How to calculate the capacity of a rear derailleur It's easy to calculate what capacity of a rear derailleur will be needed based on the size of the big and small chainring and the big and small cog. What about the converse question, how to calculate the capacity of a derailleur?

I was comparing some short-cage rear derailleurs I have lying around, and I noticed that the cage length and pulley sizes varied enough that they probably had different capacities.

Related question: is there a fairly comprehensive place to look these up?
If you are asking how to calculate the maximum capacity of the chainrings and cogs, based on looking at the derailleur, then it's not going to be as easy as just looking at them.

Finding a derailleur to fit your chainrings/cogs based on just the chainrings/cogs is going to be a lot easier than finding chainrings based on looking at your derailleur. HOWEVER, there are some standards for manufacturer, I have listed them below. I have also included the formula to find out your capacities, since I misread your question initially and decided to answer the part you weren't concerned with. I am not deleting it because, well, it took a while to type.

According to United Bicycle Institute:


Determine Maximum Chainring Difference by subracting the number of teeth in the smallest chainring from the number of teeth in the largest chainring
Determine Maximum Cassette Cog Difference by subtracting the number of teeth on the smallest cassette cog from the number of teeth on the largest cassette cog
Determine Total Drivetrain Capacity by adding Maximum Chainring Difference to the Maximum Cassette Cog Difference
Record the Maximum Cassette Cog (the number of teeth on the largest Cassette Cog)


For Shimano:

SS - Short Cage Road Double - Maximum Cassette Cog is 27 and Total Capacity is 29

GS - Medium Cage MTB/Road Triple - Maximum Cassette Cog is 34(MTB)/27(Road) and Total Capacity is 33(MTB)/37(Road)

SGS - Long MTB - Maximum Cassette Cog is 34 and Total Capacity is 45

For SRAM:

Short - Maximum Cassette Cog is 34(MTB)/28(Road) and Total Capacity is 32(MTB)/31(Road)

Medium - Maximum Cassette Cog is 34 and Total Capacity is 37

Long - Maximum Cassette Cog is 34 and Total Capacity is 45

For Campagnolo:

Short - Maximum Cassette Cog is 26 and Total Capacity is 27

Medium - Maximum Cassette Cog is 29 and Total Capacity is 36

Long - Maximum Cassette Cog is 29 and Total Capacity is 39

NOTE - THIS INFORMATION IS SUBJECT TO CHANGE BY MANUFACTURER

And a great source for all of this is Sutherland's 7th edition

http://www.sutherlandsbicycle.com/7th_Edition.html

Hope that helps