This calculator estimates your blood alcohol concentration, or BAC, from the number of standard drinks you have had, your body weight and sex, and the time that has passed, using the long-established Widmark formula. It is offered as an educational guide to help you understand how alcohol affects the body and how slowly it leaves, not as a way to judge whether you are safe to drive. In New Zealand a standard drink contains 10 grams of pure alcohol, and your body removes alcohol at a roughly fixed rate, about one standard drink per hour for many people, which is why time is the only thing that genuinely lowers your BAC. Coffee, food, water and a cold shower do not speed it up. You enter your sex, which affects how alcohol distributes through the body, your weight, the number of standard drinks consumed, and the hours since your first drink, and the calculator estimates your current BAC as a percentage, shows how it compares with the New Zealand legal driving limits, and estimates how long until your BAC returns to zero. The legal limit for drivers aged 20 and over is 0.05 percent, or 250 micrograms of alcohol per litre of breath, while for drivers under 20 the limit is zero. The crucial message is that this is only an estimate: real BAC varies widely with food, hydration, medication, health, drink strength and individual metabolism, and people routinely underestimate how affected they are. The only safe amount of alcohol before driving is none. If you have been drinking, do not drive; plan a sober driver, taxi or ride-share. This tool is for awareness, never a green light to get behind the wheel.
An educational estimate only. Real BAC varies with food, health and many factors. The only safe level before driving is zero. Never drink and drive.
The Widmark formula estimates BAC from the grams of alcohol (standard drinks times 10), divided by your body weight in grams times a distribution factor of 0.68 for men or 0.55 for women, expressed as grams per 100 millilitres. Alcohol is then removed at about 0.015 percent per hour, so the time elapsed is subtracted. Hours to zero is the current BAC divided by 0.015.
For an 80 kilogram man who has had 4 standard drinks, that is 40 grams of alcohol. Dividing by his body water gives about 0.073 percent, and after 2 hours about 0.03 percent is removed, leaving an estimated BAC near 0.044 percent, just under the 0.05 limit for drivers 20 and over. It would take roughly another 3 hours to reach zero. This is only an estimate.
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