A blood alcohol concentration of 0.08% is 80 mg/dL, 0.8 promille, and about 17.4 mmol/L: one quantity in four units. The conversions are exact arithmetic, and this guide gives the table and the worked examples so a result from any country, laboratory, or study can be read without misreading it.

What 0.08% actually means

Blood alcohol concentration is a concentration: an amount of alcohol divided by an amount of blood. When a result is written as 0.08%, it means 0.08 grams of alcohol per 100 millilitres of blood. That is the whole definition, and every other unit in this guide is the same quantity restated. Percent is the unit of American law enforcement, American media, and the Widmark formula used by the calculator on the BAC Calculator page. It is not the unit of most laboratories, most of the rest of the world, or most medical literature, which is why one number arrives in four costumes and causes so much confusion.

The confusion is practical, not academic. A traveller reads a limit of 0.5 promille and wonders whether it matches the 0.08% they know from home. A laboratory report shows 80 mg/dL and a reader wonders whether that is high, low, or the same as the legal limit. A research paper quotes 17 mmol/L and nobody at the table knows whether to worry. All three describe, or nearly describe, the same concentration, and the conversions between them are exact arithmetic, not judgement. This guide gives the definitions, the conversion table, three worked conversions in both directions, and the reading rules that stop a foreign result being misread. It gives no driving advice beyond the one rule that matters: the only BAC that is safe for driving is 0.00. A converted number is never a decision to drive.

The four units, side by side

Percent, written with the % sign, is grams of alcohol per 100 millilitres of blood. Milligrams per decilitre, mg/dL, is milligrams per 100 millilitres, which is the same volume as percent uses, so the conversion is a clean factor of one thousand: 0.08% equals 80 mg/dL. Promille, written with the sign, is grams per litre of blood, ten times the volume of percent, so the conversion is a factor of ten: 0.08% equals 0.8 promille. Millimoles per litre, mmol/L, counts molecules rather than mass: divide grams per litre by the molar mass of ethanol, 46.07 grams per mole, and multiply by one thousand. That gives 0.08% equals about 17.4 mmol/L. The table below lists the same concentrations in all four units, so any result can be found in its own row and read across.

Three conversion rules cover every case. To go from percent to mg/dL, multiply by 1,000. To go from percent to promille, multiply by 10. To go from percent to mmol/L, multiply by about 217: more precisely, multiply the promille figure by 1,000 and divide by 46.07. To reverse any of them, divide instead of multiplying by the same factor. Keep the direction straight by checking the table: the mg/dL figure is always the largest number, the percent figure is always the smallest, and promille and mmol/L sit in between, with mmol/L roughly twenty times the promille figure. If a conversion produces a mg/dL number smaller than the percent number, the direction is backwards.

Milligrams per decilitre: the laboratory unit

Clinical laboratories in the United States report blood alcohol in mg/dL, and the figure on a hospital report is usually the first one a patient actually sees. Because a decilitre is 100 millilitres, mg/dL and percent share their volume, and the only difference is grams against milligrams. That is why 80 mg/dL and 0.08% are not merely equivalent in effect but identical in quantity, written two ways. The same volume appears in British reporting as milligrams per 100 millilitres, mg per 100 mL, which is the same unit under a different name: 80 mg per 100 mL is 80 mg/dL is 0.08%. Readers who meet all three names in one evening of searching are meeting one unit three times.

The laboratory framing also explains why breath and blood figures are sometimes quoted differently. A breath test measures alcohol in breath and converts it to an equivalent blood figure using a fixed partition ratio, conventionally 2,100 to 1 in the United States, meaning 2,100 millilitres of breath are treated as equivalent to 1 millilitre of blood. The conversion is a convention, and real partition ratios vary between people and within a person, which is one reason a breath figure is an estimate of blood concentration rather than a direct measurement of it. Blood drawn and analysed directly needs no such conversion, which is why blood results are reported in mg/dL without qualification and breath results are usually converted back into percent or mg/dL for legal use. The unit on the report does not tell you which method produced it; the report itself should.

Promille: the European unit

Promille, from the Latin for per thousand, is the standard unit across much of Europe, including Germany, Scandinavia, and many countries of central and eastern Europe. One promille is one gram of alcohol per litre of blood, or equivalently one gram per kilogram of blood under the usual approximation. Because a litre is ten times 100 millilitres, promille figures are ten times the percent figures: the common European limit of 0.5 promille is 0.05%, and the stricter 0.2 promille limit found in some countries for novice or professional drivers is 0.02%. The promille sign looks like a percent sign with an extra zero, and the visual similarity is the trap: 0.5 promille read as 0.5% would overstate the concentration tenfold.

Promille limits vary by country, and the variation is real law, not rounding. Several countries set a general limit of 0.5 promille, some set 0.2 or 0.0 for all drivers, and some apply lower limits to young, novice, or commercial drivers while keeping a higher general limit. This guide does not catalogue limits country by country, because limits change and a guide that prints them risks going stale; the companion guide on BAC limits around the United States covers the American map in detail, and any driver abroad should check the current law of the country they are driving in through an official source before travelling. For conversion purposes, only the factor matters, and it never changes: divide promille by 10 to get percent, multiply percent by 10 to get promille.

Millimoles per litre: the scientific unit

Millimoles per litre counts amount of substance rather than mass. One mole of ethanol weighs 46.07 grams, so one millimole weighs 46.07 milligrams. A concentration of 0.8 grams per litre, which is 0.8 promille and 0.08%, contains 800 milligrams per litre, and 800 divided by 46.07 gives 17.4 millimoles per litre. The unit appears in research papers, in clinical chemistry in countries that use SI units throughout, and in some laboratory systems that report all analytes in molar units for consistency. It is the least intuitive of the four for everyday reading, because the numbers are unfamiliar: 17.4 does not look like a small concentration until you know the scale, and 40 mmol/L, which sounds moderate, is 0.18%, a very high concentration.

The conversion to and from mmol/L is the only one of the four that needs a constant from chemistry rather than a round factor. From percent: multiply by 10 to get grams per litre, multiply by 1,000 to get milligrams per litre, divide by 46.07. In one step, multiply percent by 217.1. From mmol/L back to percent: multiply by 46.07, divide by 1,000, divide by 10, or in one step divide by 217.1. Rounding to one decimal place is enough for reading purposes: 17.4 mmol/L for 0.08%, 10.9 for 0.05%, 21.7 for 0.10%. If a source quotes mmol/L to more precision than that, it is reporting laboratory precision, not a difference that changes any practical reading.

The United Kingdom and other local wordings

The United Kingdom reports blood alcohol as milligrams per 100 millilitres of blood, which is mg/dL under another name, and sets its drink-drive limits in those units: 80 mg per 100 mL in England, Wales, and Northern Ireland, and 50 mg per 100 mL in Scotland. In percent, those are 0.08% and 0.05%. Breath and urine limits are set separately in their own units, micrograms per 100 millilitres of breath and milligrams per 100 millilitres of urine, and the three limits in each jurisdiction are designed to describe the same level of impairment through different samples, not three different levels. A reader converting a Scottish blood result of 50 mg per 100 mL should therefore land on 0.05%, the same figure as the Utah limit in the United States and the general limit in much of Europe expressed as 0.5 promille. Different laws, different units, same concentration.

Australia, Canada, and Ireland add their own local wordings to the same set. Australian reporting commonly uses grams per 100 millilitres, written as a percentage, or milligrams per 100 millilitres in legal contexts. Canadian material often uses milligrams per 100 millilitres as well, with the federal criminal limit at 80 mg per 100 mL. Irish law uses milligrams per 100 millilitres for blood, with a general limit of 50 mg per 100 mL and a lower limit of 20 mg per 100 mL for learner, novice, and professional drivers. The pattern is always the same: find the volume the unit names, convert the mass into grams per 100 millilitres, and the percent figure follows. When a unit names 100 millilitres, milligrams convert to percent by dividing by 1,000. When it names a litre, grams convert to percent by dividing by 10. When it names moles, divide by 217.1 after converting to grams per litre.

Three worked conversions, both directions

Worked example one starts from a European limit. A driver reads a limit of 0.5 promille. Divide by 10 for percent: 0.05%. Multiply percent by 1,000 for mg/dL: 50 mg/dL. Multiply percent by 217.1 for mmol/L: 10.9 mmol/L. All four figures describe the same limit, and any of them can be checked against the table above: the 0.05 row reads 50, 0.5, and 10.9 across the columns. The practical content is that a 0.5 promille limit is stricter than the 0.08% limit familiar in most of the United States, by three hundredths of a percent, or 30 mg/dL.

Worked example two starts from a laboratory report. A blood result reads 120 mg/dL. Divide by 1,000 for percent: 0.12%. Multiply percent by 10 for promille: 1.2 promille. Multiply percent by 217.1 for mmol/L: 26.0 mmol/L. The figure sits above the common legal limits in every unit, which is the only reading that matters for understanding its seriousness, and it sits in the range where impairment is substantial. No conversion changes that reading; conversions only restate it. Note also that a laboratory result is a measurement at the time of the blood draw, not at the time of driving, and alcohol continues to be eliminated at roughly 0.015% per hour on average, so a result and a driving-time estimate are different quantities that must not be confused.

Worked example three starts from a research paper. A study reports a mean concentration of 30 mmol/L. Divide by 217.1 for percent: 0.138%, usually rounded to 0.14%. Multiply by 1,000 for mg/dL: 138 mg/dL. Multiply by 10 for promille: 1.38 promille. The rounding deserves a word: 0.138% rounded to 0.14% loses information, and a paper that reports two significant figures in mmol/L should be converted with the same care, keeping the extra digit until the final step. For reading purposes, one decimal place in mmol/L and three decimal places in percent preserve everything that matters. A calculator estimate, by contrast, is never that precise: the Widmark estimate on this site carries an error band from the population averages inside it, and quoting it to three decimal places is a statement about arithmetic, not about accuracy.

Reading a foreign result without misreading it

Four checks prevent nearly every misreading. First, identify the unit before reading the number: percent, mg/dL, promille, and mmol/L all look like bare numbers once the unit is dropped, and most misreadings begin with a dropped unit. Second, check the volume: 100 millilitres, a decilitre, and 100 millilitres again are the same volume, while a litre is ten times larger. Third, sanity-check against the table: a percent figure above 0.40 is extraordinarily high, a mg/dL figure above 400 is the same statement, and a promille figure above 4.0 is the same statement again. If a converted figure lands far outside the range of the table, the conversion direction is probably reversed. Fourth, remember what the number is not: it is not a measure of impairment in a particular person, it is not a prediction of when someone will be sober, and it is not permission. Tolerance, medication, fatigue, and food all change what a given concentration does to a given person, and the calculator on this site models none of those beyond the averages inside the formula.

That last point is the only safety content this guide needs, and it needs stating plainly. No conversion, no calculator, and no elapsed-time estimate makes a BAC safe for driving. Impairment begins below legal limits, it varies between people, and the legal limit is a legal threshold, not a safety threshold. The only BAC that is safe for driving is 0.00. If the question is whether someone can drive, the units do not matter and the arithmetic does not matter: the answer is to wait, to use another way home, or not to drink. Units exist so that results from different countries, laboratories, and studies can be understood and compared. They were never meant to be a decision procedure, and this guide does not present them as one.

Frequently asked questions

How do I convert a BAC percentage to mg/dL?

Multiply by 1,000. A BAC of 0.08% equals 80 mg/dL, because percent is grams per 100 millilitres and mg/dL is milligrams per 100 millilitres. To convert back, divide mg/dL by 1,000.

What is 0.08% BAC in promille?

0.8 promille. Promille is grams per litre, ten times the volume of percent, so multiply the percent figure by 10. The 0.5 promille limit used in much of Europe equals 0.05%.

What is BAC in mmol/L?

Multiply the percent figure by about 217. A BAC of 0.08% equals about 17.4 mmol/L, and 0.05% equals about 10.9 mmol/L. The factor comes from the molar mass of ethanol, 46.07 grams per mole.

Why do different countries use different BAC units?

History and measurement convention. The United States uses percent, most of Europe uses promille, laboratories commonly report mg/dL or mg per 100 mL, and research uses mmol/L. All four describe the same concentration, and the conversions in this guide are exact arithmetic.

Is 80 mg/dL the same as a BAC of 0.08%?

Yes, exactly. Both mean 0.08 grams of alcohol per 100 millilitres of blood. The same quantity is also 80 mg per 100 mL, 0.8 promille, and about 17.4 mmol/L.

Can I use a unit conversion to decide if I can drive?

No. A conversion restates a number; it says nothing about impairment, and impairment begins below legal limits. The only BAC that is safe for driving is 0.00. Never use a calculator estimate or a converted figure as a decision to drive.