Finally - a Clear Guide to Bike Sizing

Marin Lucas Valley bicycle bike

How sizing is done between the different types of bikes is one of the great mysteries of the world. We can put a man on the moon and have Facetime conversations with people living on the other side of the globe, but somehow can't standardize how bikes are measured. 

Here's an example of the nuttiness. I ride a 62 cm road bike but have a mountain bike with an XL frame My other bikes include a 21-inch hybrid, a 64 cm touring bike and I have a BMX bike with 20-inch wheels. Other adults have 26-inch bicycles, and some 29er sales have been displaced by 27.5 bikes, which are now all the rage in the mountain bike scene.

Frame Size vs Wheel Size

A big part of the problem comes from two different sets of sizing systems: one that uses frame measurement to gauge bike size and another sizing system based on wheel diameter.

Road bikes are one type of bikes using frame measurements (in centimeters) to indicate size, and the typical road bike size is usually between 50 and 64 cm. This number represents the distance from the center of the crank to the top of the frame at the seat tube. Same thing with hybrid bikes, where numbers people throw around also indicate frame size, only in this instance as measured in inches. Mountain bikes can be described in inches too, but you'll also see frame sizes for both hybrids and mountain bikes described as being small, medium, large and XL.

Where things start to get confusing is when a second sizing metric, wheel diameter, comes in as a way to describe bike size. Many times you'll see ads on places like Craigslist, just one of many places where people sell bikes online., that has something described as being a 26-inch bike. Well, yes, that's an indicator of wheel diameter -- fairly typical for an adult road bike -- but it doesn't tell you anything about how big the frame is.

To determine how well a frame fits when precise sizing is not known, the most common approach is to look at the standover height, which roughly corresponds with the length of one's inseam. On a traditional men's style road bike with a horizontal top tube, a good general indicator of bike fit is when a person straddling the bike flat-footed would have a couple of inches of clearance between the top tube and crotch. There are other adjustments that can be made for proper bike fitting and of course to have a more precise and accurate fit, it's always a good idea to work with a local bike shop staffed with people who are experts in these things.

20-inch, 26-inch, and 29ers

The whole mountain bike scene further adds to the muddle. At first all-mountain bikes, regardless of frame size, typically had the 26-inch wheels. But then larger wheel sizes were introduced by several mountain bike manufacturers about 15 years ago. These so-called "29ers," (referencing the larger diameter) offered superior capacity to roll over obstacles like bumps and logs. Thanks to this and the larger diameter that offered more rolling momentum, these 29ers quickly grew in popularity, making them a significant part of the mountain bike market. And again, like the "26-inch bike" when you hear somebody talk about a 29er, it's a wheel size, not a frame size.

And if that wasn't enough, an in-between size, the 27.5 mountain bike has come out in just the past couple of years. Attempting to capture the best of both sizes (26 and 29er), the 27.5 wheel has been well received by mountain bikers. Just remember that it's a wheel size, not a frame measurement.

Kids' Bikes

Kids' bikes above all use wheel sizes to describe the size of the bike, without attempting to measure or differentiate between frame size. A 20-inch bike is the wheel size for a standard kids bike, and easiest reference is to think of it as typical wheel size for a BMX bike. Diameter measurement for wheel sizes on kids' bikes go down from there, with 12-inch bike typically being the smallest bike available, ridden with training wheels by the littlest tots. There can be the occasional 24-inch bike (again, wheel size) but beyond that, it gets into the 26-inch range and the world of adult bike measurements.