Bike sizing is done prior to bike fitting and happens BEFORE you purchase your bicycle. Bike sizing deals mainly with the size of a frame. The bike shop or retailer should measure you to help determine which size frame is best suited for you. Please be aware that not all shops or retail bike stores do this! If you're thinking of buying a bike someplace that does not take the time to measure you in order to sell you a proper-sized frame, you may want to consider shopping elsewhere.
The two measurements (at a minimum) that are typically taken to start the process of finding the correct frame size are your height and inseam. Measurements are then "converted" into a range of sizes based on current industry-accepted formulas. These measurements provide a starting point, but others may be needed. Our bodies are all different. Two riders who are exactly the same height, for instance, may have vastly different upper and lower body proportions, and ride for totally different purposes – which could result in their bike sizes (and fit) being wildly different.
With modern frame geometries, the options in stem lengths, seat post lengths, crank arm lengths, etc., a cyclist could be considered to fit on up to 8 different frame sizes. For example, based on height alone, a cyclist who is 6’-0.5” (72.5”, 184cm) has an industry-accepted traditional frame size range of 55, 56, 57, 58, 59, 60, 61, 62. So the bike shop will start asking questions about what type of cycling you will be doing and possibly taking other measurements to help narrow down this range.
A Few Notes on Frame Sizing
Years ago, when frames were (a) steel, (b) built with conventional geometries (i.e., top tubes parallel to the ground), frame builders could afford to make many different sizes since they could take a given-length tube, slide it into a lug and then braze (weld) the tube to the lug. Since all it took was a different length tube, different sized frames were fairly easy to create.
Everyone in the industry measured frames along the seat tube from the center of the bottom bracket to the center of the top tube. You have probably even heard the term "center-to-center." Shortly thereafter, some frame manufacturers decided that an extended seat post lug (extended at the top) provided a stiffer frame. Now there were 2 measurements, "center-to-center" and "center-to-top." It was still pretty easy to figure out the size of a frame.
Enter carbon fiber, where each frame requires a $250,000+ mold. A separate mold is required to produce a different size frame. Frame manufacturers didn't want to invest millions of dollars into numerous molds, so they got creative and came up with an idea where they could get by with a minimum number of molds and still satisfy cyclists with different measurements. These are referred to as "compact frames" where the top tube is sloped downward toward the seat tube. This radical new design also reduced the size of the frame triangles, making for a stiffer frame.
You might now only see certain models listed as S, M, L and maybe an XL (and on women-specific frames, XS). Makers don't really want to give it a size number (54cm, 56cm, etc.) since their thought is that with different length crank arms and different length stems, each compact frame can satisfy numerous numerical sizes. It's safe to say that each size along the range of S to XL would probably correspond to 2-3 traditional sizes.
So how do you measure a frame with a dramatically sloped top tube, a dramatically short seat tube and an extra-long seat post? Nowadays, it's common to measure the "effective" top tube length for the "basic" size of your frame, where you would superimpose a traditional frame over the compact frame to get the approximate conventional measurement – knowing that you can personalize the bike fit with a different length stem and different length crank arms. This is basically how and why we have bikes looking and sized the way you see them at your LBS.
But manufacturers have gotten even more creative and now build frames in sections, saving even more money in mold costs. For example, both S and M frames might use the same seat tube and rear triangle molds but use 2 different length top tube molds. Same for L and XL. Some manufacturers even share seat tube and rear triangle molds across the range of S, M, L, XL and only differ in the length of the top tube.
3 Ways Shops Typically Look for Your Correct Frame Size
- HEIGHT – the least accurate since, for a given height, people can have long legs/short torso and/or arms or short legs/long torso and/or arms – or any combination. This method has the most variability and results in a wide range of frame sizes for a given height. As stated above, using this method alone, up to 8 traditional frame size options could exist for one height.
- INSEAM – a little more accurate and is based purely on your inseam measurement. Unlike height alone, which doesn't take into account body proportions (upper and lower), inseam is slightly more accurate in that the measure typically corresponds to an optimal +/- 2 frame sizes, so now we are down to roughly 5 frame size options. For example, let’s assume that the same 72.5” cyclist has a 34”, or 86cm, inseam. This would convert to a 59cm (+/- 2cm) frame. Of course, it doesn't take into account upper body proportions, nor how the rider would primarily use the bike (racing vs. distance riding, etc.).
- HYBRID – both HEIGHT & INSEAM – the most accurate. With this method, a frame size can be narrowed down to 2-3 frame size options, and then "fine-tuned" from there. For example, the resulting industry-accepted standard mentioned above would indicate a recommended frame size range of 58, 59 or 60 – but could also accommodate special cases such as extremely long or short legs, where frame sizes of 56, 57, 60 or 61 are also acceptable.
The good news is that stems come in many different lengths (the most popular are 70, 80, 90, 100, 110, 120mm and +/- 6, 8, 10, 17 degrees), and choosing the right one can help keep the frame choice to a minimum. But stem selection is really the job of the bike fitter.
A Proper Fitting Can Only Be Done on a Proper Bike Size
Once a correct frame size (bike sizing) has been determined, then and only then can a proper bike fitting be done. Trying to fit a rider on a bike of the wrong size is folly. So the only appropriate starting point for a good fitting is a well-sized bike.
Bike fitting takes the bike and, through tweaks or changes to different components, fits the bike to the person. The big 3 are CLEAT fitting, SADDLE height/fore/aft positioning and COCKPIT SIZING, including stem length and bar width.
For maximum enjoyment and fun, you need the correct size frame that is then custom fit to you.
Have a question about bike fitting, riding skills, power training, nutrition, fitness, tech or any aspect of road riding? Just ask at https://www.roadbikerider.com/ask-the-coach
Coach Rick Schultz is an avid cyclist who trains, races and coaches in Southern California. Rick is an engineer by trade, and in addition to being a coach, he's a bike fitter and prolific product reviewer. He's the author of Stretching & Core Strengthening for the Cyclist and Bike Fit 101: Your Toolset for a Great Bike Fit in the RBR eBookstore. Check his product reviews website, www.biketestreviews.com, and his coaching site, www.bikefitnesscoaching.com. Click to read Rick's full bio.