Body weight is one of the most significant factors determining your blood alcohol content (BAC) after drinking. But it's not just about how much you weigh — body composition, water content, and biological sex all play important roles.
Alcohol distributes throughout your body's water content. When you drink, ethanol enters the bloodstream and spreads into all water-containing tissues. The more body water you have, the more the alcohol is diluted — resulting in a lower BAC.
This is reflected in the Widmark formula, used by our BAC calculator:
BAC = (A ÷ (r × W)) − (β × t)
Where W = body weight and r = Widmark factor (proportion of body water)
A heavier person has more body water (assuming similar body composition), so the same amount of alcohol produces a lower concentration. This is why a 100 kg person will typically reach a lower BAC than a 60 kg person after the same number of drinks.
Not all body weight is equal when it comes to alcohol distribution. The key distinction is between:
Contains ~75% water. Alcohol distributes freely into lean tissue, diluting it more effectively. People with more muscle mass tend to have lower BAC for the same alcohol intake.
Contains only ~10% water. Alcohol does not readily distribute into fat tissue. Two people of the same weight but different body fat percentages will have different BAC levels — the person with more body fat will have a higher BAC.
This is why the Widmark factor differs between men and women: on average, men have a higher proportion of body water (r = 0.68) compared to women (r = 0.55), largely due to differences in body fat percentage.
Approximate peak BAC for a male after drinking standard drinks within one hour on a moderately full stomach. Values use the Widmark formula with r = 0.68 and β = 0.015/hr. Female values would be approximately 20–25% higher.
| Body Weight | 1 drink | 2 drinks | 3 drinks | 4 drinks |
|---|---|---|---|---|
| 50 kg (110 lb) | 0.04% | 0.08% | 0.12% | 0.16% |
| 60 kg (132 lb) | 0.03% | 0.07% | 0.10% | 0.13% |
| 70 kg (154 lb) | 0.03% | 0.06% | 0.08% | 0.11% |
| 80 kg (176 lb) | 0.02% | 0.05% | 0.07% | 0.10% |
| 90 kg (198 lb) | 0.02% | 0.04% | 0.07% | 0.09% |
| 100 kg (220 lb) | 0.02% | 0.04% | 0.06% | 0.08% |
| 110 kg (243 lb) | 0.02% | 0.03% | 0.05% | 0.07% |
Even at the same body weight, women typically reach a higher BAC than men after consuming the same amount of alcohol. This is due to multiple factors:
✗ "Heavy people can always handle more alcohol"
While higher weight dilutes alcohol more (lower peak BAC), it does not speed up elimination. A heavier person and a lighter person who both reach 0.08% BAC will take roughly the same time to sober up — about 5–6 hours.
✗ "Muscle helps you metabolize alcohol faster"
More muscle means more body water, which dilutes alcohol and leads to a lower peak BAC. But muscle does not increase liver enzyme activity — the elimination rate stays the same regardless of how muscular you are.
✗ "BMI is a good predictor of BAC"
BMI does not differentiate between fat and lean mass. A muscular person and a person with high body fat at the same BMI will have very different BAC responses to the same amount of alcohol.
BAC estimates based on body weight are approximations. Individual variation in metabolism, genetics, health, and other factors means actual BAC may differ from calculated values. Never use BAC estimates to decide whether you are safe to drive.