Or, you can count the number of teeth on your rear cassette, this equates to the number of speeds your cassette has- and from this you can search our website for a replacement chain- Either Shimano or a compatible 3rd party alternative. 
(CN) Chain
Or, you can count the number of teeth on your rear cassette, this equates to the number of speeds your cassette has- and from this you can search our website for a replacement chain- Either Shimano or a compatible 3rd party alternative.
Shimano ID Guide
(CN) Chain
(DH) Dynamo Hub
(RT) Disc Rotor
(SH) Shoes
(BB) Bottom Bracket
(BL) Brake Lever
(BR) Disc Brake Calliper
(BR) Rim Brake Calliper
(CS) Cassette
(FC) Crankset
(FD) Front Derailleur
(FH) Rear Hub
(HB) Front Hub
(PD) Pedal
(RD) Rear Derailleur
(SG) Alfine / Nexus Internal Gear Hubs
(SL) Shift Lever
(ST) STI Shifter Lever
(WH) Wheelset