Bike sizing

Bike sizing nomenclature

Bike sizing nomenclature is used throughout the cycling industry to help riders choose a proper fitting bicycle. Mountain bikes typically use standard sizing terms such as small, medium, or large which isn't based on any specific measurement of the bicycle frame. Some mountain bikes, generally low-end bikes, use imperial measurements based on the seat tube length to identify the bike size such as 17", 19" or 21". Road bikes typically use a metric measurement based on the seat tube length to identify the bike size such as 54cm, 56cm, or 58cm. Some road bikes used standardized terms such as small, medium, or large. 

Bike Geometry Terms and Sizing

Terms to consult when sizing a bike

A) Seat Tube Length:  Most manufacturers measure seat tube length from the center of the bottom bracket to the top of the seat tube. In road cycling, this measurement indicates frame size. For example, a 56cm road bike will have a 56cm seat tube.

B) Effective Top Tube Length:  The effective top tube is measured by a horizontal line from the top of the head tube to the seat post. Since seat tube angles on modern mountain and road bikes vary quite a lot, effective top tube length isn’t as accurate of a measurement for determining bike size as it used to be. With modern bikes, the reach and stack measurements are the best numbers to use when determining the size of a bike.

C) Stack:  Stack is the vertical distance from the center of the bottom bracket to the top of the headtube. The true stack height of a bike will be increased by spacers under the stem and the rise of the stem and/or handlebars. In layman’s terms, stack is how tall the bike feels. The lower the stack number, the more bent over the handlebars you will become.

D) Reach:  Reach is measured from the center of the head tube to the center of the bottom bracket on a horizontal plane. Many consider reach to be the most important measurement when determining the size of a bike. In layman’s terms, reach is how long a bike feels. As reach increases, the distance from your stance to the handlebars increases. Unlike top tube length, the reach measurement is standardized across all bikes. This is the best measurement to uses when comparing bike sizes regardless of brand or model.

STEPS for sizing your next bike

Follow these steps to find your perfect bike size

1) Reference the size guide:  All bike companies have a size guide for their bikes. These guides are a great place to start when determining which size of bike to purchase. The size chart will show the range of heights the company recommends for each size of their bikes. The size chart is a great place to start when sizing a bike, but it shouldn't be your only stop.

2) Look at the reach number:  The reach number is arguably the most important measurement to look at when sizing a bike. The reach number is a standardized measurement across all bicycles. When comparing the size of one bike to the size of another bike from another brand, one should always consult the reach number. It will offer the most accurate comparison between the two bikes. Once you have ridden several bikes, you will learn which reach number works best for you and your riding style.

3) Look at the stack number:  Just like reach, the stack measurement is also standardized across all bikes. It's really only important to look at the stack number if you have a point of reference. If you would like a higher or lower front end than a bike you previously had, then use the stack number of both bikes to figure out which one is taller. It's also important to remember the "effective" stack of a bike can be greatly changed with the rise of the stem and/or handlebars.

Generic Sizing Guide

As mentioned above, the best place to start your sizing journey is with a size chart from the bike brand you intend on purchasing. The size chart below is a generic size chart based on certain measurements from a geometry chart. This size chart does not contain the standard nomenclature of small, medium, and large because each bike brand has a different definition of what a "large" is. Again, analyzing the geometry chart of a bike is the best way to understand the size of it. Additionally, every rider has a personal preference of what their "ideal" frame size is. 

Rider Height (in) 4' 10" - 5' 2" 5' 3" - 5' 6" 5' 7" - 5' 10" 5' 11" - 6' 1" 6' 2" - 6' 4" 6' 4" +
Rider Height (cm) 148cm - 158cm 159cm - 168cm 169cm - 178cm 179cm - 185cm 186cm - 193cm 194cm +
Reach Below 420mm 420mm - 440mm 440mm - 460mm 460mm - 480mm 480mm - 500mm Above 500mm
Seattube Length (in) 13" - 14" 15" - 16" 17" - 18" 19" - 20" 21" - 22" 23" +
Seattube Length (cm) 33cm - 37cm 38cm - 42cm 43cm - 47cm 48cm - 52cm 53cm - 57cm 58cm +