[4] The record was set in July 2013 by an unidentified Polish man found unconscious by the side of a road in the village of Tarnowska Wola, in south-east Poland.

First responders reportedly did not believe the initial BAC readings taken at the scene, possibly due to it being almost 69 times greater than the Polish legal limit of 0.02% (0.2 g/L).

[4] BAC is generally defined as a fraction of weight of alcohol per volume of blood, with an SI coherent derived unit of kg/m3 or equivalently grams per liter (g/L).

Sweden, Denmark, Norway, Finland, Germany, and Switzerland use mass-mass concentrations in their laws,[6] but this distinction is often skipped over in public materials,[12] implicitly assuming that 1 L of blood weighs 1 kg.

[19] Forensic laboratories typically use headspace-gas chromatography combined with mass spectrometry or flame ionization detection,[20] as this method is accurate and efficient.

[25] Blood alcohol content can be quickly estimated by a model developed by Swedish professor Erik Widmark in the 1920s.

The model is most accurate when used to estimate BAC a few hours after drinking a single dose of alcohol in a fasted state, and can be within 20% CV of the true value.

[29][6] The equation varies depending on the units and approximations used, but in its simplest form is given by:[30] where: A standard drink, defined by the WHO as 10 grams of pure alcohol,[35] is the most frequently used measure in many countries.

This can make it difficult to conclusively determine the blood alcohol content in autopsies, particularly in bodies recovered from water.

[40][41][42][43] For instance, following the 1975 Moorgate tube crash, the driver's kidneys had a blood alcohol concentration of 80 mg/100 mL, but it could not be established how much of this could be attributed to natural decomposition.

[44] Newer research has shown that vitreous (eye) fluid provides an accurate estimate of blood alcohol concentration that is less subject to the effects of decomposition or contamination.

[45] For purposes of law enforcement, blood alcohol content is used to define intoxication and provides a rough measure of impairment.

Although the degree of impairment may vary among individuals with the same blood alcohol content, it can be measured objectively and is therefore legally useful and difficult to contest in court.

Most countries forbid operation of motor vehicles and heavy machinery above prescribed levels of blood alcohol content.

Retrograde extrapolation is the mathematical process by which someone's blood alcohol concentration at the time of driving is estimated by projecting backwards from a later chemical test.

In an increasing number of states, laws have been enacted to facilitate this speculative task: the blood alcohol content at the time of driving is legally presumed to be the same as when later tested.

Although subject to the same infirmities as retrograde extrapolation—guessing based upon averages and unknown variables—this can be relevant in estimating BAC when driving and/or corroborating or contradicting the results of a later chemical test.