Originally Posted by
fietsbob
Count the teeth in the rear cluster of cogs, each, and the chainrings in front.
simple math gives you the gear ratio.. that will tell you the relative size of each gear combination.
Sheldon Brown's Bicycle Gear Calculator this is a math converter you can input the data into.
Knowing the gear inches or any other measurement is good information, but not a requirement to knowing if you are in "THE RIGHT GEAR" for current conditions.
The ease or difficulty of pedaling you physically experience combined with which gear (terms for front and rear toothed gears is chainring=front & cog=rear) the chain is running will simply help you to get in the zone that gives you the greatest benefits (moving forward) with the least / most comfortable amount effort.
Example of shifting into the best gear(s) ahead of making a full stop (you cannot shift without pedaling):
1). Front on largest gear and rear on largest gear. (That causes "cross chaining" which is harder on the equipment and is not advised)
2). Front on smallest changing and rear on largest (Easiest to pedal combo) or somewhere in the middle in the rear. If you have it on the smallest in the front and smallest in the rear you are again cross chaining. Again not the best combination.
As you put more time riding selecting the best gear combination will quickly become engrained and second nature for you.