The best way to ensure that a bicycle fits your body is in-person inspection of the bicycle. By visiting a store before buying the bike, you can personally try the fit and ask for any necessary adjustments. After a road test, a bike mechanic can perform any necessary fine-tuning.
Pay attention to any signs your body may be providing. Back pain is an indication that the handlebars are possibly too low, while knee pain often signals that the saddle needs adjustment.
Standing over the bicycle is a good way to judge how well it fits your body. While straddling the top tube, the ideal amount of distance between your body and the bicycle is about 1 inch for road bicycles and at least 2 inches for mountain bicycles. Raise the seat to a height where the leg comes to 80 to 90 percent of full extension at the bottom of a pedal stroke.
If inspecting the bicycle is not possible before purchase, inseam is the most important measurement to consider. Bicycles should list a standover height in their specifications. The distance between the standover height and your inseam should be approximately 1 inch for road bicycles and at least 2 inches for mountain bicycles.