Blood alcohol content (BAC) is the percentage of alcohol present in your bloodstream, and it is the number police and breath tests use to decide whether you are legally allowed to drive. This BAC calculator estimates your approximate blood alcohol level using the classic Widmark formula — the same forensic model used by toxicologists and by NHTSA's impaired-driving worksheets. Enter your weight, biological sex, the amount of pure alcohol you have consumed, and the hours since your first drink, and the result card shows your estimated BAC alongside the 0.08% legal driving limit.
The estimate is only a rough guide. Real BAC moves with food in your stomach, your metabolism, medications, hydration, and how quickly you drank. Nothing on this page is medical or legal advice, and the only safe choice after drinking is to never drink and drive.
How to Use
- Enter your Body Weight in pounds. A larger body dilutes the same amount of alcohol across more water, so two people who drink identically can have noticeably different BAC.
- Choose your Biological Sex. This sets the Widmark distribution constant — 0.73 for male bodies and 0.66 for female bodies — which reflects average differences in total body water.
- Enter the Standard Drinks count. One standard US drink is a 12 oz beer, a 5 oz glass of wine, or a 1.5 oz shot — each contains about 0.6 oz of pure alcohol. Just tally your drinks; the calculator handles the conversion.
- Set the Hours Since First Drink. Your liver removes alcohol at a fairly steady 0.015% BAC per hour, so the calculator subtracts this for every hour that has passed.
- Read the result card. It shows your estimated BAC, the Widmark constant used, how much alcohol your body has already metabolized, and whether you are over or under the 0.08% legal limit.
If the result is near 0.08%, treat it as a warning, not a green light. Breath alcohol and actual blood alcohol also differ slightly, and individual variation is wide.
The Widmark Formula
The estimate comes from the equation first published by Erik Widmark in 1932:
BAC = alcohol × 5.14 / (weight × genderConstant) − 0.015 × hours
Where:
- drinks × 0.6 is the pure ethanol consumed, in fluid ounces (one standard drink = 0.6 oz pure alcohol).
- weight is body weight in pounds.
- genderConstant is 0.73 for male bodies and 0.66 for female bodies.
- 0.015 × hours is the amount of alcohol your liver has eliminated since you started drinking.
A 180 lb male who has four standard drinks over one hour reaches roughly 0.08% BAC — right at the legal threshold. The same person on an empty stomach, drinking faster, would spike higher; after a full meal and slower sipping, lower. Because the formula uses averages, treat the output as a planning estimate rather than a measurement.
What the BAC Levels Mean
BAC is written as a percentage. At 0.02% most people feel a slight relaxation. At 0.05% attention and judgment begin to soften. At 0.08% — the US criminal per se limit for drivers 21 and older — coordination, reaction time, and speed control are measurably impaired. At 0.15% the risk of a serious crash jumps dramatically, and above 0.30% the risk of alcohol poisoning, unconsciousness, and life-threatening respiratory depression rises sharply.
Food slows absorption but does not cancel it. Coffee, cold showers, and exercise do not speed removal — only time does, because the 0.015% per hour elimination rate is biologically fixed.
Examples
Example 1 — Just at the limit. A 180 lb male has four standard drinks (about 2.4 oz pure alcohol) and one hour has passed. BAC = 4 × 0.6 × 5.14 / (180 × 0.73) − 0.015 ≈ 0.079%, near the 0.08% limit. He should call a ride.
Example 2 — Staying under. A 140 lb female has three standard drinks over two hours. BAC = 3 × 0.6 × 5.14 / (140 × 0.66) − 0.030 ≈ 0.070%, just under the limit. Even so, the calculator recommends waiting and hydrating before any driving.
Example 3 — Time helps. The same 180 lb male at 0.079% after one hour drops to about 0.034% after three more hours of no further drinking (3 × 0.015% = 0.045% additional metabolized). That is why waiting, not rushing, is the only reliable way to sober up.
FAQ
How does this BAC calculator work?
It uses the Widmark formula: BAC = drinks × 0.6 × 5.14 / (weight × genderConstant) − 0.015 × hours. The gender constant is 0.73 for male bodies and 0.66 for female bodies, and the 0.015% per hour term represents steady liver metabolism. This is the same model used in forensic toxicology and NHTSA worksheets.
What is the legal BAC limit for driving?
For drivers 21 and older in the United States, the criminal per se limit is 0.08% BAC. Drivers under 21 face zero-tolerance limits near 0.00–0.02%, and commercial drivers are held to a 0.04% limit. These limits vary by country and state.
How many drinks does it take to reach 0.08%?
It depends almost entirely on your weight and sex. A rough average is about three to four standard drinks over an hour for a 150–180 lb male, and two to three drinks for a 120–140 lb female. Enter your own numbers above rather than relying on averages.
How long does it take for BAC to return to zero?
Your liver removes roughly 0.015% BAC per hour regardless of what you do. If you are at 0.08%, expect about five and a half hours of no further drinking to clear — coffee, food, and showers do not speed this up.
Why do males and females have different BAC estimates?
Female bodies carry, on average, a lower percentage of body water and less of the stomach enzyme that breaks down alcohol before it enters the bloodstream. The Widmark constant reflects this: 0.66 for female bodies versus 0.73 for male bodies, so the same drinks produce a higher estimated BAC for a female body at the same weight.
Is this estimate accurate enough to decide whether to drive?
No. Individual factors — food, medications, tiredness, and drinking speed — can move real BAC well above or below the estimate. Even a result below 0.08% does not mean you are fit to drive. The only safe rule is that if you have been drinking, do not drive.
Disclaimer
This BAC calculator is provided for informational purposes only and does not constitute medical, legal, or clinical advice. Estimates are based on the Widmark formula and average population constants; actual blood alcohol content varies widely by individual and cannot be measured accurately by this tool. This site is not a substitute for a calibrated breathalyzer, a blood test, or professional medical judgment. Never drink and drive — arrange a designated driver, rideshare, or taxi, and seek medical help immediately if someone shows signs of alcohol poisoning.