I have the 21oz and it weighs in at 12oz empty. But it is a very good hydroflask. I wouldnt so much call it a water bottle, as its not plastic. It holds ice for a long time and will keep cold drinks cold. I love mine. I also elected for the additional lid with straw and mouth piece, makes it easier to drink out of, it is well worth the purchase, you will not be disappointed!!
One answer you got makes no sense, if the bottle holds 21 ounces PLUS the weight of the bottle itself how can it weigh as much as a 16 ounce bottle of water? I suppose if you only had 4 ounces of liquid in the bottle will then that answer would be right… Anyway the real answer is the bottle itself weighs 12 ounces empty, add 21 ounces of liquid and it now weighs 33 ounces, of course it gets lighter from there as you drink the liquid…LOL!!