Pages on this topic: Bike size explained, How to choose.

Do bicycles come in sizes? What size do I need?

Bike size explained

Q: Do bicycles come in sizes, or are they one size fits all? How do I decide what size I need?

A: Yes, bicycles come in numerous sizes. Getting a bike that fits you well is very important for making that bike more fun and comfortable to ride.

Bikes come in multiple sizes

There are two parts to bike size: wheel size and frame size.

Wheel size is pretty simple. There are many different wheel sizes, such as 20”, 24”, 26”, 700c, and so on. That number is roughly equal to the diameter of the wheel, so for instance a 26” wheel is bigger than a 20” wheel. (The 700c size is from a different measuring system, but is close to the old 27” wheel size that isn’t used any more.)

For small bikes (for children), wheel size is all we talk about, and there is just one frame size per wheel size. (Sometimes two: “boys” and “girls”.) A child might start with a 12” or 16” wheeled bike, outgrow that and move to a 20” bike, then on to a 24” bike. In each move up the frame and some of the parts get bigger along with the wheel size, but we just talk about the wheel size.

In department stores, toy stores and discount stores, the “one frame size per wheel size” rule still holds for adults’ bikes. Toy stores will have 26” bikes, and now sometimes 700c bikes, with just one frame size for each. Somehow this is supposed to fit every adult who wants to ride, whether they are 5 foot 2 inches tall or 6 foot 4. Sort of like selling only size medium sweatpants and expecting everyone to fit in them.

Fortunately the situation for full size bikes is completely different in bike shops. Here you’ll find each bike model comes in multiple frame sizes, even though the wheel size stays the same. For instance you might look at a particular mountain bike, and find that it is available in frame sizes as small as 13” up to as large as 25”, all of those sizes using 26” wheels. That will be true for almost all the bikes for adults you see at bike shops—each model will have one wheel size but multiple frame sizes. (Well, some models will have two wheel sizes, slightly smaller wheels on the smaller frames and slightly bigger wheels for the bigger frames. But the basic point still stands, multiple frame sizes to fit different sized people.)

How size is measured

So what does “different frame size” actually mean? And what does that frame size number, 13” or 25” or whatever, mean? It’s fairly simple.

The frame size number comes from the length of the seat tube. The seat tube is that nearly vertical tube of the three big tubes that make up the “main triangle” of the bike frame. The seat tube has the bike’s saddle attached at the top, and has the pedals and crank arms attached at the bottom. A short seat tube will make the pedals closer to the saddle; a long seat tube will make the pedals further away. The frame size number is the length from the center of the crank arm spindle (the axle that holds the two crank arms together) up to the top of the seat tube (where the saddle and seatpost are attached). On some bikes this is measured in inches, on others in centimeters.

But that’s not the only dimension that changes for different frame sizes. As bicycle frames get taller, they also get longer. That means the distance from the saddle to the handlebars gets longer. This makes sense, since tall people don’t just have longer legs than short people. They usually also have longer arms, a longer torso, and so on. So the bike frame also needs to get longer in every direction for a taller rider, not just longer from pedal to saddle. The top tube gets longer, which pushes the handlebars further away from the saddle. The head tube (the frame part that the fork attaches to) gets taller, so the handlebars will be higher up. All of these dimensions and more are fine tuned in every frame size, so the right size frame for a person’s height fits well everywhere, not just in the saddle.

That is the very basic version of the story about bike frame size. Now how do you figure out which size is right for you? That’s the next topic.

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