Bike Tire Sizes Explained: ETRTO Numbers, 700c vs 29er, Tubeless & Hookless Compatibility | BikeQuiver Cyclopedia

Also called: ISO sizing, E.T.R.T.O., width-BSD

Width (mm) - bead seat diameter (mm), e.g. 57-622; rims labeled BSD x internal width, e.g. 622x25

The international standard that identifies every tire by two millimeter measurements: its inflated width and the bead seat diameter of the rim it fits. 57-622 means a 57 mm wide tire for a 622 mm rim — exactly the same wheel size a 29 x 2.25 marketing label refers to. Rims use the same numbers in reverse (622x25 = 622 mm BSD, 25 mm internal width), so tire and rim can be matched directly. The number is molded into the sidewall of every modern tire, usually in smaller print near the marketing size.

Why it exists: Inch and French sizes described the tire's approximate outer diameter, which drifted as tire widths changed — leaving several incompatible rim diameters all called '26 inch'. ETRTO (the European Tyre and Rim Technical Organisation, founded 1964) instead standardized on the one measurement that actually determines fit, and ISO adopted it worldwide as ISO 5775.

Typically found on: Every bicycle tire and rim made in recent decades, all categories

Also called: 700C, 29er, 28" (Northern Europe), 622

Bead seat diameter 622 mm

The dominant wheel size in cycling. Road, gravel, hybrid, and most commuter bikes call it 700c (a French size: '700' was the outer diameter in mm, 'c' a width class); mountain bikers call the identical rim diameter 29" and many European city bikes call it 28". A 29er MTB tire and a 700c road tire fit the same rims — only the width differs.

Why it exists: 700c was the French racing size that won out when road bikes standardized in the late 20th century. The MTB industry adopted the same 622 mm rim around 2001-2010 for its better roll-over, branding it 29" because a wide MTB tire brings the outer diameter to roughly 29 inches.

Typically found on: Nearly all road, gravel, and cyclocross bikes; most modern XC and trail MTBs; hybrids, commuters, and many e-bikes

Also called: 650B, 27.5, 26 x 1-1/2 (obsolete), 584

Bead seat diameter 584 mm

The second modern wheel size, about 38 mm smaller in diameter than 622. Mountain bikes call it 27.5"; road and gravel bikes call the same rim 650b and typically pair it with a wide 42-48 mm tire so the outer diameter matches a 700c wheel with a narrower tire. As with 622, the two names are the same rim.

Why it exists: 650b was an old French touring and utility size that the MTB industry revived around 2012-2015 as a middle ground between nimble 26" and fast-rolling 29" wheels. Gravel riders later adopted it to fit very wide tires in frames designed around 700c.

Typically found on: Most trail/enduro MTBs 2013-2019 and many current ones (often as the rear of a mixed 'mullet' setup); 650b gravel/all-road wheelsets; small-size road frames

Also called: 20 inch BMX, 20 x 1-1/8 (451), 24 inch (507)

BSD 406 mm (20" decimal), 451 mm (20" fractional), 507 mm (24" decimal)

The common small-wheel diameters. 406 mm is the standard 20" size on BMX bikes, most folding bikes (including many folding e-bikes), and kids' bikes; 451 mm is a different, larger '20 inch' used on BMX race and some recumbents and folders — the two are not interchangeable. 507 mm is the usual 24" size on kids' MTBs and some fat/utility e-bikes.

Why it exists: Same story as 26": inch labels that described outer diameter split into multiple bead seat diameters. The decimal/fractional trap is at its worst here — always read the ETRTO number on a 20" or 24" tire.

Typically found on: BMX, folding bikes (Brompton uses 349 mm 16"), Dahon/Tern (406/451), kids' bikes, compact cargo and folding e-bikes

Also called: TLR, TLE (Tubeless Easy), TR, TCS, tubeless-compatible

Standard ETRTO sizes; requires sealed rim bed (tape or solid), tubeless valve, and liquid sealant

The mainstream modern tubeless system. A tubeless-ready tire has a reinforced, precisely-molded bead and an airtight-enough casing to run without an inner tube once liquid sealant is added; the sealant closes the casing's remaining porosity and small punctures as you ride. Brands use their own labels — TLR (Trek/Bontrager, Schwalbe uses TLE, WTB uses TCS, many just say TR — but all follow current ETRTO tubeless tolerances, which is what makes cross-brand tire/rim pairing work.

Why it exists: Riders wanted UST's flat resistance and low-pressure grip without its weight. Making the tire lighter and letting sealant handle air-tightness — while ETRTO tightened bead and rim tolerances — got there, and tubeless-ready is now the default on MTB, gravel, and increasingly road wheels.

Typically found on: Nearly all current MTB and gravel tires and wheels; most performance road wheels since ~2018

Also called: TSS (Tubeless Straight Side), straight sidewall, crotchless

Rim sidewall with no bead hook; ETRTO max 72.5 psi (5 bar); ETRTO minimum tire widths per rim internal width

A rim standard rather than a tire one, but it dictates which tires you may use. Hookless rims have straight inner sidewalls with no hook to capture the bead, so the system relies on tight ETRTO tolerances and the tubeless bead lock. Rules: tubeless-ready or UST tires only (never a tube-type tire, even with a tube inside), never exceed 72.5 psi / 5 bar, and respect minimum tire widths — ETRTO guidance pairs 25 mm internal hookless rims with 28 mm or wider tires. Most carbon wheel brands (ENVE, Zipp, Giant) publish tested tire-compatibility lists; check them before buying road tires.

Why it exists: Removing the hook lets carbon rims be molded more precisely, stronger, lighter, and cheaper, and suits the wide-tire/low-pressure direction of modern riding. ENVE shipped hookless in 2016; ETRTO published formal hookless standards in 2019.

Typically found on: Many current carbon road, gravel, and MTB wheels: ENVE SES/M, Zipp 303/404 (2020+), Giant/Cadex, some alloy MTB rims

Also called: E-25, E-50, e-bike ready

Certification marking on sidewall; no dimensional change — standard ETRTO sizes

A UN/ECE motor-vehicle tire regulation applied to fast e-bikes. Tires certified under ECE-R75 are tested for the higher continuous speeds, loads, and braking forces of motor-assisted bikes and carry an approval mark; Schwalbe and others also print E-25 or E-50 ratings showing suitability for 25 km/h e-bikes or 50 km/h speed pedelecs.

Why it exists: In the EU, 45 km/h speed pedelecs are type-approved as mopeds, so their tires must legally meet the moped tire regulation ECE-R75. For ordinary 25 km/h e-bikes no special tire is required by law, but reinforced e-bike-rated tires are recommended for the extra weight and torque.

Typically found on: Speed pedelecs (legally required in EU); e-cargo bikes and commuter e-bikes (recommended) — e.g. Schwalbe Energizer/Pick-Up, Continental eContact

By reading the sidewall

Every modern tire has its ETRTO size molded into the sidewall in small print, usually near the big marketing size — look for a pattern like 57-622, 40-622, or 47-584 (it may be prefixed 'ETRTO'). That one number is all BikeQuiver needs. While you're there, note the brand and model name, the max pressure (e.g. 'INFLATE TO 35-65 PSI'), any tubeless marking (TLR, TLE, TR, TCS, UST, 'Tubeless Ready'), and any E-25/E-50 e-bike mark. If the print is worn, rotate the wheel — the size is usually molded on both sides.

By converting the marketing size

If you only have the big printed size, convert it: 700c and 29" both mean BSD 622; 27.5" and 650b both mean 584; 26 with a decimal width (26 x 2.2) means 559; 20" BMX/folding is usually 406; 24" kids is usually 507. Widths in inches convert at 25.4 mm per inch, so 29 x 2.4 = 61-622 and 27.5 x 2.6 = 66-584. Watch the traps: 27 x 1-1/4 (630) is NOT 700c, and a fractional size like 26 x 1-3/8 (590) is NOT the same as decimal 26 x 1.375 (559) — fractional and decimal inch sizes are never interchangeable.

By measuring

To confirm width, inflate the tire and measure across its widest point with calipers or a ruler — expect the real width to differ a few millimeters from the label depending on your rim (wider internal rims make the same tire measure wider, roughly 0.4 mm per extra millimeter of rim width). To confirm wheel diameter with the tire off, measure the rim's bead seat diameter: measure the rim's full outer diameter and subtract about twice the flange height, or measure the tire's inner bead circle — 622, 584, and 559 are far enough apart that a tape measure settles it.

By checking tubeless setup

Not sure if your wheel is set up tubeless? Press the valve: a tubeless valve is a separate part with a lock nut at the rim and often sealant residue; a tube's valve is part of the tube and moves with it. Deflate and pop a small section of bead off — sealant inside means tubeless. On the tire, only sidewall markings like Tubeless Ready, TLR, TLE, TR, TCS, or UST indicate a tubeless-capable tire; if there is no such marking, it is tube-type and must be used with an inner tube on a hooked rim.