Blood alcohol content is the percentage of alcohol in your bloodstream. In the U.S., 0.08% is the legal driving limit for adults 21 and over — but impairment begins well below that, and commercial and under-21 limits are lower.

Can I use a BAC calculator to decide if I can drive?

No. Never. This estimate cannot account for your individual metabolism, medications, food, health, or drink strength, and can be badly wrong. The only safe amount of alcohol before driving is none. If you have been drinking, do not drive.

BAC Calculator

Q: How is BAC estimated?

The Widmark formula: BAC = (alcohol grams ÷ (body weight in grams × distribution ratio)) × 100, minus roughly 0.015 per hour as your body metabolizes alcohol. It's only an approximation and ignores many individual factors.

A rough educational estimate of blood alcohol content using the Widmark formula. This is not a tool for deciding whether it is safe or legal to drive — the only safe amount before driving is zero.

⚠ Never use this to decide whether to drive. If you have been drinking, do not drive.

Estimated BAC (rough)

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Typical effects

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Hours to reach ~0.00

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U.S. driving limit

0.08%

The Widmark formula

BAC% = (alcohol grams ÷ (weight grams × r)) × 100 − (0.015 × hours)
r ≈ 0.68 (men), 0.55 (women); one standard drink ≈ 14 g alcohol

Why this is only an approximation

The formula ignores food in your stomach, drinking speed, medications, liver health, hydration, and the actual strength of your drinks — all of which move real BAC substantially. Two people with the same inputs can have very different actual levels. Metabolism averages ~0.015%/hour but varies person to person.

What BAC levels mean

BAC	Typical effects
0.02%	Mild relaxation; some judgment/attention decline
0.05%	Lowered alertness, impaired coordination
0.08%	Legal driving limit (U.S.); clear impairment
0.15%+	Major impairment; risk of injury
0.30%+	Potentially life-threatening

Impairment starts below the legal limit, and only time lowers BAC — coffee, water, food, and cold showers do not speed it up. If you have been drinking, use a taxi, rideshare, or designated driver. In an emergency involving heavy drinking (confusion, vomiting, slow breathing, unconsciousness), call emergency services.

Frequently asked questions

What is BAC?

The percentage of alcohol in your blood. 0.08% is the U.S. driving limit, but impairment starts lower.

How is BAC estimated?

The Widmark formula, minus ~0.015%/hour for metabolism — an approximation only.

Can I use this to decide if I can drive?

No, never. It can be badly wrong. The only safe amount before driving is none.

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Important safety & medical disclaimer: This calculator provides a rough educational estimate only and must never be used to determine whether you are fit or legally permitted to drive or operate machinery. Actual blood alcohol content depends on many individual factors this tool cannot measure and can differ greatly from the estimate. If you have consumed any alcohol, do not drive. This is not medical or legal advice. Formula: Widmark. Last reviewed: July 2026.