Why Do Two People Get Different BAC From the Same Amount of Alcohol?

Two friends order the same three drinks. An hour later, one of them may have a BAC almost twice as high as the other. Here is why.

Body weight

A heavier body has more blood and water to dilute alcohol. The same drink produces a lower concentration in a larger person.

Body composition

Muscle holds a lot of water; fat holds very little. Two people of the same weight can still differ — the one with more muscle usually ends up with a lower BAC.

Biological sex

On average, women have less body water and slightly different alcohol metabolism than men of the same weight. That is why BAC formulas use a different factor for each.

Amount of water in the body

Alcohol spreads through body water. Less water — whether from body type or dehydration — means higher concentration from the same drink.

Drinking speed

Three drinks in 30 minutes push BAC up much faster than three drinks over three hours. Your liver can only clear about one standard drink per hour, so fast drinking stacks up.

Food in the stomach

Food slows the move of alcohol into the small intestine, where most of it is absorbed. Drinking on an empty stomach usually gives a faster and higher peak. See what to eat before drinking.

Time since drinking

BAC keeps rising for 30–90 minutes after your last drink, then slowly falls. Two people measured at different moments can show very different numbers.

Individual metabolism

Genetics, liver health, medication and age all affect how quickly alcohol is processed. Some people clear it noticeably faster or slower than average.

Feeling drunk is not a BAC meter

Regular drinkers can build tolerance and feel fine at a BAC that still impairs driving. Feeling sober tells you very little about your actual level.

Don't compare drinks with friends

"I had the same as him and he's fine" is not a safe rule. Use an estimate as a guide, remember it is not a measurement — and if there is any doubt, don't drive.