BAC Calculator
Estimate your blood alcohol content from your weight, biological sex, number of standard drinks, and hours since your first drink. Uses the Widmark formula — the same model used in forensic toxicology. Educational only: never use this to decide whether to drive.
Estimated BAC
0.0438 %
- Alcohol consumed
- 28 g
- Hours until BAC reaches 0.00 %
- 2.92 hr
Currently below the 0.08 % limit used in the US and England/Wales/NI (Scotland: 0.05 %). Widmark formula; 14 g per standard drink (US/NIAAA). ⚠ Educational estimate only — never use to decide whether to drive.
How to use this calculator
Enter your body weight in kilograms, select your biological sex (this affects the Widmark distribution factor), enter how many standard drinks you have consumed, and how many hours have passed since your first drink. One standard drink is defined as 14 g of pure ethanol — roughly a 12 fl oz 5% beer, a 5 fl oz glass of 12% wine, or a 1.5 fl oz shot of 40% spirits. The result is an estimated BAC in g/100 mL (%). The breakdown shows total alcohol consumed in grams, how many hours from now until BAC reaches 0.00%, and — if you are currently above the 0.08% legal threshold — how long until you fall below it.
How the calculation works
The calculator uses the Widmark formula: BAC (%) = (A × 100) / (BW × r) − (β × t). A is the grams of ethanol consumed; BW is body weight in grams; r is the Widmark distribution factor (0.68 for males, 0.55 for females), which reflects how alcohol distributes through body water; β is the mean elimination rate of 0.015 g/100 mL per hour; and t is hours elapsed since the first drink. The result is the estimated current BAC as a percentage (g/100 mL). Time to zero is current BAC divided by β. The Widmark factor differs between sexes because women typically have lower total body water per unit of body mass.
Worked example
A 70 kg male drinks 2 standard US drinks (28 g of ethanol) and 1 hour has elapsed. Peak BAC (at t = 0) = (28 × 100) / (70,000 × 0.68) = 2800 / 47,600 = 0.0588%. After 1 hour of elimination: 0.0588 − (0.015 × 1) = 0.044%. Time to zero from now: 0.044 / 0.015 = 2.9 hours. This person is below the 0.08% legal threshold.
Frequently asked questions
What is a standard drink?
One standard drink as defined by the US National Institute on Alcohol Abuse and Alcoholism (NIAAA) contains 14 g of pure ethanol. This is roughly a 12 fl oz (355 mL) can of 5% beer, a 5 fl oz (148 mL) glass of 12% wine, or a 1.5 fl oz (44 mL) shot of 40% spirits. Note that UK "units" (8 g) and Australian standard drinks (10 g) differ — adjust accordingly if you are counting in those units.
Why does biological sex affect BAC?
The Widmark distribution factor (r) differs between males (0.68) and females (0.55) because women typically have a higher proportion of body fat relative to total body water. Since alcohol distributes into body water but not fat, the same amount of alcohol reaches a higher concentration in a female body of the same weight. This is a population average — individual variation is substantial.
How accurate is the Widmark formula?
The Widmark formula is the standard model in forensic toxicology and is used by courts worldwide, but it produces a population-average estimate. Actual BAC is affected by whether you drank on an empty stomach (food slows absorption significantly), hydration, medications, liver health, and metabolic rate. Individual elimination rates range from 0.010 to 0.020 g/100 mL/hour — the 0.015 midpoint used here could be 33% off in either direction.
What is the legal BAC limit for driving?
Limits vary by country. In the US, the federal threshold is 0.08% (all 50 states and DC). In England, Wales, and Northern Ireland the limit is also 0.08% (80 mg/100 mL). Scotland reduced its limit to 0.05% in 2014. Most of continental Europe uses 0.05%; several countries (Czech Republic, Hungary, Slovakia, Romania) enforce 0.00%. Always check the specific rules for where you are driving.
Can I use this to decide if I am safe to drive?
No. This calculator is educational — it illustrates how the Widmark formula works and provides a rough population-average estimate. It cannot account for your individual physiology, food intake, hydration, medication, or fatigue. Impairment begins well below the legal limit. The only safe choice if you have been drinking is not to drive.
Why does BAC drop at 0.015% per hour?
Ethanol is primarily metabolised in the liver by alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH). At typical drinking doses, this proceeds at a roughly constant rate — a zero-order kinetic process — rather than slowing as concentration falls. The population mean is approximately 0.015 g/100 mL per hour; heavy regular drinkers may metabolise faster (up to 0.020), while some individuals metabolise more slowly.
Does eating before drinking affect BAC?
Yes, significantly. Food in the stomach slows gastric emptying, which delays alcohol absorption. Peak BAC on a full stomach may be 30–50% lower than on an empty stomach, and the peak occurs later. The Widmark formula assumes average absorption and does not model food intake separately — treat its output as a rough upper bound if you have eaten.