Ideally youll know your bike size before you buy. If you need to convert your measurements to inches divide them by 254.

Bike Fit What Size Bike Should I Get Rei Co Op
Body measurement for bike. To find inseam length for cycle fitting purposes remove your shoes and any baggy trousers. Stand with your feet 6 inches apart then measure from the inside of your foot up to your crotch where your leg meets your waist. Now run the measuring tape up your seat tube. You no doubt already know your height but the inseam measurement you need for bike sizing is not the same as the one you use when buying jeans which will be at least two inches shorter than your actual inside leg length. Itll make the whole process easier and youll be set up for success right from the beginning. Take your inseam measurement the most important bike measurement there is.
You can have two bikes with the same frame size but differing stand over height. When you reach the end of the seat tube you have your frame size. To measure your bikes frame size take a measuring tape and place the tape in the center bolt of your bikes cranks. Listen to your body. We need 8 core pieces of information in order to calculate your optimum frame size and initial position. To calculate your bike frame size take your inseam in centimeters and multiply it by 0575 for mountain bikes 0665 for road bikes and 0645 for trekking bikes.
Find the best bike size for you. If it is just the frame this will be the center point in the bottom bracket shell. Think of the seam on the inside of a pair of jeans.