The stem length on your mountain bike determines how far you must bend at your waist, and how far you must reach to grasp the handlebars. The Colorado Cyclist says when you have the proper stem length you can breathe easier, your neck and back are more comfortable and your weight is evenly distributed over your mountain bike. Measure your current mountain bike stem to find the proper replacement or to know what size stem to place on a new mountain bike.
Look for the headset on your mountain bike. The headset rests on the top of the steering head that holds the steering column.
Measure the length of your mountain bike stem from the center of the headset to the center of you handlebars. The center of the bar is where the handlebar installs through the mountain bike stem.
Convert the mountain bike stem measurement to millimeters. For example, if your mountain bike stem measures 6 inches, multiply by 25.4 to obtain the millimeter measurement.