Blood alcohol concentration is expressed in grams of alcohol per 100ml of blood.

Twelve oz. of 5% beer, 5 oz. of 12% wine, or 1.5 oz. of 40% (80 proof) liquor contains 0.6 fluid ounces of alcohol.  Since one fluid ounce is 30ml, and alcohol weighs 0.79g per ml, one "typical" drink contains 14g of alcohol.

To convert body weight in pounds to volume in milliliters, divide by 2.2 to convert to kilograms, then multiply by 1000 to get milliliters.  Since alcohol is distributed in body water, the volume of distribution is further reduced to the body water percentage; average figures are 58% for men, 49% for women.

Since blood is about 80% water, the calculated water alcohol concentration is further reduced to this amount in the blood.  Multiply the whole by 100 to express the result in grams per 100ml.

Alcohol is metabolized at a fairly constant rate in a given person.  An average figure for the reduction in BAC per hour is 0.015; a conservative figure to leave a margin of error is 0.012.

The bottom line:  divide the number of drinks by weight in pounds, multiply this by 4.2 in men or 5 in women, then subtract 0.012 per hour from the result.

These values are approximate and are affected by numerous physiologic factors.
There is no safe blood alcohol concentration.