To use a bike sizing chart, measure your height and inseam, then choose the bike size on the chart that is closest to those measurements. Bike size charts vary depending on the type of bike, and some charts only require an inseam measurement.
Bike size charts base sizes on a bike's seat tube length. Manufacturers typically measure from the center of the bottom bracket to the point at which the seat tube and top tube meet, although measurement standards vary by manufacturer.
Bike size charts also have crank size measurements, which is the circumference of the crank shaft. Standard sizes range from 165 to 175 millimeters.
Larger bike sizes also tend to be longer, although this varies by manufacturer. If you're between two sizes, one method to decide which to purchase is to measure your wingspan. If your wingspan is greater than your height, choose the larger size. Otherwise, choose the smaller size.
Different styles of bikes have different size charts. For example, city bike size charts have different measurements than mountain bike size charts.
Keep your riding style and goals in mind when choosing a bike size. Smaller bikes are often easier to handle and maneuver, which is useful for aggressive riding or frequent turning. Larger bikes tend to have higher maximum speeds, so those who race often choose a larger size than normally necessary.