Blood alcohol content is the percentage of alcohol in your bloodstream, measured in grams of alcohol per 100 milliliters of blood

Blood alcohol content (BAC) is expressed as a decimal — for example, 0.08% means 0.08 grams of pure alcohol per 100 milliliters of blood. It is not about how much you drank, but how much alcohol is circulating in your body at a specific moment. The same drink produces different BAC levels in different people because body weight, food intake, metabolism, and sex all affect how quickly alcohol enters and leaves your bloodstream.

BAC is the only measurement law enforcement and courts use to determine impairment for driving offenses. It is measured through breath tests (breathalyzers), blood tests, or urine tests, though breath and blood are most common. The reading changes constantly as your body metabolizes alcohol — roughly one standard drink per hour for most adults — so a BAC of 0.08% at 9 p.m. will be lower at 10 p.m. even if you stop drinking.

Key Takeaways

  • The legal driving limit in all 50 states is 0.08% BAC for drivers age 21 and older; commercial drivers face a 0.04% limit, and drivers under 21 face a 0.02% limit in most states.
  • BAC is measured by breath, blood, or urine test and reflects the amount of alcohol in your bloodstream at the moment of the test, not how much you drank.
  • Your BAC depends on body weight, food in your stomach, how fast you drink, and individual metabolism — the same drink produces different BAC levels in different people.
  • Impairment can occur below the legal limit, and you can be charged with driving under the influence even at 0.05% or lower if your driving shows signs of impairment.

Legal BAC limits by driver category

The legal limit for drivers age 21 and older is 0.08% in all 50 states. This limit applies to passenger vehicles and is the threshold at which a driver can be arrested for driving under the influence (DUI) or driving while intoxicated (DWI), depending on your state's terminology.

Commercial drivers — those operating trucks, buses, or vehicles carrying hazardous materials — face a lower limit of 0.04% BAC. This stricter standard reflects the greater risk posed by large vehicles and the commercial nature of the work.

Drivers under age 21 face a 0.02% limit in most states, though some states set it at zero tolerance. A 0.02% BAC can result from a single drink, so underage drivers have virtually no legal margin. A few states explore a 0.01% limit instead.

Some states also distinguish between DUI (driving under the influence, based on impairment) and DWI (driving while intoxicated, based on BAC). The names vary by state, but the legal BAC thresholds remain consistent.

How BAC is measured and what the numbers mean

A breath test (breathalyzer) is the most common roadside measurement. You blow into a device that estimates BAC by measuring alcohol in your lungs. Breath tests are quick and non-invasive, but they estimate rather than measure directly — they assume a fixed ratio between breath alcohol and blood alcohol, which varies between individuals.

A blood test is more accurate and is often used to confirm a breath test result or when a breath test is unavailable. Blood is drawn and sent to a lab for analysis. Blood tests are the gold standard in court because they measure actual alcohol in your bloodstream, not an estimate.

A urine test is less common and less reliable than breath or blood tests. It is sometimes used when breath and blood tests are not available, but courts weight urine results more lightly because urine alcohol levels do not correlate as directly to impairment.

The numbers themselves are straightforward: 0.08% means 0.08 grams of alcohol per 100 milliliters of blood. A BAC of 0.15% is roughly twice as high and indicates greater impairment. Most people begin to show visible signs of impairment around 0.05% to 0.07%, though this varies widely.

Why BAC varies between people drinking the same amount

Body weight is the most obvious factor. A 120-pound person who drinks two beers will have a higher BAC than a 200-pound person who drinks the same two beers, because the alcohol is distributed across a smaller volume of body fluid.

Food in the stomach slows alcohol absorption. Drinking on an empty stomach causes alcohol to enter the bloodstream faster, raising BAC more quickly. Eating before or while drinking delays absorption and produces a lower peak BAC.

Metabolism and sex also matter. Women typically have lower water content in their bodies than men of the same weight, so they reach higher BAC levels from the same drink. Individual metabolism varies too — some people process alcohol faster than others due to genetics and liver function.

Drinking speed affects BAC because your liver metabolizes alcohol at a roughly fixed rate (about one standard drink per hour). If you drink three beers in one hour, your BAC will be higher than if you drink the same three beers over three hours, because your body has time to process the alcohol between drinks.

Impairment versus the legal limit

The legal limit of 0.08% does not mean you are safe to drive below it. Impairment can occur at lower BAC levels, and you can be arrested for DUI even at 0.05% or 0.06% if your driving shows signs of impairment — weaving, slow reaction time, poor coordination, or failing field sobriety tests.

This is why some states have an "impairment-based" DUI law in addition to the per se 0.08% limit. Per se means "by itself" — at 0.08%, you are presumed impaired by law regardless of how you drive. Below 0.08%, the prosecution must prove impairment through observation and testing.

Conversely, a BAC below 0.08% does not may provide you will not be arrested. If an officer observes impaired driving and you fail field sobriety tests, you can be charged even if your breath or blood test comes back at 0.07% or lower.

How your body processes alcohol and BAC over time

Your liver metabolizes alcohol at a relatively constant rate for most people — roughly one standard drink per hour. A standard drink is defined as 12 ounces of beer (5% alcohol), 5 ounces of wine (12% alcohol), or 1.5 ounces of spirits (40% alcohol). These all contain about 0.6 ounces of pure alcohol.

This means if you drink two beers in one hour, your BAC will peak shortly after the second drink and then decline by roughly 0.01% to 0.02% per hour as your liver processes the alcohol. If you drank at 8 p.m., your BAC at 9 p.m. will be lower, and at 10 p.m. lower still — assuming you stop drinking.

Nothing speeds up this process. Coffee, cold showers, food, or exercise do not lower BAC faster. Only time allows your liver to metabolize alcohol. This is why the phrase "sleep it off" has a grain of truth — your BAC will decline while you sleep, but it declines at the same rate as when you are awake.

Some medications and health conditions can slow alcohol metabolism, keeping BAC elevated longer. Liver disease, certain diabetes medications, and some antibiotics can interfere with how quickly your body processes alcohol.

Refusing a breath or blood test

You have the right to refuse a breath or blood test in most states, but refusal carries its own legal consequences. Many states impose an automatic license suspension for refusal — often longer than the suspension for a failed test. Some states also allow the refusal itself to be used as evidence of guilt in court.

A few states have "implied consent" laws, meaning that by driving on public roads, you have implicitly agreed to submit to a BAC test if stopped. Refusal can result in criminal charges separate from the DUI charge itself.

The decision to refuse is complex and depends on your state's laws and your specific situation. If you are arrested for suspected DUI, you have the right to speak with an attorney before deciding whether to submit to a test.

Frequently Asked Questions

Can I be charged with DUI if my BAC is below 0.08%?

Yes. If an officer observes impaired driving — weaving, slow reaction, poor coordination — and you fail field sobriety tests, you can be charged even at 0.05% or 0.06%. The 0.08% limit is a per se threshold; below it, the prosecution must prove impairment through other evidence.

How long does it take for one drink to show up on a breath test?

Alcohol begins to enter your bloodstream within minutes of drinking, and a breath test can detect it within 15 to 20 minutes. Peak BAC usually occurs 30 to 90 minutes after finishing a drink, depending on food intake and other factors.

Does food lower your BAC?

Food does not lower BAC once alcohol is already in your bloodstream. Eating before or while drinking slows how fast alcohol is absorbed, so it delays when your BAC peaks and produces a lower peak level. Eating after drinking does not reduce your current BAC.

What is the difference between DUI and DWI?

The terms vary by state. Some states use DUI (driving under the influence) for impairment-based charges and DWI (driving while intoxicated) for BAC-based charges. Other states use only one term. Check your state's law to know which applies to you.

Can I be arrested for DUI if I was not driving, just sitting in a parked car?

Some states have "physical control" laws that allow DUI charges even if the car is parked, as long as you have the keys and the engine is running or the car is in a position to be driven. Other states require actual driving. Laws vary significantly by state.