BAC Calculator (Blood Alcohol Content)

What is blood alcohol content?

Blood alcohol content (BAC), also called blood alcohol concentration, measures the grams of ethanol per 100 milliliters of blood, expressed as a percentage. A BAC of 0.08% means 0.08 grams of alcohol per 100 mL of blood, or 80 mg/dL in clinical units.

Alcohol distributes throughout total body water after absorption from the stomach and small intestine. Peak BAC typically occurs 30 to 90 minutes after the last drink on an empty stomach, later with food. Once absorbed, elimination proceeds at a relatively fixed rate. Coffee, cold showers, and exercise do not speed metabolism.

Impairment of reaction time, judgment, and coordination tracks with BAC. Legal driving limits, workplace policies, and medical decisions still use those thresholds. Tolerance can hide how drunk someone looks. It does not make driving safer.

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The Widmark formula explained

Swedish physician Erik Widmark published the foundational BAC estimation equation in 1932. The simplified form: BAC = (alcohol consumed in grams ÷ (body weight in grams × r)) × 100 − (β × hours elapsed), where r is the Widmark distribution ratio and β is the elimination rate.

Distribution ratio r accounts for sex differences in body water fraction: about 0.68 for men and 0.55 for women.

A 70 kg man has more total body water per kilogram than a 70 kg woman, so the same alcohol dose produces a higher BAC in women. Standard elimination rate β is roughly 0.015% BAC per hour, about one standard drink metabolized per 60 to 90 minutes. The range spans 0.010 to 0.020% depending on liver enzyme activity, genetics, medications, and chronic drinking history.

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How this BAC calculator works

Enter body weight, sex, number of US standard drinks consumed (each containing 14 grams of pure ethanol), and hours since your first drink. The calculator converts drinks to grams of alcohol, applies the Widmark distribution ratio for your sex, and subtracts estimated metabolism over elapsed time.

One US standard drink equals 12 oz beer at 5% ABV, 5 oz wine at 12% ABV, or 1.5 oz spirits at 40% ABV. Mixed drinks often contain two or more standard drinks. Count the pour, not the glass.

Actual BAC depends on gastric emptying rate, food intake, hydration, liver function, and concurrent medications. Never use a calculated BAC to decide whether you are safe to drive.

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Effects at different BAC levels

At 0.02 to 0.04%, many people notice mild relaxation and slight warmth, with some decline in visual tracking. From 0.05 to 0.07%, coordination, judgment, and steering ability drop measurably. Impairment begins well below legal limits and below what many people perceive as a buzz.

0.08% is the per se legal driving threshold in most US states. NIAAA defines binge drinking as a pattern that typically raises BAC to 0.08% or higher, often after four drinks for women or five for men within about two hours. At 0.10 to 0.15%, slurred speech, balance problems, and nausea are common in non-tolerant drinkers.

From 0.16 to 0.30%, severe intoxication, blackout risk, vomiting, and loss of consciousness become likely. Above 0.30%, respiratory depression, coma, and death from alcohol poisoning are possible, especially with rapid binge consumption. Place an unconscious breathing person on their side and stay with them.

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BAC reference table

The ranges overlap because BAC is an exposure measure, not a complete neurologic examination. Fatigue, cannabis, opioids, benzodiazepines, antihistamines, head injury, and low blood glucose can produce more impairment than the estimated BAC suggests. A person with tolerance may look less intoxicated while retaining the same driving impairment and poisoning risk. Do not raise or lower the result based on how someone appears.

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Worked examples and uncertainty

| Scenario | Arithmetic | Calculator estimate before uncertainty | | --- | --- | --- | | 180 lb man, 3 standard drinks, 2 hours | 42 g alcohol divided by body weight and distribution factor, then subtract 0.030% | About 0.04% to 0.05% | | 140 lb woman, 3 standard drinks, 2 hours | Same 42 g dose with a smaller average distribution volume, then subtract 0.030% | About 0.07% to 0.09% | | Either person immediately after the last drink | Absorption may still be occurring | The displayed value may be below the later peak |

These examples show why one drink count can produce different estimates. The simplified Widmark equation uses population averages for distribution and elimination. Modern forensic guidance uses ranges, including elimination rates around 0.010% to 0.025% per hour, rather than claiming an exact value. A field validation of 109 drinkers found that common estimated-BAC equations explained only about 54% to 55% of measured breath-alcohol variation and tended to overestimate on average. The calculator is unsuitable for legal reconstruction or permission to drive.

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Measurement, screening, and diagnosis

A calculator estimates concentration from self-reported dose, weight, sex category, and time. A calibrated breath instrument estimates breath alcohol. A laboratory blood test directly measures alcohol in a specimen. None of these alone diagnoses alcohol use disorder, poisoning, concussion, or driving competence. Alcohol use disorder requires a clinical history of symptoms and consequences over time. Alcohol poisoning is a clinical emergency judged from breathing, consciousness, temperature, vomiting, seizures, and other findings, not from a calculator threshold.

For health screening, clinicians ask about standard drinks, heavy-drinking days, and loss of control, then assess withdrawal risk and DSM criteria when indicated. A high estimated BAC can prompt a safer plan for the current episode, but a low estimate cannot clear symptoms such as confusion or unsteady walking. If a measured result and the person's condition conflict, clinicians evaluate other drugs, trauma, metabolic illness, and timing of absorption.

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How to use the estimate safely

Enter the actual container size and ABV, convert each serving to US standard drinks, and measure time from the start of drinking. Recalculate only to understand a possible range, not to count down toward driving. If any alcohol was consumed, choose a sober driver, public transit, a taxi, or a place to stay. Food may delay and flatten the peak, but it does not remove alcohol already consumed. Coffee and exercise do not accelerate hepatic elimination. Mixed drinks often contain two or more standard drinks, so count each pour instead of assuming one glass equals one drink.

Call emergency services for inability to wake, seizures, repeated vomiting with reduced alertness, blue or pale skin, or breathing slower than eight times per minute or with pauses of ten seconds or more. Place an unconscious breathing person on their side, stay with them, and do not induce vomiting. Alcohol mixed with opioids, benzodiazepines, sleep medicines, or other sedatives can suppress breathing at a BAC that might otherwise appear modest. People who drink heavily every day should obtain medical advice before abruptly stopping because withdrawal can also be dangerous.

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Legal limits and safety

All US states enforce 0.08% BAC as the per se limit for drivers 21 and older. Utah uses 0.05%. Commercial drivers face 0.04%. Zero-tolerance laws apply to drivers under 21. Penalties escalate with higher BAC and prior offenses. CDC data show alcohol-impaired driving accounted for about 32% of US traffic deaths in 2022. NIAAA notes that impairment can occur below the legal limit and below what many people perceive as a buzz.

Do not drive after any drinking. Use a sober driver, rideshare, or stay overnight.

Signs of alcohol poisoning requiring emergency care: confusion, vomiting while semi-conscious, seizures, slow or irregular breathing below eight breaths per minute, hypothermia, and inability to wake. Do not leave an intoxicated person alone to "sleep it off."

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Limitations and when to seek help

Widmark estimates cannot replace breathalyzer or blood test results used in legal proceedings. Results remain estimates only and cannot clear a person to drive.

Gastric absorption continues for up to two hours after the last drink in some cases, so BAC may still be rising when you calculate. Regular heavy drinking increases tolerance to outward signs of intoxication without reducing BAC or organ damage. If you drink daily, experience withdrawal when stopping, or fail to cut down despite consequences, consult a healthcare provider or SAMHSA helpline (1-800-662-4357).

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How it works

Widmark: BAC = (alcohol grams / (weight × r)) × 100 − 0.015×hours. r = 0.68 men, 0.55 women.

Frequently asked questions

• What BAC is legally impaired?0.08% is the legal limit for driving in most US states. Impairment begins well below this.

Related calculators

References

1. CDC. Alcohol Use and Your Health
2. Gullberg, 1992. Widmark equation for blood alcohol concentration
3. NIAAA. Rethinking Drinking: Alcohol Metabolism
4. Moskowitz & Fiorentino. Effects of alcohol on driving performance
5. CDC. Impaired Driving Facts
6. NIST OSAC. Guidelines for Performing Alcohol Calculations in Forensic Toxicology
7. Hustad & Carey. Using calculations to estimate blood alcohol concentrations
8. NIAAA. Understanding the Dangers of Alcohol Overdose