Alcohol by Weight (ABW) is often used in the United States for indicating the alcohol content of beer (while Alcohol by Volume is still preferred for wine.) This is somewhat ironic, considering that Americans insist on measuring everything else in God’s creation from flour to macaroni by volume, but never mind.
Alcohol by Weight gives lower percentage numbers, because alcohol weighs only 79.6% as much as water. The number will look lower than that given by ABV; this makes American beers look lighter in alcohol than they really are.
The starting and finishing gravities are used for the calculations:
76.08 * (Original Gravity – Final Gravity) / (1.775 – Original Gravity) = Alcohol by Weight
When given ABW numbers, to convert to alcohol by volume, multiply by 1.25.