Before you shop for a bike, you'll need to take several measurements of your own body. Bike sizes are based on a person's height and body frame dimensions — not on their weight. You'll want to know your height, inseam, torso length and arm length. Be sure to remove your shoes prior to taking these measurements. Also, measuring can be easier with a helper.
Measure Your Height
If you don't already know your height, measure yourself while standing on a flat surface with your back against a wall. Feet should be spaced slightly less than shoulder width apart.
Measure Your Inseam
The most important measurement when sizing a bike is your inseam length. Don't base your inseam simply on your jeans size. The more precise, the better.
- Stand with your back against a wall and spread your feet 6-8" apart.
- Place a book between your legs and up against the wall, spine up; raise it until snug against your crotch (this mimics your bike seat).
- Have another person measure from the top of the book (spine) down to the floor.
Measure Your Torso and Arms
You may need your torso length and arm length when shopping for a road bike. To determine your torso length, simply measure from your groin to the small "V" in your throat, just above your sternum. To determine your arm length, measure from your collarbone to roughly the center of your palm.
While many bikes offer a size you're familiar with, such as S, M, L or XL, others don't. Many bikes instead provide a size listed in inches or centimeters as one number (e.g. 18"). The “frame size” of a bike refers to the length of its seat tube. This measure is given in one of two ways: from the center of the bottom bracket to the top of the top tube (C-T) or from the center of the bottom bracket to the center of the top tube (C-C).
There is no industry standard for gathering bicycle measurements or fitting cycle to rider, and different brands may measure their bikes in slightly different ways. Every bike brand should offer some specifications based on their own measurement standards, however, so you can check out the brand's website for their preferred standards. For this guide, we'll focus on a C-T measurement standard. The C-C measurement standard is usually only about 1 to 1.5 cm shorter than the C-T standard for a bike, so you can make rough comparisons.
Women and Children's Sizing
Compared to men, women and children (especially young girls) have shorter arms and longer legs. This means that they will have a slightly different fit on a bike, particularly on road bikes. In easy rule of thumb for women and children is as follows: If you fall somewhere on the border between two bike sizes, go with the smaller size. It's easier to control a smaller bike, and the saddle height can be increased easily.
Road bikes are more difficult to accurately fit than other bikes, and require a few more measurements to hone the fit. Besides the seat tube length, you need to have just enough horizontal length on a road bike to let you comfortably stretch forward into your pedaling stance. To obtain your ideal seat tube height (or “size”) for a road bike, multiply your inseam (in cm) by .67 to get C-T length (or by .65 to get the C-C size for your seat tube). If you measured your height and inseam in inches, convert inches to centimeters by multiplying inches by 2.54 (Example: 30 inches x 2.54 = 76 cm). For the best fit on a road bike, you also need to know your ideal "total reach," which is the combined length of the top tube and stem. Unfortunately, there are many different ways to convert upper-body measures into the total reach for a road bike. To get an estimate of your total reach, consider using the formula below taken from the Lemond System. (Be sure to measure in inches).
Total Reach = [(Torso Length + Arm Length) / 2] + 4 inches = (Top Tube + Stem)
Saddle height, which is measured from the center of the bottom bracket to the top of the bike seat, is another measurement that helps you custom fit your bike. However, since saddle height is adjustable on most road bikes, this measurement is not as important as seat tube length.
Road Bike Sizing Table
Inseam(in.) Inseam(cm) Seat Tube(cm) Seat Tube(in.) Saddle Height(cm) 27 69 46 18 61 28 71 48 19 63 29 74 49 19 65 30 76 51 20 67 31 79 53 21 70 32 81 54 21 72 33 84 56 22 74 34 86 58 23 76 35 89 60 23 78 36 91 61 24 81 37 94 63 25 83
Note: Taller riders (over 6’1”) may want to lean toward a slightly larger frame size (approx. 1-4 cm extra seat tube length) when selecting a size based on this table. For example, a tall person with a 36-inch inseam would require a 61 cm seat tube (size 61) based on the table, but might actually prefer a 63 cm seat tube.
“Stand-over-height” is the most important thing to look at when sizing a mountain bike or commuter bike. The stand-over height of a bike frame, or the distance from the center of the top tube to the ground, should be 2-5 inches less than your inseam. Aggressive mountain bikers will need 4-5 inches difference between stand-over height and inseam length, while commuters only need about 2 inches of clearance. Another way to find your MTB size is to simply subtract about 10 cm (4 inches) from your ideal road bike frame size. Thus, a rider with a 33-inch inseam would need a 22-inch road frame and an 18” mountain bike frame. Just keep in mind that most mountain bike measurements are given in inches, while the majority of road bike specifications are only offered in metric units.
Since mountain bike seats are easily raised and lowered, and the rider almost always stays in an upright position, the length of a mountain bike’s top tube is as important as the frame size. This is important because if the top tube is too long, it will be difficult to keep your weight over the rear wheel when riding uphill. You'll want to calculate your best top-tube length (measure in inches) and be sure the bike you're interested in offers a similar length. If you're looking at a bike's specifications on this website or any individual brand's website, this spec may be listed as the "effective top tube." This term describes the imaginary horizontal top tube length for any bike with a top tube that is not perfectly horizontal.
Approximate top-tube length = [(Torso Length + Arm Length) / 2] - 6 inches
MTB/Commuter Bike Sizing Table
Inseam(in.) Stand-over Height(in.) Seat Tube(in.) 27 22-25 14 28 23-26 14 29 24-27 15 30 25-28 16 31 26-29 17 32 27-30 17 33 28-31 18 34 29-32 19 35 30-33 19 36 31-34 20 37 32-35 21+
Cruiser bikes don't require a custom fit, but the closer the frame is to the perfect size, the more you'll enjoy your rides. Sizes are general, usually listed simply as S, M, L or XL. Base the size on your approximate height and inseam measures.
Cruiser Bike Sizing Table
Height Inseam Approximate Size 5'3" to 5'6" 27-29" S 5'6" to 5'10" 29-32" M 5'10" to 6'1" 32-34" L 6'1" to 6'4" 34-37" XL
Once your new bike arrives, you may need to fine-tune a few things depending on your personal riding style. Here are a few examples of minor alterations you can do at home or have done at a shop.
Adjust Saddle Height
Adjust your saddle height so you reach a near-full leg extension with every pedal, and experience a slight bend in the knee at the bottom of a pedal stroke. The correct saddle height will keep rides easier on your knees and allow for powerful downstrokes.
Adjust Saddle Tilt
Saddle tilt angle is simply based on personal preference. Adjust the saddle tilt so that A) you are comfortable, and B) you don't slide off the front of the saddle. Men tend to prefer the saddle nose tilted slightly up, while women tend to prefer it tilted slightly down.
Bike Handlebar Size and Height
Handlebars come in multiple widths and configurations. If you want to customize your handlebars, select bars that are the same width as your shoulder joints. Go narrower for a more aerodynamic ride, or wider for easier breathing and added leverage. If your back is getting sore or cramped after riding, you may need to raise the handlebars. Bear in mind, however, that you'll most likely need to install a new set of handlebars or a different stem, which should be done by a qualified bike tech.
For any custom fits, such as adjusting your seat tube angle or crank length (i.e., bigger cranks for slow pedaling in higher gears), head to a bike shop and get help from a pro.