Bicycle Mechanics - What cassette do I need?

Hey guys, Long time lurker first time poster. I didn't find anything on the search so I apologize if this is a repeat question.. I need a new cassette and don't know which one to get. I have an ultegra 6600 RD and 105 shifters. Any ideas?

Within the limitations of your derailleur - 11-27t max and 29t of chain takeup capacity (the sum of the differences between the largest and smallest chainrings, and the largest and smallest rear sprockets, ie. 39/53 & 11/25 = 14t + 14t or 27t total) - the cassette choice is entirely yours and should be made based on what's right for you and the terrain you're riding in.

If it's hilly and you're not that strong, you'd lean toward a wider cassette, if flat and you don't need low gearing for hills a narrower cassette gives you closer spaced gears.

What's on the bike now and is it satisfactory? If so, just match the gearing. You can use use any model Shimano or SRAM cassette as long as you match the number of cogs you currently have. You said you have 105 shifters but didn't specify the number of "speeds" which could be 8,9 or 10.

Within a given makers line, the higher level, more expensive cassettes (Ultegra and Dura Ace for Shimano) are slightly lighter and prettier than the lower line (105, Tiagra) cassettes but don't work any better.

Within the limitations of your derailleur - 11-27t max and 29t of chain takeup capacity (the sum of the differences between the largest and smallest chainrings, and the largest and smallest rear sprockets, ie. 39/53 & 11/25 = 14t + 14t or 27t total) - the cassette choice is entirely yours and should be made based on what's right for you and the terrain you're riding in.

If it's hilly and you're not that strong, you'd lean toward a wider cassette, if flat and you don't need low gearing for hills a narrower cassette gives you closer spaced gears.

Thanks for your quick answer. I'd prefer something in the middle.. I'm in Arizona so there are not many hills, however there are some climbs I go out of my way to do and I'd like to not die trying to make it up them.

Assuming a 10-speed cassette, my recommendation is Shimano's 12x27 for the most versatility. It is a "straight block" (i.e. one tooth steps between cogs) from 12T to 17T and only 2 tooth steps from 17 to 21, then it has 24 and 27T cogs for hills. Very few riders can really use an 11T smallest cog so the 12T is plenty high.

If you buy a cassette with a bigger cog than you have now, you will probably need a longer chain too.

Awesome, thanks for the help guys =) It's a shame my LBS isn't this helpful

If you buy a cassette with a bigger cog than you have now, you will probably need a longer chain too.
Unless the current chain has very low mileage a new chain is always a good idea when the cassette is changed.