A bike tire’s size is printed on the sidewall, and the most reliable measurement is the ETRTO/ISO number — two digits separated by a dash that tell you exact width and rim diameter.
One wrong tire size can mean a frustrating ride or a dangerous fit. The good news is that every bicycle tire has its dimensions stamped right on the rubber — you just need to know where to look and what the numbers mean. Whether you’re replacing a worn tire or upgrading to something wider, reading that sidewall code correctly is the first step.
Where To Find The Size On Your Tire
The tire size is printed or embossed along the sidewall, usually on both sides. If the text is faded, rotate the wheel into good light — wiping off dirt or running a finger over embossed lettering can make it readable again. The size line is typically the longest string of characters on the tire.
Which Number System Matters Most: ETRTO/ISO
You’ll likely see one of three formats on the sidewall, but the ETRTO/ISO code is the only one that guarantees a perfect fit. This standard uses two numbers separated by a dash: the first is the tire width in millimeters, the second is the bead seat diameter (BSD) in millimeters. That BSD must match your rim exactly.
- ETRTO/ISO example: 25-622 means a 25 mm wide tire that fits a rim with 622 mm BSD.
- Inch-based example: 29 x 2.3 means a tire roughly 29 inches in diameter and 2.3 inches wide — a common mountain-bike marking, but less precise for fit.
- Hybrid example: 700 x 25mm is a road-bike standard that usually maps to the 622 mm BSD, but the ISO code eliminates guesswork.
If you see only the common name, use it as a starting point, then verify against the ISO/ETRTO equivalent before buying. A reader ready to buy can explore our roundup of top 27 x 1.25 bike tires for tested options on that specific size.
How To Decode The Sidewall Step By Step
- Look for the longest printed line on the tire sidewall — this is the size marking.
- Find the ISO/ETRTO code first (two numbers with a dash, like 32-622). If present, that is your definitive reference.
- Identify the first number as tire width and the second as the rim bead seat diameter. The rim diameter must match your wheel exactly; width can vary within limits.
- If only common sizing is visible (e.g., 700 x 23c), check a conversion chart against the ISO equivalent before purchasing.
- If no tire is mounted, check your rim for an ISO label or measure the bead seat diameter directly — that BSD is the non-negotiable fit number.
Fit Rules And Safety Checks
Matching the BSD is mandatory. A tire with the wrong diameter will not seat on the rim. Width is more flexible but comes with limits. Before buying a wider tire, confirm clearance around the frame, fork, brakes, and fenders — a tire that rubs any of these parts creates a safety hazard.
REI’s guidance for U.S. consumers states that matching rim diameter is the hard rule while width can vary somewhat, but always verify frame clearance. When sidewall markings are unreadable, measure the rim carefully and compare it against a size chart rather than guessing from the common name.
| ETRTO/ISO Code | Tire Width | Rim BSD | Common Name |
|---|---|---|---|
| 23-622 | 23 mm | 622 mm | 700 x 23c (road) |
| 25-622 | 25 mm | 622 mm | 700 x 25c (road) |
| 28-622 | 28 mm | 622 mm | 700 x 28c (road) |
| 32-622 | 32 mm | 622 mm | 700 x 32c (hybrid) |
| 47-622 | 47 mm | 622 mm | 29 x 1.75 (touring/mtb) |
| 65-584 | 65 mm | 584 mm | 650b (mountain/gravel) |
| 37-622 | 37 mm | 622 mm | 700 x 37c (commuter) |
FAQs
What do the two numbers in an ETRTO code mean?
The first number is the tire width in millimeters; the second is the bead seat diameter in millimeters. The BSD must match your rim, while the width can vary within frame-clearance limits.
Can I use a 29-inch tire on a 700c rim?
Yes, because both systems typically share the same BSD of 622 mm. The 29-inch designation is just a different naming convention for wider tires on the same rim diameter, but check for frame clearance before mounting.
What if my tire sidewall markings are completely worn off?
Measure the rim’s bead seat diameter directly. You can also look for an ISO label stamped on the rim itself. If you find neither, compare the wheel’s diameter against a known size chart to narrow it down.
References & Sources
- Michelin. “Bike Tire Sizes.” Explains ETRTO/ISO numbering and sidewall reading.