This BAC calculator estimates blood alcohol concentration using the Widmark formula, the standard method used in research and forensic settings for approximating how much alcohol is in someone’s bloodstream. Enter your gender, weight, the drinks you’ve had, and the time elapsed, and it returns an estimated BAC percentage, compares it against a legal limit you choose, and shows roughly how long it would take to reach 0.00%. It is built for education and awareness, not as a tool for deciding whether it’s safe or legal to drive.
⚠ This tool gives a rough estimate only. Real BAC depends on many factors it cannot measure. Never drive after drinking, no matter what number this calculator shows. If impaired, use a taxi, rideshare, or a sober friend.
How to use this BAC calculator
- Choose your Units: US (lb, fl oz) or Metric (kg, ml).
- Select your Gender, Male or Female, since this affects the body water distribution constant used in the formula.
- Enter your Body Weight.
- Enter the Time Since First Drink in hours and minutes.
- Choose a Legal Limit To Compare Against from the dropdown: US Standard (0.08%), Common International (0.05%), US Commercial Driver (0.04%), Zero-Tolerance/Under-21 (0.02%), or Zero Tolerance (0.00%).
- In the drinks table, set the Amount, Size, and ABV for each drink type (beer, wine, liquor), or select “+ Add Another Drink” to add a custom row with its own Type, Brand (optional), Amount, Size, and ABV.
- Review your results: Estimated BAC, a safety message comparing it to your chosen legal limit, the stats row, a full Drink Breakdown table, and an Estimated Time to Reach 0.00% timeline.
- Use “Save Calculation” to keep a record, or export the result as an Image, PDF, or CSV.
What this BAC calculator does
Blood alcohol concentration measures the percentage of alcohol in a person’s bloodstream. This calculator estimates that percentage using the total alcohol content of every drink you log, your body weight, your gender, and how much time has passed since you started drinking, then applies the Widmark formula to produce a single estimated BAC figure. It also runs the same math forward in time to estimate when your BAC would fall back to 0.00%, based on a standard elimination rate.
The Widmark formula explained
BAC = (Alcohol consumed in grams / (Body weight in grams × r)) × 100 − (Elimination rate × hours elapsed)
where r = 0.68 for men, 0.55 for women (body water distribution constant)
Elimination rate ≈ 0.015% per hour
The formula works in two parts. First, it estimates your peak BAC by dividing the total grams of alcohol consumed by your body weight adjusted for a body water distribution constant, r, which differs between men and women because body composition affects how alcohol distributes through body water. Second, it subtracts the amount your body has already metabolized, using a standard elimination rate of about 0.015% per hour, the rate at which most people’s livers clear alcohol from the bloodstream over time.
Why gender changes the result
The r constant (0.68 for men, 0.55 for women) exists because, on average, women have a higher percentage of body fat and lower percentage of body water than men at the same body weight, and alcohol distributes through body water rather than fat tissue. This means two people of identical weight who drink the same amount can end up with meaningfully different estimated BAC, which is exactly why this calculator asks for gender as a required input rather than treating everyone the same.
Why body weight matters
A given amount of alcohol is diluted across a larger volume of body water in a heavier person than in a lighter one, so body weight and estimated BAC are inversely related when the amount of alcohol consumed stays the same. This is also why the calculator asks for your specific weight rather than using an average figure, since the difference between, say, 130 lb and 220 lb meaningfully changes the resulting estimate for identical drinks.
How the drinks table calculates standard drinks
Each row in the drinks table converts a real-world drink, a beer, a glass of wine, a shot of liquor, into a standard measure of pure alcohol based on three inputs: the Amount (how many), the Size (volume, with a customizable unit if your drink doesn’t match a preset), and the ABV (alcohol by volume percentage). The calculator multiplies these together to work out the grams of pure alcohol each row contributes, then sums every row before running the Widmark formula.
Grams of alcohol per drink = Volume × ABV (%) × alcohol density (0.789 g/mL)
This is why entering an accurate ABV matters: a craft beer at 7% ABV contributes meaningfully more alcohol than a light beer at 4%, even at the same serving size, and the calculator’s per-row breakdown reflects that difference rather than treating all “beers” as equivalent.
Reading the legal limit comparison
The Legal Limit dropdown lets you compare your estimated BAC against several commonly referenced thresholds, since legal limits vary by jurisdiction and by driver category.
| Limit option |
Value |
Typical context |
| US Standard |
0.08% |
The standard legal limit for drivers in the United States and many other countries |
| Common International |
0.05% |
A lower limit used in many countries outside the US |
| US Commercial Driver |
0.04% |
The stricter limit applied to commercial vehicle operators in the US |
| Zero-Tolerance / Under-21 |
0.02% |
Limits applied to drivers under the legal drinking age in many US states |
| Zero Tolerance |
0.00% |
Some jurisdictions and driver categories require no measurable alcohol at all |
Selecting the limit that actually applies to your jurisdiction and situation gives a more relevant comparison, but the specific legal limit that applies to you should always be confirmed against your local law, not assumed from this list.
The time-to-sober timeline
Because the Widmark formula includes an elimination rate over time, the calculator can also project forward to estimate when your BAC would return to 0.00%. This is shown as an “Estimated Time to Reach 0.00%” timeline, built on the same roughly 0.015% per hour clearance rate used in the main calculation. This figure moves only in one direction, forward, and nothing (coffee, cold showers, food, exercise) speeds up how quickly the liver actually processes alcohol, despite common belief otherwise.
Why this is an estimate, not a measurement
The explainer on the calculator itself is direct about this: real BAC is affected by many factors the Widmark formula cannot account for, including food in your stomach, medications, liver health, genetics, and how quickly you drank. Two people with identical weight, gender, drink count, and elapsed time can have meaningfully different actual BAC. This calculator is not a legal or medical tool, and it cannot tell you whether it is safe or legal for you to drive. The only reliably safe choice after drinking is not to drive, regardless of what any calculator, app, or personal estimate suggests.
Frequently asked questions
Can I use this BAC calculator to decide if I’m okay to drive?
No. This calculator provides a rough estimate only and cannot account for factors like food intake, medications, liver health, or genetics that meaningfully affect actual BAC. The only guaranteed-safe choice after drinking is not to drive, regardless of the number this or any calculator shows.
Why does the calculator ask for my gender?
The Widmark formula uses a body water distribution constant that differs between men and women (0.68 for men, 0.55 for women), since body composition affects how alcohol distributes through body water. This means the same weight, drinks, and time can produce a different estimated BAC depending on gender.
Does eating food or drinking coffee lower my BAC faster?
No. This calculator’s elimination rate assumes a standard clearance of about 0.015% per hour, the rate at which the liver processes alcohol. Food, coffee, cold showers, and exercise do not speed up how quickly your liver actually metabolizes alcohol, even though they may affect how impaired you feel.
Why do I need to enter the ABV for each drink?
Alcohol by volume varies significantly between drinks, a craft beer at 7% ABV contains meaningfully more alcohol than a light beer at 4% at the same serving size. The calculator multiplies volume by ABV to calculate the actual grams of alcohol each drink contributes, so an accurate ABV entry directly affects the accuracy of your estimated BAC.
