Alcohol by Volume (ABV) is a way of measuring alcohol content.
Out of a certain amount of liquid, measured by volume (as opposed to weight), what portion of that liquid is actually alcohol? You apply this formula to see how much alcohol there is.
(Original Gravity – Final Gravity) * 131 = Alcohol by Volume
(1.100 – 1.005) * 131 = 12.45%
The “131” is the magic number makes the formula work.
To convert to Alcohol by Weight, multiply by 0.80 (actually, .7608.)