A normal blood alcohol level in someone who has not been drinking is essentially 0 mg/dL. The human body produces trace amounts of ethanol through gut fermentation, but these amounts are so small (typically well under 1 mg/dL) that they don’t register on standard testing and are clinically considered zero. Any measurable reading above 0 mg/dL indicates alcohol consumption or, rarely, a medical condition.
How mg/dL Relates to BAC Percentage
Blood alcohol is reported in two common ways: as a percentage (BAC) and in milligrams per deciliter (mg/dL). The conversion is straightforward. A BAC of 0.08% equals 80 mg/dL, because mg/dL literally measures how many milligrams of alcohol are present in 100 milliliters of blood. So 0.10% is 100 mg/dL, 0.20% is 200 mg/dL, and so on.
In the United States, the legal definition of intoxication for driving is a BAC of 0.08%, which equals 80 mg/dL (or 17.4 mmol/L for labs using international units). Most people will see their results reported in one of these formats depending on whether the test was done in a clinical lab or a legal context.
What Different Levels Feel Like
Because 0 mg/dL is normal, any number above that reflects how much alcohol is circulating in your bloodstream. The effects escalate predictably with concentration, though individual tolerance varies based on body weight, drinking history, genetics, and whether you’ve eaten recently.
- 20 to 50 mg/dL: Mild relaxation, slight warmth, and lowered inhibitions. Most people feel sociable but not noticeably impaired.
- 50 to 80 mg/dL: Judgment and reaction time begin to decline. Fine motor skills start to slip, even though you may feel fine.
- 80 to 100 mg/dL: The legal impairment zone. Coordination, balance, and speech are clearly affected. This is where most states draw the line for drunk driving.
- 100 to 200 mg/dL: Significant impairment in motor control, blurred vision, and poor decision-making. Nausea and vomiting are common at the higher end.
- 200 to 300 mg/dL: Confusion, disorientation, and difficulty staying conscious. The risk of choking on vomit increases substantially.
- 300 to 400 mg/dL: In people without heavy alcohol tolerance, this range often causes unconsciousness and alcohol poisoning, a life-threatening emergency.
- Above 400 mg/dL: Risk of coma and death from respiratory arrest. Breathing can slow or stop entirely at this concentration.
These ranges apply to people without significant alcohol tolerance. Heavy, long-term drinkers can sometimes remain conscious at levels that would be fatal for others, but this does not mean they are safe from organ damage.
Serum vs. Whole Blood Results
If you’re looking at a lab report from a hospital, there’s an important detail: most clinical labs measure alcohol in serum or plasma, not whole blood. Serum readings run about 20% higher than whole blood readings because serum contains more water (and alcohol dissolves in water). Labs and forensic experts divide the serum value by 1.20 to estimate the true whole blood concentration.
This matters if you’re comparing a hospital lab result to a legal BAC standard. A serum reading of 96 mg/dL, for example, converts to roughly 80 mg/dL in whole blood, right at the legal limit. If your lab report seems higher than expected, the serum-to-blood difference is likely why.
How Fast Alcohol Clears Your System
The liver breaks down alcohol at a fairly constant rate of about 15 mg/dL per hour (equivalent to a BAC drop of 0.015% per hour). This rate doesn’t change much regardless of how much you’ve had. Coffee, cold showers, and food won’t speed it up.
In practical terms, if your blood alcohol level is 80 mg/dL, it will take roughly five to six hours to return to 0 mg/dL. At 160 mg/dL, you’re looking at closer to ten or eleven hours. This is why people can still be legally impaired the morning after heavy drinking.
When a Non-Zero Reading Appears Without Drinking
In rare cases, a condition called auto-brewery syndrome causes gut fungi to ferment carbohydrates into ethanol, producing measurable blood alcohol levels without any alcohol consumption. People with uncontrolled diabetes or certain gastrointestinal conditions are more susceptible. These cases are uncommon but well-documented in medical literature, and they can produce readings high enough to cause actual intoxication symptoms. If you consistently test positive for alcohol without drinking, this is worth investigating with a gastroenterologist.