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Bike frame sizes, how to measure and pick the correct size bicycle

Bicycles come in a variety of frame sizes and styles. One model of bike might have two or three frame sizes meant to fit different sizes of riders. While people of average height can ride most model sizes comfortably, and while the seat height can be raised or lowered several inches in each direction to customize your ride, both taller riders and shorter riders want to pay particular attention to the frame sizes they get. If a taller rider gets a frame size too small for them, they will experience an uncomfortable ride and they will pedal in an inefficient manner. If a shorter rider gets a frame size too large for them, they will find it difficult to get on the bike and will experience problems with balance when coming to a stop (as they won't be able to get their feet on the ground without tipping over.)

When riding a bike, your body should be situated as follows:

When the pedals are in their lowest positions, your legs should be almost completely extended.

Notice the bold text? That's because it's important. It allows for efficient pedaling (why waste energy?) and protects your knees from undue strain.

"So, keeping that in mind, what frame size would be right for me?"

A 16" frame is most suitable for riders ranging from 5' to 5'8" in height.
An 18" frame is most suitable for riders ranging from 5'7" to 6'1" in height.
A 20" or 21" frame works for riders 6' and up.

Keep in mind, these ranges are just approximations. You'll find varying ranges listed elsewhere, as there is no set standard. In reality, no matter your height, people have different builds. Two people of the same height can have drastically different proportions: one might have long legs, while the other might have shorter legs. To be as accurate as possible, you'll want to measure your inseam (standing up, along the inside of your leg from your crotch to the floor.)

"Ok, I've taken my measurements. Now how do I translate that to the bike?"

The frame measurements (16", 18", 20", 21" or what have you) measure the vertical seat tube (not to be confused with the adjustable seat post) on the main frame. The measurement begins from the bottom bracket (the cylindrical part of the lower frame in which the pedal cranks are affixed) and goes up to the top of the seat tube, the point at which the seat post enters the main frame.

Another point of reference would be the following: When standing in front of the seat, the highest point of your inseam should be at least one inch above the top tube of the frame.

Note: The frame measurements are not to be confused with the wheel sizes. Wheel sizes can come in at 16", 18", 20" (most BMX's), 24", 26" (the standard wheel size in the U.S.), and 700C/27" (often found on racing bikes and European models.) The wheel measurements are independent of the frame size; when a bike is labelled as an 18" bike, the measurement is referring to the frame size specifically.