What is bead seat diameter?

Bead seat diameter (BSD) is the exact diameter, measured in millimeters, of the part of the wheel rim where the tire's bead sits and seals. It is essentially the same measurement as the rim diameter (the "17" in 225/45R17), just expressed in millimeters instead of inches — 17 inches = 431.8 mm.

Why does bead seat diameter need to match exactly?

The tire bead and wheel rim need an airtight, secure seal for a tubeless (or tubed) tire to hold pressure safely. Even a small mismatch between the tire's marked bead seat diameter and the wheel's actual bead seat diameter can prevent the tire from seating properly, causing air leaks, blowouts, or the tire coming off the rim under load.

What is the bead seat diameter for a 29-inch rim?

For bicycle wheels, a 29-inch wheel (also called 700c) has an ETRTO bead seat diameter of 622 mm — this is the same BSD as most road bike wheels, which is why 29er mountain bike tires and 700c road tires can share the same rim diameter despite very different tire widths.

Is bead seat diameter the same as rim diameter?

For standard automotive and motorcycle tires, yes — they refer to the same measurement, just in different units (inches for the nominal size, millimeters for BSD). For bicycles, the relationship is less direct because wheel "size names" like 26", 27.5", and 29" are marketing conventions that map to specific ETRTO bead seat diameters (559 mm, 584 mm, and 622 mm respectively) rather than a literal inch-to-mm conversion.

Bead Seat Diameter Calculator

Name: Bead Seat Diameter Reference Dataset (Automotive & Bicycle)
Creator: TireCalculatorHub
Keywords: bead seat diameter calculator, bead seat diameter chart, etrto bead seat diameter

Convert any rim size to bead seat diameter in millimeters — plus automotive and bicycle (ETRTO) reference charts.

By TireCalculatorHub Editorial Team·Updated: September 5, 2026

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Quick Example

A standard 17-inch rim has a bead seat diameter of 431.8 mm — this is the number that must match exactly between a tire and wheel, not just the nominal "17" label. Bicycle wheels use a separate ETRTO system: a 29-inch (700c) wheel has a bead seat diameter of 622 mm, the same BSD used by 27.5" (650B) wheels is 584 mm. Enter your rim size below for the exact figure.

For standard automotive and motorcycle tires, bead seat diameter (BSD) is simply the rim diameter converted to millimeters — the number that must match exactly between tire and wheel.

Rim diameter (inches)

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.

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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

Bicycle wheels use the same bead-seat-matching principle, but the common "wheel size" name (26", 29", 700c) doesn't always match its true BSD in mm — always match the ETRTO number, not the marketing name.

Wheel Size	ETRTO BSD (mm)
20" (BMX / kids)	406
24" (MTB / juniors)	507
26" (MTB)	559
27.5" / 650B (MTB)	584
29" / 700c (MTB / road)	622

Bead Seat Diameter Guide

Background on why bead seat diameter matters for safe tire mounting.

Read the Guide

How to Use This Calculator

Enter your rim diameter in inches.
Read the exact bead seat diameter in millimeters.
Check the reference charts below for common automotive and bicycle sizes.

Bead Seat Diameter Calculator

Quick Example

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