From the
Indiana Prevention Resource Center website:
BLOOD ALCOHOL LEVELS
Conversion Factors for Alternative Measurements
The most common system for measuring and reporting Blood Alcohol Levels (BAL) is calculated using the weight of alcohol (milligrams) and the volume of blood (deciliter). This yields a Blood Alcohol Concentration that can be expressed as a percentage (ie. 0.10% alcohol by volume), or as a proportion (ie. 100 mg. per deciliter). This system is the one prescribed by almost every state, and is sometimes referred to as the "weight by volume" or "w/v" method.
A few states prescribe a "weight by weight" or "w/w" method (milligrams of alcohol in milligrams of blood). For forensic evidence purposes, the w/w can be calculated by dividing the w/v by a factor or 1.055 (the average specific gravity of blood).
Although most evidentiary rules specify that the proportion of alcohol in the whole blood be used to measure the level of intoxication, most hospitals routinely calculate the proportion of a drug in the plasma portion of the blood. While this measurement is the most useful one for medical purposes, it is not the appropriate one for most legal purposes. Since the addition of red and white blood cells would add to the blood volume that dilutes the alcohol, using only the plasma fraction would result in a percentage of alcohol that is higher than would be found in whole blood. Although there is some variation from person to person, depending upon the density of their bloods, the average adjustment factor is 1.16. In other words, to estimate the alcohol level in whole blood using the alcohol level in blood plasma, divide by 1.16. For example, a plasma alcohol level of 0.15% would convert to a blood alcohol level of 0.13%.
Comparison of Blood Alcohol Level Reporting Systems Six different methods of reporting the same blood alcohol measurement.
Code:
Method of Stating the Measurement Result
Blood Alcohol Concentration (percent weight to volume) 0.10% Milligrams per milliliter 1.0 Milligram percent (milligrams per 100 ml.) 100 parts per million 1000 Percent by weight (weight by weight) 0.09%