Alcoholic proof and temperature The US defines the "proof" of an alcoholic beverage to be twice the alcohol per volume percentage at 60° F (about 15.5° C)

As someone who does not have a chemistry background, I am wondering what the 60°F requirement means.

Given a mixture of alcohol and water that is 80 proof (40% alcohol by volume at 15.5°C), what is the alcohol by volume at room temperature 20°C?
Briefly, since ethyl alcohol expands more rapidly with increasing temperature than does water, then measuring the mix by volume at a warmer temperature would give you less alcohol and more water, because the alcohol would be less by weight for the same volume.

Thermal Coefficient of Expansion (approximate, measured at room temperature)

Alcohol, ethyl (ethanol):   0.00109 / K

Water:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;0.00021 / K

Using these constants, you can figure out the answer to your question. But note the wording: given a mixture. If the EtOH/H2O is already mixed, changing the temperature does not change the concentration or proof, and you would have to separate the ingredients to measure their respective volumes. BTW the volume of the mix is slightly less than the sum of the volumes of its ingredients.

Moral: if your measuring your drinks by shot-glass rather than with a scale, you'll get a weaker drink in a warm bar ;-)