BAC Calculator — estimate blood alcohol content (Widmark)

This is an estimate, not a measurement. Do not use it to decide whether you are safe or legal to drive, operate machinery, or do anything else. Individual BAC varies widely with food, medication, metabolism, hydration and health, and only a breath or blood test measures it. If you have been drinking, the safe choice is to not drive.

How it works

Blood alcohol content (BAC) is the percentage of alcohol in your blood — 0.08% means 0.08 g of alcohol per 100 ml. This calculator uses the Widmark equation, the standard estimate:

• BAC = 100 × A ÷ (W × r) − 0.018 × t

where A is the grams of pure alcohol consumed (14 g per US standard drink), W is body weight, r is the alcohol-distribution ratio (0.68 for men, 0.55 for women, reflecting differences in body water), and t is hours since the first drink. The last term is elimination: the liver clears roughly 0.018% BAC per hour on average, and there is no way to speed it up — coffee, water, food and a cold shower do not lower BAC, only time does.

Two people who drink the same amount can reach very different levels: a lighter person, and (on average) a woman, will reach a higher BAC than a heavier person or a man, because the alcohol is distributed through less body water.

Worked example

A 160 lb man, 4 standard drinks, 1 hour after starting:

• Alcohol: 4 × 14 g = 56 g
• BAC = 100 × 56 ÷ (72,575 g × 0.68) − 0.018 × 1 ≈ 0.113 − 0.018 = 0.095%

That is above the 0.08% limit — this person should not drive.

Frequently asked questions

What is the legal BAC limit to drive?

In most US states it is illegal to drive with a BAC **at or above 0.08%** if you are 21 or over; **Utah is stricter at 0.05%**. Commercial drivers and drivers under 21 face much lower or zero limits, and many other countries use 0.05% or 0.02%. Being under a limit does not mean driving is safe — impairment begins with the first drink.

How accurate is this estimate?

It is a **rough estimate**. The Widmark formula ignores food in the stomach, drinking pace, drink strength, medications, body composition and individual metabolism, all of which shift real BAC. Treat the number as a ballpark, never as proof you are under a limit.

How long does it take for BAC to reach zero?

At about **0.018% per hour**, a BAC of 0.08% takes roughly 4–5 hours to clear, and higher levels take proportionally longer. Only time lowers BAC — nothing speeds up the liver.

Does coffee, water, or food sober you up?

No. Food eaten **before or while** drinking slows how fast BAC rises, but nothing lowers BAC once alcohol is in your blood. Coffee makes you more alert but just as impaired.

When is drinking a medical emergency?

Very high BAC can be life-threatening. Warning signs of an **alcohol overdose** (alcohol poisoning), per NIAAA, include mental confusion, trouble staying conscious or being unable to wake, vomiting, seizures, slow or irregular breathing, slow heart rate, clammy or bluish skin, and a very low body temperature. **Call 911 (or your local emergency number) right away** — do not wait for all the signs, and do not leave the person to "sleep it off." While you wait, keep them awake and sitting up if you can, or turn them on their side with an ear toward the ground to keep the airway clear. Mixing alcohol with opioids or sedatives (including some sleep, anxiety and allergy medicines) sharply raises the risk.

Why do weight and sex change the result?

Alcohol spreads through body water. A heavier person has more water to dilute it, so a lower BAC; women on average have a lower proportion of body water than men of the same weight, so a higher BAC from the same drinks — which is why the formula uses a different ratio for each.

What counts as one standard drink?

In the US, one standard drink is about **14 g of pure alcohol** — 12 oz of 5% beer, 5 oz of 12% wine, or 1.5 oz of 40% spirits. Many cocktails and large pours count as more than one.