There are a few things that go into choosing the right size to fit you.

Bike Type
Your Height
Your Leg Length

Measuring Yourself

Height

Your height is easy to measure. All you need is a flat surface, an upright wall, a pencil and a measuring tape. Stand with your legs together and shoulders back and draw a mark where your head reaches on the wall. Make sure your shoes are off, though.

Inside Leg

Similar to height, you need to be flat against a wall without shoes on. Place a book or similar item between your legs (at your crotch) and have a friend measure from the floor to the top of the item. You do not need this size for buying a kids bike.

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