How a BAC Calculator Works
A blood alcohol content (BAC) calculator estimates the percentage of alcohol in your bloodstream based on how much you have had to drink, your body weight, your gender, and how long ago you started drinking. This tool uses the Widmark formula, the standard model behind most BAC estimators.
The calculation works by converting your drinks into grams of alcohol, dividing by the volume of water in your body, and then subtracting the amount of alcohol your liver has already processed. Because everyone metabolizes alcohol differently, the result is only an approximation and should never be used to decide whether you are safe to drive.
The Widmark Formula
- BAC % = (grams of alcohol ÷ (body weight in grams × r)) × 100 − 0.015 × hours
- Each standard drink contains about 14 grams of pure alcohol
- r is the body-water constant: 0.73 for men, 0.66 for women
- Your body eliminates alcohol at roughly 0.015% BAC per hour
Understanding the Numbers
In most of the United States the legal driving limit is 0.08% BAC, but impairment begins well below that level. Even a BAC of 0.02% can slow reaction times. The only thing that reliably lowers your BAC is time — coffee, cold showers, and food do not speed up the process.
- A standard drink is about 12 oz of beer, 5 oz of wine, or 1.5 oz of spirits.
- Drinking on an empty stomach raises BAC faster.
- Medications and health conditions can change how alcohol affects you.
- The estimate cannot account for individual tolerance or metabolism.
Frequently Asked Questions
Is this BAC estimate accurate enough to drive?
No. This calculator provides a rough estimate for educational purposes only. Actual BAC varies with countless personal factors that the formula cannot capture. Never rely on any calculator to decide whether you are fit to drive — if you have been drinking, do not drive.
How long does it take for alcohol to leave my system?
The body eliminates alcohol at a fairly steady rate of about 0.015% BAC per hour. The calculator estimates time to reach a near-zero reading, but this is approximate. Sleep, hydration, and waiting are the only ways to sober up.
Why do men and women get different results?
The Widmark formula uses a different body-water constant for men (0.73) and women (0.66) because women, on average, have a higher proportion of body fat and less body water, which leads to higher alcohol concentrations from the same amount of alcohol.