What Size Are My Road Bike Tires? – Decoding Sidewall Numbers

Bethany: “I have no clue what tire size to ask for at my bike shop. What do all those numbers on my sidewall mean?”

The great range of mysterious numbers on your sidewall are only complicated by the weird numbers you’ve also got on your wheel rim. Plenty of people are just as confused by these numbers as you, and I’m sure a few of them are still pretty dang good bike riders.

Assuming we’re only talking road bikes, you really only need to know what two of the numbers mean. Take the photo to the right, for example, and you’ll see two lines: 23-622 and 700x23c.

Reading 23-622: The first number refers to the width of the tire once it’s fully inflated. So, in this case, the tire to the right is a 23mm tire. The 622 is what’s called a “bead seat diameter,” which means it’s the diameter where the bead of the tire–basically the edge of the sidewall–meets and is held into the rim (an internal measurement of sorts).

Reading 700x23c: The second line is really all you’ll need to remember when you head into a bike shop. On road bikes, you’ll generally find one of two different sizes here: 650c and 700c. The 700c tire is more common on road bikes (let’s all avoid the whole 650 vs. 700 debate here, because 700’s are clearly cooler, right? *note hint of sarcasm), and that number technically refers to the outside diameter of the tire. I’m not going to go into all the confusing realities of the tire measurements and such, because the actual sizes get a bit messy. But that’s a general idea of what these numbers mean. Just know that a 700c won’t interchange with a 650c and vice versa.

I will tell you, however, that if you don’t know what to ask for at the bike shop, simply tell them you need this second number: 700x23c. They’ll point you to a range of tires that will certainly fit. If you want to impress the person at the bike shop, nonchalantly mention that the ISO number is 23-622. You’re essentially telling them the same thing, you’re just acting cooler about it. If, on the other hand, you’ve got a rim with no tire, you can read the corresponding numbers on the rim, or you can just take it into the shop and they’ll get you fitted up.

Tire Width: Now I just briefly touched on widths earlier. But this is a trendy topic these days. I recently made the switch to 25mm tires. Why? Because they’re cushy, so they make for a super comfortable ride when you’re really putting in the kilometers. But the wider tire, in relation to your rim, will often reduce the aerodynamic property of your rims (assuming they are aero) and add some grams to your weight. For most people, the trade-off is a no-brainer. In fact, plenty of pros these days race on 25’s. If you’re on 23mm tires now, you might consider making the switch–it’s a noticeable difference.

I’ve even heard predictions that the 28mm tire will soon be the thing, but not every road bike these days will fit a 28. So make sure you know they’ll squeeze into your frame and between your brakes before you empty your pockets for those uber-comfy 28’s.

Fantastic question, Bethany. Are you thinking of switching up tire widths? Let me know in the comments!

Got a burning question? Send it over to me, and I’ll answer to the best of my ability. Because it’s important to remember, there are no stupid questions. And anonymity is really cool too.