BAC Calculator
Estimated blood alcohol from drinks, weight, sex, and hours.
What it does
Blood alcohol concentration weighs alcohol grams against body water, then subtracts metabolism over time. More drinks raise it, more body water dilutes it, and steady liver work lowers it about 0.015 per hour on average.
The tool above takes standard drinks, weight, sex, and hours using Widmark math (r 0.68 male, 0.55 female). It returns the estimated percent with a limit verdict and hours to clear.
The default of a 180 lb man with 4 drinks over 2 hours reads about 0.071%, under the 0.08 driving limit but well into impaired judgment. The same drinks in a 140 lb woman over 1 hour read about 0.105%, over the limit.
The equation follows NHTSA alcohol guidance with university Widmark references such as the Indiana University BAC calculator behind it. Both stress the same limit: estimates guide planning, never driving decisions after drinking.
How to use
- Count standard drinks honestly: 12 oz beer, 5 oz wine, 1.5 oz spirits each.
- Enter body weight in lb or kg.
- Pick the sex factor the formula was validated for.
- Enter hours from first sip to now.
- Plan the ride home before the first drink, whatever it shows.
Formula
- 14 g
- Alcohol grams in one US standard drink.
- r
- Water distribution ratio by sex.
- 0.015
- Average hourly metabolism in BAC points.
Zero drinks always reads zero. Food slows absorption without changing the total, so full stomachs delay the peak rather than lowering it much.
Worked example
Example: 180 lb man, 4 drinks, 2 hours
Imperial defaults: 4 drinks, 180 lb, male, 2 h.
- Alcohol: 4 x 14 = 56 g.
- Weight: 180 lb = 81,646.6 g. Times r 0.68 = 55,519.7.
- Raw: 56 / 55,519.7 x 100 = 0.1009%.
- Metabolism: 0.015 x 2 = 0.03. Result 0.0709%, shown 0.071%.
- Verdict under 0.08 with 4.7 h to zero. Tool shows 0.071%, verdict, 4.7 h.
Reference table
| Drinks | Estimate | Verdict |
|---|---|---|
| 2 | about 0.020% | Under 0.08, still impaired |
| 4 | about 0.071% | Under 0.08, still impaired |
| 6 | about 0.121% | Over 0.08 limit |
| 8 | about 0.172% | Very high |
Limits
- Planning estimate only, never a drive or not test.
- Averages for sex, water share, and metabolism assumed.
- Food, pace, medication, and illness shift real values.
- Meters and lab tests beat every equation.
Common mistakes
- Counting strong pours as single standard drinks.
- Forgetting shots hidden in mixed cocktails.
- Reading under 0.08 as safe to drive.
- Trusting coffee or cold air to lower BAC.
- Using morning after math without enough hours.
Try it yourself
Quick checks. Answers sit tucked under each question.
140 lb woman, 3 drinks, 1 hour?
42 g over 34,926.6 x 100 = 0.120%, minus 0.015 = 0.105%, over.
70 kg woman, 2 drinks, 1 hour?
28 g over 38,500 x 100 = 0.0727%, minus 0.015 = 0.058%, under but impaired.
Zero drinks?
Always 0.000%: zero verdict, zero wait.
Questions
What is a standard drink?
About 14 g alcohol: 12 oz of 5% beer, 5 oz of 12% wine, or 1.5 oz of 40% spirits.
What is the driving limit?
0.08% in most US states, lower for commercial and young drivers. Impairment starts below it.
How fast does BAC fall?
About 0.015 per hour on average. Only time sobers, never coffee or showers.
Why does sex matter?
Different average body water shares (r 0.68 vs 0.55) dilute the same drinks differently.
Does food help?
It slows the rise and lowers the peak a little, but the total alcohol still counts.
Can I drive at 0.07%?
No. Judgment and reaction already suffer there; arrange another ride.