What Is BAC? Blood Alcohol Content Explained

Your car has a speedometer not because you're a reckless driver, but because driving without one would be absurd. BAC is that same kind of number, and most people spend their entire drinking lives without ever seeing it.

Whether you're nursing a glass of wine at dinner, out with friends, or wrapping up a happy hour, that number is already moving. How you feel and what it actually reads are two different things.

The question of what is BAC turns out to be the easy part. Here is what the number actually means, and why it varies.

What BAC Actually Is

BAC stands for blood alcohol content (also written as blood alcohol concentration). It measures one thing: grams of alcohol per 100 milliliters of blood, expressed as a percentage. A reading of 0.08 means 0.08 grams of alcohol per 100 mL of blood, or 0.08% alcohol by volume in the bloodstream.

Alcohol enters the body through the stomach and small intestine. BAC starts rising within 15 to 30 minutes of the first drink.[1] Food slows that process by competing with alcohol for access to the stomach lining. An empty stomach accelerates it.

The scale runs from 0.00% (no alcohol present) to 0.40% and above, where readings reach life-threatening levels. Most people encounter the 0.08% mark in conversations about driving laws. But blood alcohol content covers the full range of what a drink or two actually does in the body, and the effect varies more than most people expect.

How BAC Is Measured

Three methods exist for measuring BAC: blood testing, breath testing, and urine testing.

Blood testing is the most accurate of the three. It requires a lab draw and a professional collection. Urine testing is the least accurate and rarely used as a standalone legal measure.

Breath testing is how most people encounter the technology. Law enforcement uses it at the roadside, and personal devices use the same underlying method. It works because alcohol from the bloodstream crosses into the lungs and exhales at a predictable ratio. A fuel-cell sensor measures the alcohol in that exhaled breath and converts it into a BAC reading in seconds.

BACtrack breathalyzers use the same fuel-cell sensor technology as law enforcement. That means a personal device delivers the same kind of measurement without a lab, without a needle, and without a wait. For a closer look at what separates one device from another, see BACtrack's guide to choosing a breathalyzer.

What the Numbers Mean

Different readings correspond to different physical effects. Here is how the levels break down.

• At 0.02%, mild warmth and relaxation are commonly reported. Most states set zero-tolerance thresholds for drivers under 21 at or below this mark.
• At 0.05%, reduced inhibitions and mildly slowed reaction time are frequently associated with this range. Utah sets its legal driving limit here, effective December 2018.
• At 0.08%, coordination changes are common and judgment is frequently affected. This is the legal driving threshold in 49 states.[2] Federal law sets the limit for commercial drivers at 0.04%.[3] For more on what the legal context around that number looks like, see BACtrack's overview of DUI laws.
• At 0.10%, coordination, reaction time, and balance are more clearly affected.
• At 0.15%, motor control, speech, and balance are significantly altered.
• At 0.40% and above, readings reach critical territory. This range is associated with alcohol poisoning and life-threatening complications.

One thing worth knowing: tolerance affects how alcohol feels, not what it measures. A person with high tolerance can feel entirely clear-headed while at or above a legal threshold. The number and the feeling are two separate signals. A BACtrack breathalyzer shows you the actual number in seconds.

Why Two Drinks Hit Differently for Two People

BAC is a personal number. Two people sitting at the same table, drinking the same wine, can finish the night at very different readings.

Five factors determine where someone lands:

• Body weight: A lighter person has less body mass to dilute the alcohol, so BAC climbs faster and higher with each drink.
• Biological sex: Women typically have lower body water percentage than men and lower levels of the enzyme that begins breaking down alcohol before it enters the bloodstream. Both push BAC higher, faster, at the same intake.
• Food: Eating before or during drinking slows absorption. An empty stomach accelerates it.
• Rate of consumption: The faster the drinks go down, the faster BAC rises. The liver can only process alcohol at a fixed rate.
• Individual metabolism: There is natural variation in how quickly different people process alcohol, even at the same weight and biological sex.

Body weight and biological sex are the two largest variables. A person at 120 lbs reaches a notably higher BAC from the same drinks as someone at 200 lbs. Same table, same bottle, different numbers.

"I feel fine" and "my BAC is fine" are not the same statement. That gap is the whole point of having a number. A BACtrack breathalyzer closes it in seconds.

How Fast BAC Drops

Once alcohol is in the bloodstream, the body clears it at a roughly fixed rate. The liver processes alcohol at approximately 0.015 to 0.020% per hour for most people.[4] A BAC of 0.08% takes roughly four to five hours to return to 0.00% at that rate.

Coffee, water, food, and sleep do not meaningfully speed up the process. Time is the only reliable variable.

The BACtrack Z9 includes a feature called ZeroLine that predicts the exact time BAC will reach 0.00% based on the current reading. Instead of estimating, you get time on screen.

The feeling of being clear-headed often arrives before the number actually reaches zero. How someone feels and what the number says are two different signals. The BACtrack Z9 shows both at once.

From Guessing to Knowing

The gap between guessing and knowing is exactly what a personal breathalyzer closes. Two drinks hit differently at 120 lbs vs 200 lbs. Body weight, biological sex, food, and metabolism all move the reading. Understanding what BAC is one thing. Seeing the actual number is another.

BACtrack breathalyzers use the same fuel-cell sensor technology as law enforcement and give a reading in seconds. No lab. No wait. See the full lineup of BACtrack breathalyzers and find out where you stand.

Frequently Asked Questions

What does BAC stand for?

BAC stands for blood alcohol content, also written as blood alcohol concentration. It measures grams of alcohol per 100 milliliters of blood, expressed as a percentage. What is BAC in practical terms: a reading of 0.08 means 0.08 grams of alcohol per 100 mL of blood in the bloodstream.

What is a 0.08 BAC?

A 0.08 BAC means 0.08 grams of alcohol per 100 milliliters of blood, equal to 0.08% alcohol by volume. This is the legal driving threshold in 49 US states. Coordination changes and affected judgment are commonly reported at this level, though individual responses vary by body weight, biological sex, and tolerance.

How long does it take for BAC to go down?

The liver processes alcohol at approximately 0.015 to 0.020% per hour. At that rate, a BAC of 0.08% takes roughly four to five hours to return to 0.00%. Coffee, water, food, and sleep do not meaningfully speed up the process.

What affects blood alcohol content?

Five factors move BAC: body weight, biological sex, food consumed before or during drinking, rate of consumption, and individual metabolism. Body weight and biological sex are the two largest variables. A lighter person reaches a higher reading from the same drinks than a heavier person, and women typically reach higher BAC faster than men of the same weight due to differences in body water percentage and enzyme levels.

How do breathalyzers measure BAC?

Breathalyzers measure BAC by detecting alcohol in exhaled breath. Alcohol from the bloodstream crosses into the lungs and exhales at a predictable ratio, which a fuel-cell sensor converts into a BAC reading in seconds. BACtrack breathalyzers use the same fuel-cell sensor technology as law enforcement devices.

References

1. NHTSA. "The ABCs of BAC." National Highway Traffic Safety Administration, 2022. Accessed June 2026.
2. NCSL. "Driving While Under the Influence (DUI) Laws by State." National Conference of State Legislatures. Accessed June 2026.
3. FMCSA. "Blood Alcohol Concentration and Commercial Driver Standards." Federal Motor Carrier Safety Administration. Accessed June 2026.
4. NIAAA. "Alcohol Metabolism." National Institute on Alcohol Abuse and Alcoholism. Accessed June 2026.