Bike Tire Sizes: Making Sense of the Numbers

Walked into a bike shop to buy replacement tires. The guy asked what size. I said “the same as these” and pointed at my bike. He sighed. Turns out there’s a system to tire sizing, and knowing it saves everyone’s time — including mine when ordering online.

The Two Systems

Tire sizing uses two different systems simultaneously, which causes endless confusion. The traditional system shows numbers like 700x25c or 26×2.0. The ISO/ETRTO system shows numbers like 25-622 or 50-559.

The ISO system is more precise. The first number is tire width in millimeters. The second number is the bead seat diameter — the internal dimension that must match your rim. That’s what makes ISO numbers endearing to us when ordering tires — they tell you exactly what will fit.

Common Road Bike Sizes

700c

The standard for road bikes. 622mm bead seat diameter in ISO terms. Called “700c” because old French sizing measured outer diameter, and there was a 700a, 700b, and 700c — only 700c survived.

Width varies from 23mm race tires to 35mm or more for rough roads. Modern road bikes trend toward wider tires than a decade ago.

650b

Smaller diameter — 584mm bead seat. Common on gravel bikes for running even wider tires with similar overall diameter to 700c. Also the standard for modern mountain bike wheels (as 27.5″).

Mountain Bike Sizes

29″ / 700c

Same rim diameter as road 700c (622mm). Called “29 inch” in mountain biking. Rolls over obstacles well and maintains momentum. Standard for cross-country and increasingly for all mountain bikes.

27.5″ / 650b

The 584mm diameter. More maneuverable than 29ers, more rollover than 26″. Popular for trail and enduro riding. Some riders prefer the feel.

26″

The old mountain bike standard — 559mm bead seat. Still works fine; just hard to find tire variety. If your bike uses 26″ wheels, it’s not worth changing just for tire selection.

Width Matters

Narrower tires: lower rolling resistance, less comfort, higher pressure required. Wider tires: more grip, more comfort, can run lower pressure.

The sweet spot depends on your riding. I’m apparently someone who prefers slightly wider than typical — 28mm on road bikes, 40mm on gravel. Probably should have experimented earlier instead of running whatever came stock.

Tire Clearance

Your frame and fork limit tire width. Check before ordering. A tire that doesn’t fit is just money wasted. Some frames accommodate 35mm; others max at 28mm.

Also check brake clearance if you have rim brakes. Disc brakes don’t affect tire width limits.

Reading the Sidewall

Tires print their size on the sidewall. Look for two sets of numbers. Something like “700x28c” plus “28-622”. The ISO number (28-622) tells you exactly what you need for replacements.

Also note pressure recommendations (usually in PSI range) and directional arrows if present.

Tubes and Tubeless

Inner tubes need to match tire size but have range. A tube labeled “700×23-28” fits any tire from 23mm to 28mm wide. Don’t buy exact-size tubes — get ones that cover your size.

Tubeless eliminates tubes. Requires compatible rim, tire, sealant, and valves. Worth it for mountain and gravel; debatable for road depending on your puncture frequency.

Pressure Considerations

Max pressure printed on sidewall isn’t optimal pressure. Wider tires run lower pressure — often 40-50 PSI for gravel, 25-35 for mountain. Narrower road tires run 80-100 PSI but even that’s trending lower than it used to.

Experiment with pressure. Small changes affect comfort and grip noticeably.

When to Replace

Visible wear indicators (some tires have them), exposed casing, frequent flats, or squared-off profile. Front tires last longer than rear — swap them periodically if you’re frugal.