Most of the world measures alcohol as a percent of volume (abv). In
     the U.S., alcohol in beer is measured by weight (abw). Since alcohol
     weighs roughly 20% less than water, abw measures appear 20% less than
     abv measures for the same amount of alcohol. In Europe, beer strength
     tends to be measured on the basis of the fermentables in the wort.

     Until recently, Britain used OG (original gravity), which is 1000
     times the ratio of the wort gravity to that of water. Thus a beer
     with an OG of 1040 was 4% more dense than water, the density coming
     from dissolved sugars. You can generally take one tenth of the last
     two digits to estimate the percentage alcohol by volume once the
     dissolved sugars are fermented. In the example used, the abv would be
     approximately 4% (40/10 = 4%)  Currently, British beer is being taxed
     on its actual %ABV rather that the older OG so you'll often find both
     displayed.

     Continental Europe tends to uses degrees Plato. In general, the
     degrees Plato are about one quarter the last two digits of the OG
     figure. Hence, in our example above, the beer would be 10 degrees
     Plato. To get the expected alcohol by volume, divide the degrees
     Plato by 2.5.