Bike Fit Basics: Frame Size

I cringe when I hear a well meaning person say “Oh, my friend is the same height as me so I definitely need the same frame size” or “I fit on a 54cm <insert random bike brand> so that’s what frame size I need in <some other bike brand>.” I’m here to tell you that a nominal frame size means almost NOTHING these days. Sorry.

There is SO MUCH to know about bike sizing and it can feel really intimidating when you’re a new cyclist. You desperately want a bike that you’ll love, and there are numbers being thrown all about. Ultimately what matters is finding a bike that you feel fits YOUR body and riding style. But, there ARE some things you can read up on that will make it easier to find that bike. I’ll focus this series on “drop bar” bikes, but the concepts apply to mountain, fat, cruisers, etc.

Even if you focus on nothing else, paying attention to standover distance and top tube lengths on bike geometry charts will help you hone into the “frame size” you should try in any given model.

What are the sizing basics you should know before making a major purchase decision? I’ll attempt to tackle a few of the high points in this blog post series, wrapping up with a final post including links to some of the better articles I’ve read recently. I am not an expert, but I’ve learned a ton buying many drop bar bikes over the last six years (8 bikes total, if I count tandems!). Reading something on the internet can’t replace a one-on-one bike fitting experience with a master, but a little background knowledge will help you better advocate for what you need.

What size bike do I need?

Want to know a secret? The “Frame Size” written on a bike is pretty much a made up number. Seriously. I have drop bar bikes in different brands ranging in size from a 44 cm to a 50 cm, and all fit my body well. If you want, read the next few paragraphs (paraphrased from an in-depth discussion with my “bike shop boyfriend”) for why frame size doesn’t matter. Else, skip to the next section where I talk about what numbers I DO focus on when buying a bike.

When bikes were more standard, manufacturers used to measure from the bottom bracket to the top of the seat tube, or to the top of the top tube. In the way-before, almost all frames had a top tube that went straight across, so this measurement gave you a rough idea of size across all brands. Now, with the advent of compact frames, sloped top tubes, and bikes designed for “women” with a shorter reach, “frame size” isn’t meaningful anymore.

Today, manufacturers guess at a frame size a few different ways. Some simplify by going with a small, medium, large system. Others still try to use centimeter measurements. Some of those times, the measurement will be the actual seat tube length. Other times, they will estimate the size of the frame as if it wasn’t a compact frame. What IS standard is the measurement of an imaginary top tube that is completely horizontal, or the “effective top tube” measurement.


Project - Drawing 11755054883 - Edited
This is my attempt to show how two people of approximately the same height can have drastically different proportions. It’s likely that these two folks are not riding the exact same size of bike.



Ok, so now that I just killed your dream of picking a frame size and sticking with it, what CAN you count on when sizing a bike? Standover is one of the easiest measurements to grasp. Can you comfortably “stand over” your bike with your feet flat on the ground without any of your bits getting smashed into metal? Can you do this with cycling shoes and shorts on? Will you be doing any “emergency dismounts” such that you may need extra clearance in case things get gnarly? Despite being the same height, the stick figure on the right needs a much lower standover measurement than the leggy stick figure on the left.

Once you know how much space you need you can evaluate bike size charts accordingly.  Warning: different manufacturers measure “standover” at different points along the top tube. Not all measurements are equivalent – with sloped top tubes, it really depends where they measure it. Your best bet is to go physically see the bike OR be conservative if you’re ordering something without test riding it first.

Effective Top Tube Length

Effective top tube length is an imaginary measurement of how long the top tube (the one you straddle when you stand over the bike) would be if it were straight across and completely horizontal. On this bike I drew, the “effective” top tube measurement shown in blue would be identical to the “actual” top tube measurement since I made this bike’s top tube as flat as I could.

This distance gives you a glimpse into how far away the handlebars will be when seated on the bike. Based on the stick figures above, the one with the shorter torso on the left would probably want the bars closer to them than the longer torso individual on the right.

There are other measurements that play into how far away the bars feel when you’re on the bike such as “stack” and “reach”. Plus, there are things you can fiddle with after you’ve selected a frame size — like stem length and pitch, saddle position, etc. I’ll cover these in future posts.

Even if you focus on nothing else, paying attention to standover distance and top tube lengths on bike geometry charts will help you hone into the “frame size” you should try in any given model.

Next Up

Want to read more? I’ve got a series of “Bike Fit Basics” posts planned for the upcoming weeks. Topics for next time will expand on the top tube length, plus introduce “stack” and “reach”. Subscribe to my blog so you get an email straight to your inbox when the next post comes out. Scroll down to the bottom and enter your email address!

8 thoughts on “Bike Fit Basics: Frame Size

  1. Shawn says:

    I’m still learning. I have two bikes both are Trek “size 52”. One is Emonda and other is Domane. They have different geometry for sure. I’m going to look at and note the different measurements and compare with my fit measurements. IM actually thinking of getting fit done again

  2. mel says:

    Can’t wait for the next post! This definitely leaves me wanting more! I have a Kona Jake that’s the right standover height for me, but something is wrong with the fit. Not sure if it’s reach (I recently converted it to a straight bar from drops), the saddle, the stack, or what. After about an hour on the bike, the toes on my left foot start to go numb and tingly. After 2 or 3 hours, by butt hurts so bad I can barely walk. As soon as I can get back on my bike I plan on having a professional bike fitting done so I can ride it comfortably for my first long gravel ride ever (100miles!!!). Thanks for keeping your blog alive for us!! Your story is inspiring and you have brought a lot of value to my biking experiences over the last year or so.

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