Blackouts from alcohol typically begin at a blood alcohol concentration (BAC) of around 0.16%, though partial memory gaps can start at levels as low as 0.06%. That’s a wide range, and where you fall within it depends on how fast you drink, your body size, your sex, and individual brain chemistry. A blackout isn’t the same as passing out. You’re still awake and functioning, but your brain stops recording new memories.
The BAC Range for Blackouts
Neither type of alcohol blackout occurs below a BAC of 0.06%, which is actually under the legal driving limit of 0.08% in most U.S. states. That said, blackouts at such low levels are uncommon. The more typical threshold falls between 0.16% and 0.30%, a range where confusion, difficulty walking, nausea, and memory gaps all become likely.
To put that in perspective, the legal limit for driving is 0.08%. A BAC of 0.16% is exactly double that, and 0.20% is roughly two and a half times the legal limit. At 0.20% and above, vomiting, dizziness, and blackouts become increasingly common. These are dangerously high levels of intoxication, and they carry real risks beyond memory loss, including physical injury, sexual assault, and overdose.
Fragmentary vs. Complete Blackouts
There are two distinct types of blackouts, and they feel very different the next morning. Fragmentary blackouts, sometimes called “brownouts,” are episodes of partial memory loss. You remember some of what happened but have gaps. If a friend describes a conversation you had, you might recall pieces of it once prompted. These are the more common type and tend to happen at lower BACs within the blackout range.
En bloc blackouts are the kind most people picture when they hear the word “blackout.” During an en bloc blackout, your brain forms zero new long-term memories for a stretch of time, sometimes hours. No amount of prompting or cues will bring those memories back, because they were never stored in the first place. These typically happen at higher BACs and represent a more complete failure of the brain’s memory system.
Why Your Brain Stops Recording Memories
Alcohol doesn’t erase memories. It prevents them from being created. The part of your brain responsible for converting short-term experiences into lasting memories is the hippocampus, and alcohol disrupts a critical process there called long-term potentiation, which is essentially how brain cells strengthen connections to store new information.
At blackout-level BACs, alcohol interferes with how brain cells communicate using their primary excitatory signaling system. It blocks a key receptor that neurons need to “lock in” new memories. At the same time, alcohol amplifies inhibitory signaling, which further suppresses the hippocampus. The result is that your brain can still process what’s happening in the moment (you can hold a conversation, walk around, make decisions) but none of it gets written to long-term storage. It’s like a camera that’s showing a live feed but not recording.
Why Some People Black Out More Easily
BAC alone doesn’t determine whether you’ll black out. Two people at the same BAC can have completely different experiences, with one remembering the entire night and the other losing hours. Research from the University of North Carolina found that body size, body composition, and drinking speed all matter, but they can’t fully predict blackouts on their own. Individual predisposition plays a significant role.
Sex is one important factor. Women appear to have distinct neurological vulnerability patterns compared to men. Brain wave studies show that men and women who have experienced blackouts exhibit opposite patterns of brain activity at rest: men with blackout histories show decreased slow-wave activity and increased fast-wave activity, while women show the reverse. This suggests the underlying biology of blackout susceptibility differs between sexes, not just the pharmacokinetics of how alcohol is absorbed.
Behavioral factors also matter enormously. Taking shots, pregaming (drinking before arriving at a bar or party), and playing drinking games all spike your BAC rapidly. Speed of consumption is one of the strongest predictors of blackouts because it overwhelms your liver’s ability to metabolize alcohol, causing BAC to shoot past the blackout threshold before you feel the full effects of what you’ve already consumed.
The Dangers at Blackout-Level BAC
Memory loss is the defining feature of a blackout, but it’s far from the only risk at these BAC levels. People who are blacked out are still making decisions and interacting with the world, just without the ability to form memories or exercise sound judgment. This creates a dangerous combination. Blackout drinking is associated with significantly greater odds of physical injury, sexual assault, and alcohol overdose.
At BACs of 0.20% and above, the body’s protective reflexes start to weaken. Vomiting is common at these levels, and a suppressed gag reflex raises the risk of choking. Breathing can slow as alcohol depresses the brainstem. The gap between a BAC that causes a blackout and a BAC that becomes life-threatening is not large, and because tolerance varies so widely between individuals, there’s no safe way to “aim” for a particular level of intoxication.
Repeated blackout drinking also appears to take a toll on everyday thinking. Research on young adults who frequently drink to blackout levels has found associations with self-reported problems in day-to-day cognitive function, suggesting that the effects may extend well beyond the night in question.