Bike fit is one of the most important considerations when making a bicycle purchase. It is also one of the biggest factors in the comfort of your bike-riding experience. For serious bicyclists, a trip to a bike-fit specialist is essential. For beginning and intermediate bicyclists, basic fit is enough to get you started. Your local bike shop can help adjust the details of your fit, but you should be familiar with your standover height before you begin shopping for a bicycle. The standover height is, quite literally, the height of your body as you straddle or "stand over" the bike.
Measure your height. Stand with your back against a wall, feet shoulder-width apart, and mark your height on the wall with a pencil. Measure the height of your pencil mark. Alternatively, you can have a helper measure you. You can omit this step if you already know your height.
Measure your inseam length. Stand against the wall again, with a book between your legs as high as it will go, spine side up. Have your assistant measure from the floor to the top of the book's spine. This is your inseam length.
Stay in that position and have your assistant measure from the spine of the book to the small "v" in your throat, just above your sternum. This is your torso length.
Find the end of your collarbone closest to your shoulder on either side of your body, and extend your arm while holding a pencil in your fist. Have your assistant measure from the end of the collarbone to the pencil. This is your arm length.
Multiply your inseam in centimetres by .67 to get your ideal seat tube height, or standover height. To calculate your total reach, add your torso size to your arm length, then divide that number by two and add four inches to it.
Be prepared to change the bike's adjustable settings as necessary. Measurements give a good guide but are not exhaustive.