To use a bicycle frame size chart, first measure the rider's overall height and leg length. On the chart, find the rider's height first along the left side, and then find the leg length along the top. Where the two values intersect is the rider's recommended bicycle frame size.
There are different types of bicycle frame size charts depending on the type of bicycle a rider is looking for. Make sure to use the correct chart that corresponds to the desired bicycle type, as the frame sizes do differ by bicycle style.
To measure the rider's leg length or inseam accurately, stand the rider against a wall holding a large book between his legs with the spine of the book pressed closely against the rider's crotch. Then, measure the length from the floor to the spine of the book to get the leg length number.
Note that measuring a rider for a bicycle is not an exact science, so a bicycle frame size chart is used more as a guideline than a hard-and-fast rule. To confirm the correct bicycle size, consult a professional to fit a rider to a bicycle. Professional fittings can be done at a local bicycle shop as part of the bicycle buying process.