Chain lenght

I am rebuilding an old bike (7 speed STI)...it doesn't have a chain, how do I figure out what the chain lenght should be without being able to compare it to the old one???

Thanks....

Jay Krause
Almost done, and my wife still doesn't understand...

 

Put the chain on the big ring and smallest sprocket(cog) and remove enough links so that the derailleurs top jockey wheel is dead straight above the bottom one. Another method is put the chain on the big ring and biggest sprocket, and remove enough links so the rear derailleur is almost at full stretch.

 

better way is big/big without going thru RD,pull chain till it overlaps and is tight and then add 1"