In order to convert alcohol measured in the breath to estimates of alcohol in the blood, Breathalyzers assume that the subject being tested has a 2100-to-1 partition ratio. If the instrument estimates the BAC, it will effectively measure grammes of alcohol per 2100 ml of breath given because it measures weight of alcohol to volume of breath. This metric is proportional to the number of grammes of alcohol in one millilitre of blood. As a result, the ratio of alcohol in blood to alcohol in breath is 2100-to-1. However, the assumed partition ratio varies between individuals and within a given individual over time, ranging from 1300:1 to 3100:1. Assuming a true (and US legal) blood-alcohol concentration of 0.07%, for example, a person with a partition ratio of 1500:1 would have a breath test reading of 0.10%—over the legal limit.
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