Guides
Blood alcohol concentration and how it is measured
Blood alcohol concentration, or BAC, is the number at the center of every drink driving law, every breath test and every BAC calculator. It describes how much alcohol is in your blood at a given moment, and it rises and falls as your body absorbs and clears what you drank. This guide explains what the number actually measures, how breath and blood tests arrive at it, what a standard drink is, and what typically happens to the body at each level, from the first drink to alcohol poisoning.
Per se driving limits in the US, % BAC
What BAC measures
In the United States, BAC is expressed as grams of alcohol per 100 milliliters of blood, written as a percentage. A BAC of 0.08 means 0.08 grams of alcohol in every 100 milliliters of blood, or 0.08 percent. Put another way, it is about eight parts alcohol per ten thousand parts blood.
That small number is enough to change how the brain works. Alcohol reaches the brain within minutes and slows communication between nerve cells, which is why judgment, reaction time and coordination change long before a person feels drunk.
Other countries use different units. Many European countries express the same idea in grams per liter, so 0.5 grams per liter equals 0.05 percent. When you read results from outside the US, check the unit before you compare.
Standard drink definition
To link drinks with BAC, US health agencies use a standard drink containing 14 grams of pure alcohol. In practice that is roughly:
- 12 fluid ounces of regular beer at about 5 percent alcohol.
- 5 fluid ounces of wine at about 12 percent alcohol.
- 1.5 fluid ounces of distilled spirits at about 40 percent alcohol.
Many real drinks hold more than one standard drink. A pint of strong craft beer, a generous pour of wine or a cocktail with several shots can count as two or more, which is why counting glasses often underestimates BAC.
How BAC is measured in blood and breath
A blood test measures alcohol directly in a blood sample, usually in a laboratory using gas chromatography. It is the reference method, but it needs a trained person to draw the sample and time to process it.
A breath test measures alcohol in exhaled air from deep in the lungs and converts that into an estimate of BAC. In the lungs, alcohol moves from the blood into the air sacs, so breath carries a predictable share of what is in the blood. US law uses a partition ratio of 2100 to 1, meaning the alcohol in 2100 milliliters of deep lung air is treated as equal to the alcohol in 1 milliliter of blood. Breath testers apply this ratio to show a BAC style number.
Real people do not all match 2100 to 1 exactly. The true ratio varies from person to person and changes during the hours after drinking, which is one reason a breath reading is an estimate of blood alcohol rather than a copy of it. Our breathalyzer test guide explains how the measurement works inside the device.
This is an estimate. It cannot tell you whether you are safe or legal to drive. If you have been drinking, do not drive.
What different BAC levels do to the body
Effects vary with tolerance, body size, health and whether BAC is rising or falling, but research and agencies such as the NHTSA describe a general pattern.
| BAC | Typical effects |
|---|---|
| 0.02 | Some loss of judgment, relaxation, slight warmth, reduced ability to track moving objects and to do two things at once |
| 0.05 | Lowered alertness and inhibition, exaggerated behavior, reduced coordination, slower response to emergencies on the road |
| 0.08 | Poor muscle coordination, impaired reasoning, memory and self control, harder to detect danger; the per se limit for adult drivers in every state except Utah |
| 0.10 | Clear slowing of reaction time, slurred speech, poor coordination, trouble keeping a lane and braking when needed |
| 0.15 | Far less muscle control, major loss of balance, vomiting may start, serious impairment of the attention needed to drive |
| 0.30 and above | Confusion, stupor, possible loss of consciousness; at these levels and higher, breathing and other vital functions are at risk |
Impairment starts well before 0.08. That is why Utah sets its limit at 0.05, why commercial drivers are held to 0.04, and why drivers under 21 face zero tolerance laws, usually at 0.02 or lower. The full overview is on our legal limit page.
Signs of alcohol poisoning
Alcohol poisoning is a medical emergency. Call 911 if someone who has been drinking shows any of these signs:
- Confusion or stupor, or cannot be woken.
- Vomiting, especially while drowsy or semi conscious.
- Seizures.
- Slow breathing, fewer than eight breaths a minute, or long gaps between breaths.
- Pale, bluish or cold, clammy skin.
- A very low body temperature.
Stay with the person, keep them on their side to protect the airway if they vomit, and do not give them coffee or a cold shower. BAC can keep rising after someone stops drinking, so waiting to see is never safe.
How to estimate your own BAC
Two simple tools give a rough idea. Our BAC calculator uses your drinks, weight, sex and time to estimate a level and a timeline, and the BAC chart shows typical levels in a table. Both rely on averages, so your true level can be higher or lower.
A calculator only estimates. To measure, a fuel cell breathalyzer reads your actual breath.
If you want a personal tester, our ranking of the best breathalyzer options explains which suits which use, and the guide to a breathalyzer for car owners covers keeping one in the vehicle. Read how long does alcohol stay in your system to see how fast BAC falls over time.
Blood alcohol concentration questions
What does a BAC of 0.08 mean?
It means 0.08 grams of alcohol in every 100 milliliters of blood, which is the per se limit for adult drivers in every state except Utah.
What is a standard drink in the US?
A standard drink contains 14 grams of pure alcohol, roughly 12 ounces of beer, 5 ounces of wine or 1.5 ounces of spirits.
How does a breathalyzer turn breath into BAC?
It measures alcohol in deep lung air and applies the 2100 to 1 partition ratio used in US law to estimate the alcohol in the blood.
Is a breath test as accurate as a blood test?
A blood test is the reference method, while a breath test gives an estimate that can differ because the breath to blood ratio varies between people.
At what BAC does impairment start?
Measurable changes in judgment and divided attention can appear around 0.02, well below the 0.08 legal limit.
What BAC is dangerous?
Levels of 0.30 and above can cause loss of consciousness and threaten breathing, so call 911 if someone shows signs of alcohol poisoning.