A correctly sized mountain bike will be safer, more comfortable and more efficient. Choosing the correct mountain bike size for your proportions is something you can do from home with a few key measurements. Mountain bikes use the standard system of measurement, so you can use a tape measure marked for inches to match your inseam, arm length and hip width to the size charts provided by mountain bike manufacturers to get the perfect fit before you buy your new bike. If you fall outside the frame sizes offered by most manufacturers, a custom-sized bicycle may be your best bet to finding that perfect fit.
Find your pubic bone height. This measurement is similar to your inseam and is the most important measurement for determining frame size. Place your feet shoulder-width apart, and measure from the bony protrusion between your legs, just behind your genitals, to the floor by your heel. You'll need to be wearing the shoes you intend to cycle in for the best measurement. Follow the inside of your leg when you take this measurement. Most mountain bike frame sizes fall between 13 and 22 inches.
Match your pubic bone height to a frame size. Mountain bikes can come in a frame size based on inches, or generic sizes like small, medium, and large. Since each bike manufacturer uses their own system for determining generic sizes, you can use the sizing chart provided on their website or at your local bike shop to match your inseam measurement to a specific frame. Women's specific designs will be better matched to female proportions, so choose a frame size based on gender as well as your measurements.
Check the stand-over height of the frame. Swing a leg over the bicycle, and plant both feet on the ground with your cycling shoes on. There should be 2 inches of clearance between your pubic bone and the top tube of the bike. Riders who encounter particularly rough terrain may choose a frame with greater stand-over height clearance to ensure they aren't injured on jarring impacts.
Match the differences in frame size using a stem and seat height. Your frame size might not be perfect, but seat height and stem length can make up the difference. To determine your ideal seat height, mount your bicycle and rotate the crank so that it points straight at the floor. In this position, your knee should be just slightly bent. Stem length corresponds to your arm and torso length; choose a stem that puts you in a comfortable position where you're not reaching too far forward and straining your lower back.