A quick-and-dirty method of determining your BAC is to consider one 12-ounce beer (at 4% alcohol-by-volume), one 8-ounce glass of wine (at 12% ABV), or one shot (at 40% ABV) as being the same thing.
The problem with this is that there are all sorts of different beers, of different strengths. Enter the "Standard Beer". One Standard Beer is equivalent to one 12-ounce can of ordinary American or Canadian beer, such as Budweiser or Labatt Blue.
One Standard Beer will increase an ordinary man's BAC by .02% and an ordinary woman's BAC by .03%.
Take, for example, one 24-oz. can of Steel Reserve. At 8.1% ABV, it is twice as strong as a Standard Beer. At 24 ounces, it is also twice as large as a Standard Beer. 2 x 2 = 4. Therefore, one big Steelie is equal to four Standard Beers, and will raise the average man's BAC by .08%.