Chain cassettes

[buddiegolf]

Hi
Can anyone tell me how to know what ratio means in relation to chain cassettes example 31-21 etc and how can I tell what mine is as I wish to replace it.
Thanks

[cyclerUK]

To get a new cassette you need to know:-
1) Manufacturer (Shimano, Campagnolo, Sram etc,?)
2) The number of sprockets (5,6,7,8,9,10 or 11)
3) The ratio (count the number of teeth on the smallest sprocket, count the number of teeth on the largest sprocket ie. 12/28 )
And that's it.
So as an example = Shimano, 7speed, 12/28.