ETRTO and Inch Tire Sizes Explained
Yes, a tire can say 622 and 28 inches, and both can describe the same wheel.1 The two codes just measure different things. One is exact. The other is a habit from history.
The code that fits the rim
The ETRTO code, also called the ISO code, has two numbers. The first is the tire's width in millimeters. The second is the diameter of the rim's bead seat, also in millimeters.1
The bead seat is the shelf on the rim where the tire's edge sits. The second number is what decides things. Sheldon Brown's tire sizing article calls it the critical one, and says that if it matches, the tire will generally fit the rim. If it doesn't match, the tire won't fit.1
Michelin puts it the same way: the ETRTO bead seat diameter must match exactly between tire and rim.2
So when your sidewall says 40-622, the 622 is the part you carry to a shop or compare with a spare. The 40 is how wide the tire is.1
Why inch codes are slippery
Inch and old French codes began as outside-diameter measurements. Over time the tires and rims changed, and the old measurements lost contact with reality.1
That's why "28 inch" and "29 inch" can both turn up on a 622 mm wheel:1
| Code you see | What it can mean | Source |
|---|---|---|
| 28 x 1 5/8 x 1 1/2 | A Northern European name for the 622 mm size | 1 |
| 29 inch | A marketing term for wide 622 mm tires | 1 |
| 700c | A road tire that shares the 622 mm bead seat | 2 |
Michelin makes the same point. A 700c road tire and a 29-inch mountain bike tire share the same 622 mm bead seat diameter, but their widths and clearances are very different.2
The decimal trap
Inch codes can show width as a decimal, like 26 x 1.75, or as a fraction, like 26 x 1 3/4. The numbers look equal. Sheldon Brown's article says tires marked this way will not be interchangeable.1
If your sidewall prints only an inch code, look for an ETRTO code too. If it's missing, the maker's product page for your model is the place to find it.
Width still matters
Matching the bead seat gets you a tire that goes on the rim. It doesn't tell you the width is a good pairing.
The same article warns about extremes. A very narrow tire on a wide rim risks pinch flats and rim damage from road hazards.1 A pinch flat is a puncture where the tube is squeezed against the rim. How E-Bike Tires Get Punctures shows what one looks like.
Frame room is a separate limit. Michelin says to check the bike maker's specifications for the maximum tire width the bike will accept.3 Michelin also says the wheel diameter must match exactly.3
When you're unsure, your bike's manual and the tire maker's size table beat any rule of thumb.
Reference list
- Tire Sizing Systems, Sheldon Brown (Harris Cyclery), revised by John Allen
- Technical glossary, Michelin
- How to choose a bike tire, Michelin
More tire notes
How to Read an E-Bike Tire SidewallA plain walk through the codes molded into an e-bike tire sidewall: size, pressure, rolling direction, build type and e-bike marks.
Max and Min Pressure Markings on a TireWhat the pressure numbers on a tire sidewall mean, how bar and psi relate, and why your bike maker's manual is the place for advice.
Rotation Arrows and Mounting DirectionWhat the arrow on a bike tire sidewall means, why front and rear arrows can differ, and how makers say to mount a tire that has one.