The United States measures alcohol as Blood Alcohol Concentration expressed as a percentage — grams of alcohol per 100 millilitres of blood. A BAC of 0.08% means 0.08 grams per 100 mL, which is the same quantity Europe writes as 0.8 ‰ or 0.8 g/L. The number differs by a factor of ten; the alcohol does not.

How the US writes BAC

Two things about the American figure catch people out. The first is that 0.08% is not 0.08% of your blood. It is a mass-per-volume measure — 0.08 grams of alcohol in every 100 millilitres — not a proportion of blood volume. The second is that the same physical quantity has three common written forms, and all three mean exactly the same thing:

Written as US per se limit Used by
Percent BAC 0.08% United States, Canada, Australia, New Zealand
Grams per litre 0.8 g/L Spain, Portugal, France
Promille (‰) 0.8 ‰ Germany, Poland, the Nordics, the Baltics

The conversion that goes wrong: mg/L is not blood

Converting percent to promille is simple arithmetic — multiply by ten. The mistake happens with milligrams per litre, because in Europe that unit almost always describes breath, not blood.

0.08% BAC is about 0.4 mg/L of breath — not 800 mg/L. The arithmetic that gives 800 is correct for blood (0.8 g/L is 800 mg/L), but no European limit is written that way. Spain sets its limit at 0,5 g/l in blood and 0,25 mg/l in breath: the same offence, two media, and a number 2,000 times apart. Compare a US percentage against a European breath figure without converting media and the US limit appears thousands of times higher than it is.

Breath figures rest on an assumed blood-to-breath ratio, and that ratio is written into law rather than discovered. US statutes fix it at 2,100 to 1 — Texas and Utah both define the offence as grams per 210 litres of breath alongside grams per 100 millilitres of blood, which is the same ratio stated as a volume pair. Spain's pairing of 0,5 g/l with 0,25 mg/l implies 2,000 to 1. The ratios are close, but they are legal conventions, and they differ.

The legal limits, and where they are not uniform

0.08% is not the limit in all fifty states. Utah is 0.05% — the lowest in the country and the only state below 0.08. Any table or article stating a single national figure has simply not accounted for it.

  • 0.08% — drivers 21 and over, in 49 states and the District of Columbia. Congress obtained that uniformity by conditioning federal highway funding on it (23 U.S.C. § 163), not by passing a national DUI law.
  • 0.05% — Utah, which lowered its limit effective 2018. See our Utah page for the detail.
  • 0.04% — commercial drivers, set federally by 49 CFR § 382.201. This one is widely misdescribed: it applies to holders of a commercial driver's licence operating a commercial motor vehicle, which 49 CFR § 383.5 defines as a vehicle over 26,001 pounds, or one designed to carry 16 or more passengers including the driver, or any vehicle carrying placarded hazardous materials. An ordinary taxi is none of those, so taxi drivers are not covered by the federal 0.04% rule.
  • Under 21 — varies by state. The often-quoted 0.02% is a federal ceiling, not a national standard: 23 U.S.C. § 161 withholds highway funds unless a state treats an under-21 driver at 0.02% or above as impaired. States may go lower and several do — California sets 0.01% under Vehicle Code § 23136.

What one standard drink actually contains

A US standard drink is 14 grams — about 0.6 fluid ounces — of pure alcohol. That is a quantity of ethanol, not a size of glass, which is why servings of different drinks look nothing alike:

Drink Serving Typical strength
Regular beer 12 oz (355 mL) 5% ABV
Wine 5 oz (148 mL) 12% ABV
Distilled spirits 1.5 oz (44 mL) 40% ABV

Each of those delivers roughly the same 14 grams. It is also why a poured drink at home is so often more than one standard drink — the measure is the alcohol, not the glass.

"Non-alcoholic" does not mean zero

Under 27 CFR § 7.65(e), a malt beverage may be labelled non-alcoholic only if the label also states that it "contains less than 0.5 percent (or .5%) alcohol by volume". So a non-alcoholic beer can legally contain alcohol — just under that threshold. A separate rule in the same section bars labelling a malt beverage as 0.0% unless it genuinely contains no alcohol.

How BAC is measured

  • Breath. The roadside method, and an estimate rather than a direct reading — it infers blood concentration from breath using the statutory ratio above.
  • Blood. The most direct measurement, used where the result has to hold up evidentially.
  • Urine. Used less often; it lags behind current blood concentration rather than tracking it.

To estimate your own level from what you have drunk, our BAC calculator will give you a figure in either percent or promille — as an approximation, never as permission to drive.

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