Most bicycles have a sticker somewhere on the frame indicating the size of the frame. For some people, the number 17 or 54, or the letters XS, S, M, L or XL cause confusion. Determining that number--the bike's frame size--takes just a few seconds. Finding a bike you feel comfortable on can require much more time.
Lean the bike against a wall or some other stable surface. Locate the seat tube portion of the frame. From the floor, the seat tube extends up from the bottom bracket. The bottom bracket is the portion of the frame the crank arms fit into. The crank arms are the part of the bike your pedals fit into. The seat tube ends at the seat collar, which is where the bicycle's seat post enters the frame.
Measure the seat tube from top to bottom or bottom to top. You can begin at either end of the seat tube. Just be sure not to extend into the seat post area or too far to the ground.
Place the end of the tape measure at the centre of the bottom bracket. This is directly in the middle of the bolt that connects the crank arm to the bottom bracket. Extend the tape measure to the seat collar. The seat collar, or clamp, holds the seat post in the frame. In most cases, your result will be somewhere from 13 to 23 inches or 48 to 63cm. Frame geometry and style will impact your result, therefore do not look for a clean conversion between inches and centimetres.
Bicycle frame sizes vary from company to company. Different bike styles have different frame geometries. Many manufacturers use a system of letters that equate to the numbered sizes to make the process more intuitive. Just because you ride a small in one bike does not mean you will ride a small in another type or brand of bike.