The concentration of alcohol (ethanol) present in blood. It is usually expressed as mass per unit volume, but different countries may express it differently or use different units; examples include milligrams per 100 millilitres (mg/100 ml or, incorrectly, mg percent), milligrams per litre (mg/1), grams per 100 millilitres (g/100 ml), grams percent, and millimoles per litre. A concentration of 8 parts per thousand would be expressed in legal terminology in USA as .08%, in Scandinavia as 0.8 promille, and in Canada and elsewhere as 80 mg/100 ml. National differences also exist in the BAL set as the legal limit for driving under "per se" laws, with most limits in the range 50-100 mg/100 ml.

The BAL is often extrapolated from measurements made on breath or urine or other biological fluids in which the alcohol concentration bears a known relationship to that in the blood. The Widmark calculation is a technique for estimating BAL at a given time after alcohol ingestion by extrapolating from BALs at known times and assuming a fixed rate of alcohol elimination (zero order kinetics). In some jurisdictions this is considered a dubious assumption, and estimates of BALs at previous points in time are not accepted.

For information on blood alcohol levels and driving in Ireland see citizensinformation.ie-drink driving offences