OK, Peace. I did a search at http://www.brittanica.com
In Great Britain, the Customs and Excise Act of 1952, declared proof spirits (100 proof ) to be those in which the weight of the spirits is 12/13 the weight of an equal volume of distilled water at 51º F (11º C). Thus, proof spirits are 48.24 percent alcohol by weight or 57.06 percent by volume. Other spirits are designated over or under proof, with the percentage of variance noted. In the United States, a proof spirit (100 proof) is one containing 50 percent alcohol by volume.
Now we all know. It’s the weight of the same weight of an equal amount of distilled water at 51 degrees F and I guess the specific gravity of vodka is around 2.