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.

What Size Bike Do I Need – Why the Correct Size is VITAL!

Before buying a bike, ask: what size bike do I need? Why?

bike shopThe year was 1993, when I was 14 years old and bike crazy. A bike meant more than a way to get around, the road passing by and wind in my hair was freedom! I had saved nearly every penny from my chores and lawn mowing route.

Finally, I had enough to buy a new bike. Early Saturday afternoon, I remember the nervous anxiety and the cash in my pocket as my dad drove to the local shop. I had studied every book and magazine and was ready to buy.

At the shop, the shiny bikes and smell of tires was amazing. I chose an upgraded model on sale that would have been out of my price range ordinarily. It seemed a tad large, but…it was on sale. The sales associate asked me to “stand over the top tube so we can see if it’s the correct size”. He assured me “it looks great!”. I asked if it was too big and he said “uh…you may need to grow into it…a little”.

Before I knew it, money changed hands and the bike and I were headed home. I didn’t realize it at the time, but that would be the most uncomfortable bike. Was the seat too hard? No, the bike was far too large for me. So, who was at fault?

The sales associate? He may have genuinely thought that it was the right bike for me, I can’t judge that; however, I do know now that most bike store employees are not trained in correct bike sizing and some earn sales commissions. Not the ideal recipe for a good bike fit, especially if you need good advice. In addition, the economy has caused many shops to cut staff and as a result cannot provide the service they did in the past, including bike fit sessions.

Looking squarely in the mirror, I had studied up on bikes, including bike sizing, beforehand. But, I got greedy for the shiny new bike on sale, one that I wouldn’t have been able to afford otherwise and didn’t listen to my inner voice warning me that the size was wrong. I was anxious to buy a bike, wanted that one that day, and caved in. Yes, I’d say now that it was my fault when I should have and did know better. I paid for it. Never really enjoyed that bike and eventually sold it. Since then, I have found the right size bike to be very comfortable.

I’ve ridden thousands of miles in comfort, and so can you! Only if the bike is the size you need!

To be honest, finding your new bike can be very intimidating!

Learning about the wide variety of bike types, number of speeds, wheel sizes, etc takes time and research. Does it seem like speaking another language? When you buy your next bike, the type, speeds, wheel size, etc are important factors, but most people just want to get out and ride! So, matching your body to the correct size is of the utmost importance.

The reason is comfort.

Bike fits determines comfort and comfort determines whether you will ride it regularly.  Many people want to bike for exercise. Let’s face it, exercise is difficult enough already! You don’t need an uncomfortable bike discouraging you further.  I look forward to riding and you should too.  A good fit will do that. Use the resources on this site to determine for yourself: what size bike do I need?

Edit: Check out this informative post on setting up a spin bike correctly, just had to share:

Skippy @ What Size Bike Do I Need