It's important to note that in the U.S., proof doesn't equal alcohol by volume. Let's say the vodka you pick off the shelf ...

An 80-proof bottle of vodka is 40% alcohol by volume (ABV), with the rest being water (unless flavors or sweeteners have been added post-distillation). Let it sit long enough and you end up with ...

For example: 80-proof vodka (40% alcohol; the most common type) has 64 calories per 1oz 86-proof vodka (43% alcohol) has 70 calories/1 oz 90-proof vodka (45% alcohol) has 73 calories/1 oz 100 ...