rim Size to Bead Seat Diameter
Rim size in inches to mm, plus the bicycle ETRTO chart including 29 inch, 27.5 inch and 26 inch.
Open the Bead Seat Diameter CalculatorSources: USTMA, Care and Service of Passenger and Light Truck Tires (2017); Michelin USA, Bike Tire Sizes Explained. Tire changing should be done by trained personnel.