Many people put emphasis on the look and accessories of a bicycle, but frame size is one of the most important features of the bike you choose. The frame needs to be in proportion to your body. When the frame is the correct size, you reduce the risk of discomfort that can lead to injury and you pedal more efficiently.
Height
Height is the first aspect of your body's size that needs to be measured in order to determine proper bike frame size. Most bike frame sizing charts recommend bike frame sizes that are anywhere between 30 and 36 percent of the height of the rider in centimeters. To determine your height, remove your shoes and stand with your back against a wall, spreading your feet to just less than the width of your shoulders. Have a partner measure from the floor to the top of your head, recording the measurement in centimeters.
Inseam
The inseam is an important measurement -- it helps determine how high the crossbar should sit on a male bike, and it determines some dimensions of a bike frame that can improve riding efficiency and ease of use of the bike. According to eBicycles.com, the inseam as it relates to a bike is different from the inseam measurement used for clothing. This measurement can be taken by removing your shoes and standing against a wall, spreading your feet just as you would for a height measurement. Place a thick book between your legs with the spine up against the crotch, then have your partner measure from the top of the spine of the book to the floor. Record this measurement in centimeters.
Crank Sizes
The crank size, or crank length, is the diameter of the circle created by the pedals when the bike is being ridden. The larger the crank size, the more strength and power needed in the knees and thighs to turn the cranks. Crank size recommendations can be made based on body size, but typically it is a matter of personal preference as to which crank size you prefer. There are four common crank sizes available -- 165 mm, 170 mm, 172.5 mm and 175 mm. You will often find these overlapping one another in terms of ideal frame size on a sizing chart ranging from frame sizes 43 to 66.
Considerations
Using a sizing chart on your own can be an effective means of determining the approximate dimensions of a bike frame that are ideal for your body. Before buying, though, it's recommended you visit a store and try out the frame for yourself to evaluate the comfort. You may also want to seek out guidance from a bike specialist -- this person will be able to guide you through seemingly minor differences in bike frame sizing and help you choose the ideal frame for your body.
Calculating For Your Bike Frame
To calculate the proper height for a road bike, multiply your inseam in centimeters by .65. The resulting number is both the height in centimeters of your ideal frame and the size of frame you should ask for in a bike store. Depending on the size of your upper body, the best fit may be within 2 or 3 cm on either side of this measurement. Try the bike out for yourself and choose what is most comfortable. To calculate the best mountain bike frame size, measure your inseam in centimeters then subtract the number by 10. Convert this measurement from centimeters into inches by multiplying the number by 2.5 to determine the best frame size for this type of riding.
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