Go here and run the scanner.
It will show what fits your machine.
If your machine can accept 2 sticks it's best to have a matched pair.
I have an HP laptop (linked below) and I want to add 4GB of RAM. I know I need a DDR3L 204-pin SO-DIMM running at 1600mhz, but I don't know the voltage or the model of the current stick I have. I don't want to open my laptop to check either because it is a pain and I only want to do it once. Any help is greatly appreciated.
RAM I was looking at buying: http://www.ebay.com/itm/Hynix-4GB-DDR3-Memory-SO-DIMM-204pin-PC3L-12800S-1600MHz-HMT451S6BFR8A-PB-/142385442073?hash=item2126d56d19:g:7FwAAOSwcgNZGwjK#rwid