When you buy a bike, it’s best to try them on for size.
That, coupled with the charts atBike Size Chart, can help you find the right bike size.
They have charts for city, mountain, road, BMX, and children’s bikes.
They explain how to measure your inseam like so:
Take off your shoes.
Stand against the wall.
Place your feet apart.
Put a book between your legs, with it’s spine firmly up against your crotch.
You should feel as if you are seated on a bicycle saddle.
Have someone measure from the floor to the spine of the book.
But this should give you a good starting point.