How to Measure a Bike Frame – Terminology

Understanding how to measure a bike frame is easier if you know the terminology.

Finding what size bike do I need is easier if you understanding how bike frames are measured.

Let’s define the parts of a bike frame:How to Measure a Bike Frame - Terminology Diagram

  1. Fork: Connects the front wheel to the frame.
  2. Head Tube: the vertical frame tube at the very front of the bike
  3. Top Tube: the horizontal tube from the handlebar area to the seat area
  4. Down Tube: the tube between the head tube and the bottom bracket
  5. Bottom Bracket: the short, stubby horizontal tube at the pedal area
  6. Seat Tube: the vertical tube from the bottom bracket to the seat area
  7. Chain stays: the horizontal frame tubes between the bottom bracket and rear dropout
  8. Seat stays: the frame tubes between the rear dropouts and the seat tube.
  9. Rear Dropouts: the rearmost part of the frame. It’s where the rear wheel attaches to the frame.

With that terminology out of the way, the question remains: how to measure a bike frame?

Whoa! Grab the brakes for a second, because we need to define one more tube before we continue, an imaginary one 😉

ETT (Effective Top Tube): The length, measured horizontally from the center of the top of the head tube to where the seat tube would be if it extended past the top tube.

Can you see why I said imaginary tube? Most, if not all, top tubes slope slightly down toward the rear of the bike, so it’s not an actual tube you can measure. In fact, you probably will be measuring to the seat post (defined in a minute) if you check this distance on your own bike.

Seat post: The tube attached to your seat and inserted into the seat tube. It’s secured by a clamp to allow up and down adjustment.

So, how are bike frames measured? Bike frames are measured by seat tube length, either:

  • Center to Center (often written as C-C): From the Center of the bottom bracket, along the seat tube, to the Center of the top tube.
  • Center to Top (often written as C-T): From the Center of the bottom bracket, along the seat tube, to the Top of the seat tube.

What units of measure are used for bikes?

  • Mountain bikes: whole and half inches, such as: 18″ or 18.5″
  • Road bikes: in whole centimeters, for example: 49cm or 56cm
  • Other bike types: one or both of the above units.

Although bike frames are measured by the length of the seat tube. The role of the ETT is very important, the most important measurement for bike sizing, and will be explained further in a future article.

Okay, so these are the parts of a bike frame, the terms we use when speaking of them, and how to measure a bike frame.

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