Gear Ratio Calculation: bicycle gear range and rollouts

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Gear ratio calculation and its development in meters (also known as roll-out), together with the gear range are often bypassed, especially by beginenrs or amateurs, when choosing the right bicycle. If you don't know what I'm talking about and you want some answers, take a look below at some information and the calculation tool.


What is the gear ratio calculation in a bicycle?

The combination of your chainring and cog determine the gear ratio. Gear ratio calculation, combined with the circumference of your wheel and tyre, determines how far you will travel with each revolution of the cranks. 

This data is objective and not influenced by external conditions, such as friction, slope, weather conditions and cyclist's training condition or other external factors influencing the ride, which are not taken into consideration.

Gear ratio calculation and its consequent data, roll-outs, may help understanding how agile our ride is with that specific gear combination. 

The lower a gear ratio and the roll-out are, the more agile the ride will be and the speed of the rider. Gear ratio calculation roll out

How can you obtain your gear ratio?

Basically, we can obtain gear ratio calculation (GR) and the final roll-out of a specific gear (S) with a simple 4-factors mathematic formula:

  • number of the teeth on the cog (NTc)
  • number of the teeth on the chainring (NTp)
  • the size of the rim
  • the width of the wheel

Gear ratio calculation is the first step:

GR = (NTc / NTp)

The last two elements listed above are used to determine the roll-out of specific gear. In order to obtain the final data, you will need the circumference of the wheel (measured in meters, Cr), which you can obtain (2πr) after having measured the radius of the wheel system (rim + wheel).

The final formula for the roll-out is:

S = GR x Cr

Roll-out and gear range: the higher, the better

Gear range is a percent value allowing you the right choice depending on your riding style.

It is calculated by dividing the number of teeth of the biggest cog by the number of the teeth on the smallest; then repeat the same operation with the chainring. By multiplying the two results and the again x100, you'll obtain your gear range. Our Trek 920, one of the 2021 Trek bicycle touring bikes, has an 11x36 cassette, and a 42x28 chainring.

Its gear range will be: (36/11) 3,27 x (42/28) 1,5 x 100 = 491%.

The higher the range is, the more differentiated the roll-outs will be (from the hardest and the lightest gears): this means better adaptability to different terrains and cyclist's condition.

Pay attention, though. This range is a comparison between the smallest and the biggest gears while, especially on fully loaded touring bicycles it is very important to have a good range in the middle gears, to be used in every hard situation.

Online gear ratio calculation (and roll-out)

Down below you can play with our tool to obtain your gear ratio calculation and its consequent roll-out

[Form meter_of_development not found!]

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Francesco G

ITA - Ho 33 anni e sono piemontese, anche se da qualche anno vivo e lavoro in Lombardia. Dopo un inizio da totale inesperto in questo campo, mi sono avvicinato al mondo dei cicloviaggi e della bicicletta sempre più. Oggi posso definirmi "cicloviaggiatore", e assieme all'altra mia passione - il videomaking - non mi fermerei mai! Cyclo ergo sum, pedalo quindi sono, per cercare di capire perché andare in bici sia così bello, terapeutico, ricco... E ogni volta che provo a capirlo, non ce la faccio, e sono costretto a ripartire sui pedali!

ENG - I'm from Piedmont and I'm 33 years old, I have been living and working in Lombardy for a few years. After a start without any competence in this field, I then approached the bicycle world more and more. Today I can call myself a bicycle traveller and videomaker who would never ever stop. Cyclo ergo sum, I cycle therefore I am. I ride my bike trying to understand why it is so beautiful, rich, therapeutic. And every time I try, I do not understand it. So I must leave again...