Blood alcohol concentration, or BAC, is the proportion of alcohol in your bloodstream at a given moment.
In nearly every state, 0.08 percent is the general threshold for driving a standard vehicle. The phrase "nearly every" is doing real work there: at least one state has set its general limit lower, and the thresholds that apply to commercial drivers and to drivers under 21 are different again.
What makes BAC hard to reason about in the moment is that the same amount of alcohol produces meaningfully different results in different people, and in the same person on different days.
See the thresholds that apply in your state →
Why "a couple of drinks" is not a measurement
BAC depends on much more than how many drinks you had.
Body size. Alcohol distributes through the water in your body. A smaller person generally reaches a higher concentration from the same amount.
Body composition. Alcohol does not distribute into fat tissue the way it does into water, so two people of the same weight can land in different places.
Biological sex, through average differences in body composition and in how alcohol is metabolized.
Food. What you ate, how much, and when. Food in the stomach slows absorption substantially. The same drinks on an empty stomach produce a different curve.
Time, in both directions. Your body clears alcohol at a roughly fixed rate no matter how much you drank. You cannot speed it up.
What you were drinking. Strength varies enormously, and a home pour or a craft beer bears little relation to the "standard drink" any chart is built on.
Medication and health. Some medications affect absorption or add their own impairment.
Two people of different sizes drinking identically over the same evening can end up on opposite sides of the limit. The same person can, on two different nights.
The part that catches people out: BAC keeps rising
This is the single most useful thing on this page.
Alcohol takes time to move from your stomach into your bloodstream. Your BAC does not peak when you stop drinking. It peaks some time afterward, and it can still be climbing while you are driving home.
Two practical consequences follow.
Feeling like you are sobering up is not evidence that your BAC is falling. Early on it may be doing the opposite.
And a test taken at the station some time after you were stopped may not show the same figure as one taken at the roadside. Whether your level at the time of the test reflects your level at the time you were driving is a genuine question in some cases, and it is one of the things a defense lawyer looks at.
Nothing speeds the process up. Not coffee, not a cold shower, not food after the fact, not sleep, not exercise. The only variable is time.
What actually gets measured, and how
Breath testing does not measure your blood. It measures alcohol in your breath and converts it using an assumed ratio between breath and blood. That ratio is an average, and individuals vary from it. This is the most common method, both on a handheld device at the roadside and on a larger instrument after arrest.
Blood testing measures alcohol in a blood sample directly and is generally regarded as more precise. It takes far longer to produce a result and shows up more often in cases involving a crash, an injury, or a refusal. More on blood draws and warrants →
Both methods carry procedural requirements. Calibration and maintenance schedules for breath instruments. Chain of custody for blood. Observation periods before a breath test to rule out residual mouth alcohol. Qualifications for the person conducting it.
Departures from those procedures are a common and legitimate area for a defense attorney to examine. A number produced by an instrument is only as good as the process that produced it.
Things that can affect a breath reading
Worth knowing, because people assume the machine is simply right.
- Residual alcohol in the mouth from a recent drink, mouthwash, or a breath spray, which is why observation periods exist.
- Vomiting or reflux shortly before the test, which can bring stomach contents into the airway.
- Certain medical conditions, including some affecting the digestive system.
- Breathing pattern during the sample.
- Instrument calibration and maintenance history.
None of these is a magic defense. They are the reasons the surrounding procedure exists, and the reasons the records behind a result are worth requesting.
Thresholds beyond the general limit
The general limit is not the only line that matters, and this is where the figures genuinely vary.
An enhanced or aggravated threshold. Many states set a higher level that triggers more serious penalties even on a first offense. Where that line sits differs by state.
Commercial drivers face a lower threshold than other drivers, under federal rules that apply while operating a commercial vehicle.
Drivers under 21 face a zero tolerance standard in every state, though the exact figure states use is not identical.
Because these differ, this page will not give you a number for them.
See your state's exact thresholds and how they affect penalties →
Why the number matters beyond over or under
BAC does not only decide which side of a line you are on.
In most states how far over the limit you were affects the severity of the charge, the penalties, whether you are eligible for a diversion program, whether a reduction is realistically available, and how long any license consequence lasts.
A result barely over the general limit and a result well above an enhanced threshold can produce very different outcomes from the same offense category.
More on how results affect cost and penalties →
More on diversion eligibility →
What this page is not
This is not a guide to working out how much you can drink and still drive.
There is no reliable formula, because too many variables you cannot track affect the outcome. Charts and phone apps that estimate BAC from drinks and body weight are rough approximations built on assumptions that may not describe you, and treating one as permission to drive is a serious mistake.
The only way to know your BAC at the moment you would need to know it is to be tested, and by then the decision to drive has already been made.
If you are not sure whether you are under the limit, you do not know, and not driving is the only reliable answer.
If you are trying to understand a result you already have
The number is only part of the picture, and often not the most important part.
What matters more is which threshold it falls into under your state's law, whether the testing procedure was followed properly, how long after driving the sample was taken, whether the instrument's records are in order, and what your history looks like.
Those are answerable questions, and they are answered from documents rather than from memory. Requesting the maintenance and calibration records, the observation log, and the full report is ordinary practice, and it is the point at which a result stops being a fact and starts being evidence that can be examined.
More on whether you need a lawyer →
Find your state
Pick the state you were arrested in, not the one you live in.
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All states →This is general information about how blood alcohol concentration is measured and regulated, not guidance on safe drinking levels or legal advice about a specific test result. Thresholds and their consequences differ by state. Confirm anything you rely on with a lawyer licensed in your state.