It doesn't matter how tall you are; when you're on a bike, the length of your legs is all that matters. Bicycles are meticulously designed to get the most power and motion out of your legs' movement, which is why a well-fitted bicycle can help you go faster. Finding the right frame size for your leg length can be complicated, but it's well worth the work.
Standover Test
Whether you've carefully calculated your ideal frame size or just grabbed a bike at random off a rack, you won't get far without doing a standover test. Stand with the bike between your legs, but don't sit on the seat. Holding the bike upright, check the amount of space between your body and the top tube of the frame, which runs between your legs.
For road bikes, you should have about 1 inch of clearance. For a mountain bike, it should be at least 2 inches, and up to 5 inches for aggressive riders. Also make sure you account for the angle of the top tube. If it angles downward toward the rear of the bike, measure the standover clearance from the highest point.
Measuring Leg Length
To buy a bike online or from a catalog, know the length of your inseam, or the inner side of your leg. This is not the same as your pant inseam. To measure your inseams, stand against a wall with your biking shoes on. Place a large book between your legs so that its spine meets your crotch and one end is flush against the wall. Mark lightly on the wall with a pencil at the top of the book's spine, then measure from the floor to your mark. In some cases, you'll need to convert your leg length from inches to centimeters. To do so, multiply it by 2.54.
Road Bike Frame Size
Bike frames are typically measured in centimeters; however, the measurement actually indicates the length of the seat tube, not the height of the entire bicycle. The easiest way to find the right size of frame for your height and leg length is to use a sizing chart or speak to a professional. If you do choose to calculate your own frame size, find an approximate number by converting your inseam measurement to centimeters, then multiplying that number by 0.65. However, this number varies based on how the bicycle itself is measured, and doing your own calculations can result in an uncomfortable fit.
Fitting a Mountain Bike
Unlike road bikes, mountain bikes are often measured in inches. To fit your mountain bike, start by subtracting 12 inches from your inseam length. Subtracting another inch if you are an aggressive rider. In some cases, mountain bikes may use a letter sizing system. A size small should fit a rider with a 27- to 29-inch inseam; medium will fit a 29- to 31-inch inseam; and large will fit a 31- to 33-inch inseam.
Other Considerations
Bike frame sizes may be marked "C-T" or "C-C." This indicates the way they were measured. A C-T measurement is 1 to 1.5 cm larger than a C-C measurement of the same frame. When you're checking frame sizes, find out what measurement method was used.
Many other factors, such as frame length, seat position and crank length, can help you find the bike size that best suits your body and riding style. Talk to a bike fit professional before buying a bike, to be sure you find one that fits you comfortably.
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