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Pages on this topic: Bike size explained, How to choose.
Do bicycles come in sizes? What size do I need?
How to choose
So now you know that bikes for adults come in multiple sizes. (See the previous section of this answer if you skipped it and want the details.) But how to decide which size is right for you?
Why size is important
It is clear that wearing a shirt two sizes too small would be uncomfortable, and wearing a shirt two sizes too large wouldn’t be good either. In the same way, riding a wrong size bike will fit badly and make you uncomfortable, too. There are two main problems, one obvious and one a little more subtle.
The first big problem with a bike that doesn’t fit is saddle height. You need the saddle to be just high enough to give your legs the right extension when you pedal. Too low and your legs stay bent too much all the way around; this doesn’t use your leg muscles well and you wear out quickly. Too high and you rock back and forth on the saddle as you pedal, very uncomfortable. Either extreme (too high or too low) can also be very hard on your knees.
Saddle height is adjustable, of course, and if the bike frame is at least close to the right size, you’ll be able to adjust the saddle to the proper height. If the frame is a lot too small, you can usually overcome this problem by buying a longer seatpost to move the saddle back up to where you want it. (But that causes other problems we’ll talk about soon.) If the frame is much too large, you won’t be able to drop the seat far enough to be usable, and there is no practical way around that problem.
The saddle on a bike can be adjusted up and down several inches, even as much as a foot if you consider replacing the original seatpost with a longer one. But the handlebars can’t be moved nearly as far as saddles can. Most handlebars can be adjusted up and down or forward and back only two inches or so, and even this small change is often a complicated operation of replacing parts, not just loosening a few bolts.
This can lead to all sorts of problems. If the handlebar is too far away from the saddle, you have to bend down further and reach far out to grab the bars. This puts more weight on your back, arms and hands, which is uncomfortable. Having the handlebar too close to the saddle is less of a problem, but in extreme cases it can cause your knees to bump the handlebars when you are standing up to climb a hill. Having the handlebar a lot lower than the saddle is similar to having it too far away—you have to bend far over and reach further to grab the bars, an uncomfortable position. Having the handlebars high up is not a big problem, except that sitting upright slows you down. Your legs aren’t as strong when you are sitting up, and that position causes more wind resistance when riding fast or into a wind.
Bike size has a big effect on handlebar position. If the frame is too small, you’ll have to push the seat up high to fit your legs. You won’t be able to move the handlebar up as much, so it will be a long reach back down to grab the bars, and your back will be quite bent over. If on the other hand the frame is too large, the bars will be up high but might be too far forward (since the frame gets longer as well as taller). This may also make you bend over more than you want, depending on how big the frame is.
Choosing a size: multiple methods
There is no “one true way” to decide proper bike fit. Actually there are hundreds of methods, tables, charts, computer programs, and cycling gurus, each of which claims to have the best way to determine what bike size and setup best fits you. Part of this confusion is because most ideas about proper bike fit come from experience or tradition, not scientific study. What scientific study there is on the topic, usually looks at what makes a rider fast and efficient, not necessarily comfortable. Most of the tradition and most of the scientific study comes from the bike racing community, not average recreational riders. And that’s the underlying problem: riding a bike is a compromise, just like everything else in life. What makes you more comfortable on the bike mostly makes you slower. What makes you faster on the bike usually makes you less comfortable. So “proper bike fit” depends partly on your choice on that question. Do you want to go faster, or do you want to be more comfortable?
Many experienced bike riders might read those last few sentences and scoff. All their cycling heroes ride with backs perfectly flat, arms stretched out, handlebars much lower than their saddles, on very small frames. Surely this can’t be uncomfortable, they ride almost every day of the year, usually several hours at a time?
It’s true, these professional racers are reasonably comfortable riding in that position. But they are professional athletes, paid to be fast and get results, not to be comfortable. Their bodies have adapted to this position because they do ride several hours a day, every day, year-round. Their upper bodies don’t suffer as much because their legs are pushing hard all the time, which takes weight off their arms even when quite bent over. Their body type is suited to it—very little body fat. And to put it simply, they put up with some discomfort to go faster. It is fine to look up to bike racers, but their bike fit and riding position is an unfortunate model for the rest of us to try to copy.
A simple method
Here is a simple method for deciding what bike size and setup is best for you. Following it will give you a riding position that is a good compromise between speed and comfort. The position is what many experienced long distance bike tourists find works best—comfortable enough to make riding all day a pleasure, but efficient enough that you aren’t worn out by wind or hills sooner than you should be.
But first, two cautions. One, don’t take this method as a set of rules you absolutely have to follow. Every body is different, everyone’s notion of comfort is different, some people pedal faster, some slower, and so on. This method will get to you a set up that is good for the average rider on the average bike. Use it as a starting point, not and end. Make adjustments until you are happy, then maybe make even a few more changes. Ride a lot. Don’t be afraid to experiment, and before long you will know when you bike setup is right. Second, don’t expect that the sizes and settings you come up with for one bike will automatically transfer to another bike. If you are shopping around for bikes, be aware that different brands fit differently, even different models within the same brand fit differently. Don’t decide that the fact you need a 21” bike in one model means that the 21” size in every other model will work as well. Keep an open eye when trying different bikes and an open mind.
Step one—Pick the likely frame size. For the bike model you are considering, stand over the frame with your feet flat on the ground, slightly apart. You should be standing over the bike in front of the saddle, not sitting on the saddle itself. You should have about 1” clearance from the frame to your crotch, no more. If you can pick up the front wheel more than about 2”, the frame is probably too small. If you are resting on the bar without lifting at all, the frame is probably too large.
This step assumes a traditional bike frame with the top tube parallel to the ground or nearly so. It doesn’t work with women’s frames or frames with steeply sloping top tubes. For those, just pick a frame that seems right and go on to the next step. We’ll figure out if you picked the right one soon enough.
Step two—Set the seat height. Have a friend hold up the bike from in front, while you sit on the saddle, hold the handlebars, and pedal backwards. Pedal with your heels on the pedals, not the balls of your foot as you normally would. As you pedal backwards, stop with one of the pedals all the way down. Check the position of your knee. If it is all the way straight but not quite locked, you have the saddle at the right height. If your knee is locked our you can’t even reach the pedal, drop the saddle down a bit and try again. If your knee isn’t straight, lift the saddle. Repeat until you get it right. Then try pedaling (backward) with the balls of your feet like normal. Your knees should come close to straightening out but not get straight, and your hips should be still on the saddle, not rocking.
Step three—Handlebar height. Get back off the bike now that the seat is in the right spot. Look at the handlebar position in relation to the saddle. You want the handlebar to be somewhere between as high as the saddle to 2 inches higher. If it is not and the bike design permits it, move the handlebar up to where you want it.
Step four—Handlebar reach. Get back on the bike, sit on the saddle and grip the handlebars. Bend your elbows a bit. Pedal backwards a few times to get settled. If you have two friends with you, have one of them hold you up from in front while the other looks from the side. If you have just one friend with you, you can put the bike in a trainer to hold you up, or lean against a wall, or ride around while your friend watches.
What they are looking for is the angle of your back. If 0 degrees is a flat back parallel to the ground, and 90 degrees is a back sitting straight up at the dinner table, you want your back to be somewhere between 45 degrees and 60 degrees up from horizontal (more upright than bent over). That is a good compromise position that doesn’t put too much strain on the back, arms and hands, while being bent over enough to reduce wind resistance somewhat and make your leg muscles more efficient.
Step five—Evaluation. Ride around a bit and see how the bike feels. If you weren’t able to get the handlebar to the right spot to put your back where you want it, consider what could be done to fix the problem. If the handlebar was too low, try a larger frame size, returning to step two and repeating the rest. If the handlebar was just a little too low, possibly a different stem would put the handlebar where you want it. If the handlebar is too far away from the saddle, this can usually be fixed with a shorter stem, or by trying a smaller frame. (Remember that a smaller frame will put the handlebar lower, somewhat counteracting the effect of moving it back nearer the saddle.)
That’s it, the five step simple method to bike fit. Now go out and ride, make small adjustments to your saddle and handlebar position, ride some more, repeat until your bike feels comfortable to you. That is what matters, not some formula about leg length ratios or whatever. Ride, have fun.
More reading
Most fit formulas yield a bike setup similar to what you see in the Tour de France. One of the best known fit formula systems was publicized by Greg Lemond in his 1987 book “The Complete Book of Bicycling”. You can read about this system at the Cyclemetrics website. Cyclemetrics is a company that markets a body measuring tool (the FitStik).
Grant Petersen and the Rivendell Bicycle Works company advocate a more upright riding position, much like the position described above. You can read more about their views at the Rivendell company site.