In most of the world alcohol content is measured by volume. This gives us the familiar 12.5% on the side of a wine bottle. But what does this percentage really mean?
The alcohol in alcohol is ethanol, and it's ethanol that diffuses into our cells and inhibits our neuronal functions (makes us drunk). Most vodka is 40% alcohol by volume (ABV), meaning that in a normal-sized 750 mL bottle of Vodka, 300 of those milliliters are ethanol, and the other 450 mL are water.
More interesting though is alcohol by weight (ABW), which is preferred by some states. 300 mL of ethanol is about 237 g. So if your average gin and tonic is made with 60 mL of gin, you’re only drinking 24 mL of ethanol or 19 g. The rest is just H2O.
But if your gin has more water than ethanol, why are drinkers always dehydrated? It turns out that ethanol inhibits the production of antidiuretic hormone, whose function is to instruct your kidneys to reabsorb excess water from urine. With this hormone’s production suppressed, your kidneys pass too much water into your urine, and you’re left dehydrated, and constantly running to the bathroom.