What Size Bike Do I Need?

The first bike I bought as an adult was at a used bike co-op, a great little nonprofit in Portland, Ore., that repairs donated bikes and resells them at low prices for a good cause. It was a great little shop, and the people were very helpful, but the selection was a bit irregular and I’m a pretty lousy shopper.

I tend to want to make a fast decision and get a good deal (ok I’m a little cheap). The result was a half-decent hybrid bike at a half-decent price with a half-decent fit. I could ride it fine, but it wasn’t ever just right, and looking back it really deterred me from enjoying it as much as I would have liked.

Buying a bike is a big decision, and few things are as important when doing so as making sure that your bike fits you just the way you like it. You’re going to spend a lot of time riding the thing, and it’s going to be transporting your most precious commodity (you) long distances. A bike that is too small will be cramped and inefficient to ride, wearing the rider out. A bike that is too big can be clumsy and unsafe (ever catch a top tube to the crotch?). And if the reach to the handlebars isn’t right, the ride will be no fun at all, eventually making your shoulders and back sore.

Make the same mistake I did and settle for a bike that only mostly fits, and it’s a recipe for either discomfort, danger, or at best a bicycling experience that’s just not as rewarding as it should be.

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Advice on Sizing a Bike

Let’s get the bad news out of the way first—there is no consistent, universal way to size a bike. No magic number, no golden rule, no secret formula in which you multiply your weight by your height, divide by pi and that’s the diameter of both wheels combined. That’s because there are two facts here that can’t be ignored.

First, while there was a time when bikes were pretty standard in size, those days are long gone. Bikes are all kinds of shapes, proportions, styles, custom fit, custom parts, special shoes, special pedals. People hand-build bikes out of bamboo and fold them up to go in a trunk. The old standard number for sizing isn’t very practical. So if you have it in your head that you are a “17” based on a bike you had in college, don’t get too attached to it. And if a salesperson tries to sell you a bike based on a standard sizing system, don’t buy it.

Second, every person is shaped quite differently. Some people have short torsos, other people have big bellies, and still others have arms that go on for miles. Think of five people you know, how their bodies are shaped, and then try to think of one standard rule to determine anything they use, bike, shirt, hat, car, whatever. Not very practical. Certainly there are many such formulas, and they may very well work with some people and some bikes. But for the average person, there are some more straightforward and effective approaches.

One important tip is to seriously consider not buying a bike on the Internet. Internet shopping can be wonderful, but this is really one of those areas where the service on the sales floor can make all the difference. Buying a bike online is a great way to spend a lot of money on a bike, not to mention shipping, only to get something that’s not comfortable, despite all of the measuring and algebra you may have done in preparation.

Instead, find a good shop, where the staff specializes in bikes, as opposed to somewhere like Target where the employees likely have more generalized knowledge. At a good, locally owned bike shop, the people behind the counter live and breathe bikes. They have thousands of miles under their belts, so take advantage of that wisdom. Don’t be afraid to ask questions, for fear of sounding stupid or offending someone you assume to be a bike snob. They want nothing more than to induct another person into their bike-loving community, so let them do it.

The other advantage to patronizing a good shop is that you’ll be able to physically experience sitting on, and very likely, riding a potential bike. And that is really the only way to make sure a bike feels right.

Finally, while you don’t need to spend a fortune, keep in mind that paying a little extra for a good bike at a good shop is paying for peace of mind and many years of enjoyment. If you try to cut costs at the expense of the right bike and the right fit, you may find yourself wasting money on a bike you never ride, or having to buy a new one prematurely.

Some Key Sizing Factors

I said I would spare you the numbers and formulas, but there are a couple of measurements to keep in mind that can take out some of the guesswork in the beginning.

Inseam and Stand-Over – Inseam is the term for the length from the floor up the inner leg to the no-no-special place. Don’t mistake this for your pants inseam, since they are often different. To measure, in bare feet, take a hardcover book and place the spine firmly right up into your crotch sort of like it’s your bike seat. Then position yourself so the book touches a wall, and make a pencil mark on the wall where the spine meets it. Then measure the mark to the floor and that’s your inseam.

This is a helpful measurement, since it allows you to determine what the stand-over height should be. Stand-over just means the height of the bike, or the distance from the ground to the top of the top tube of the frame. Your inseam should be 2.5 to 5 cm greater than the stand-over height. For a mountain bike, the distance should be more toward the 5 cm length. This is a good place to start when choosing a frame size.

Distance from the Pedals – When sitting on the bike, the frame height and seat adjustment should be such that when one pedal is extended as low as it will go, your knee is only slightly bent, almost fully extended but never completely locked.

Handlebar adjustment and reach – This will vary based on the type of bike, but in general you don’t want to be putting strain on your back, shoulders or wrists. With road bikes, the handlebars will be a bit lower than the seat to make your body more aerodynamic. With a hybrid or a cruiser, the handlebars will be higher than the seat, since you’ll be sitting more upright. Here’s another area where there’s no perfect fit, since different people have different preferences and riding styles. In general, when you are gripping the handlebars, you’ll want your elbow to be bent just slightly, which will provide some shock absorption and a smoother ride.

Custom Fitting – Finally, if you are looking to do some competitive or marathon cycling, it may be worth the time to customize a bike to your size. This may mean swapping out parts at the shop, and allowing your bike shop to dip a bit more into some body measurements and math. When done by a trained professional, this will typically cost $75-$250, so again, only really necessary if there is some intense riding happening.

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