Blood Alcohol (BAC) Calculator

Formula

BAC % = [Alcohol Consumed (g) ÷ (Body Weight (g) × r)] × 100 − (0.015 × Hours)

Alcohol Consumed (g) is total pure ethanol consumed, calculated as standard drinks × 14 grams. Body Weight (g) is body mass in grams (kg × 1,000). The gender distribution ratio (r) accounts for total body water content, typically set to 0.68 for biological males and 0.55 for biological females. The constant 0.015 represents the average rate of alcohol clearance from the bloodstream per hour (0.015% BAC per hour). Hours is the elapsed time since the first drink.

Worked example

Standard drinks (14 g alcohol each)

3

Body weight

75

Sex

m

Hours since first drink

2

• Estimated BAC: 0.052%
• Hours until about 0.00%: 3.5
• US driving limit: Under 0.08% (still may be impaired)

What the BAC Calculator Measures and Who It Is For

Blood Alcohol Concentration (BAC) measures the percentage of alcohol present in your bloodstream, expressed as grams of alcohol per 100 milliliters of blood. A BAC of 0.05% means an individual has 0.05 grams of alcohol per 100 milliliters of blood.

This calculator utilizes the widely established Widmark formula, a standard toxicological model developed by Swedish physician Erik Widmark. It estimates BAC based on:

• Standard drinks consumed (each containing 14 grams of pure ethanol)
• Body weight (in kilograms)
• Biological sex (which determines the body water distribution coefficient, $r$)
• Elapsed time (in hours since the first drink was consumed)

This tool is designed for health educators, students, and curious adults seeking to understand how the body processes alcohol over time. It helps illustrate how body mass, biological sex, and pacing alter alcohol concentration, highlighting why identical drink counts affect individuals differently.

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Step-by-Step Worked Example

To see how the Widmark formula works in practice, consider the following baseline scenario:

• Standard drinks ($d$): 3 drinks
• Body weight ($w$): 75 kg
• Biological sex ($s$): Male ($r = 0.68$)
• Hours elapsed ($h$): 2 hours

Step 1: Calculate Total Alcohol Consumed in Grams

Each standard US drink contains approximately 14 grams of alcohol: $$\text{Alcohol (g)} = 3 \times 14 = 42\text{ g}$$

Step 2: Calculate Effective Body Weight in Grams

Convert body weight to grams and multiply by the gender distribution ratio ($r = 0.68$ for males): $$\text{Weight (g)} = 75 \times 1{,}000 = 75{,}000\text{ g}$$ $$\text{Water Distribution Mass} = 75{,}000 \times 0.68 = 51{,}000\text{ g}$$

Step 3: Calculate Peak Theoretical BAC Percentage

Divide alcohol consumed by the distribution mass and multiply by 100 to get the percentage: $$\text{Peak BAC} = \left(\frac{42}{51{,}000}\right) \times 100 \approx 0.08235%$$

Step 4: Subtract Metabolic Elimination

The human liver metabolizes alcohol at an average rate of 0.015% BAC per hour. Over 2 hours, the clearance is: $$\text{Metabolized BAC} = 0.015 \times 2 = 0.030%$$

Subtract the metabolized amount from the peak figure: $$\text{Estimated BAC} = 0.08235% - 0.030% = 0.05235% \approx 0.052%$$

Step 5: Estimate Time to Reach 0.00% BAC

To find the remaining time until complete clearance, divide the current estimated BAC by the hourly elimination rate: $$\text{Hours remaining} = \frac{0.05235%}{0.015%} \approx 3.49 \approx 3.5\text{ hours}$$

At 0.052%, this individual is numerically under the 0.08% US legal driving limit, but reaction times, depth perception, and coordination remain measurably impaired.

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How to Read BAC Benchmarks

Alcohol impairs brain function and motor skills in predictable stages, even if an individual feels completely sober:

• 0.020% to 0.039% (Mild Relaxation): Slight body warmth, minor mood elevation, and subtle loss of fine motor control. Visual tracking begins to decline.
• 0.040% to 0.059% (Lowered Inhibitions): Minor coordination loss, reduced alertness, and release of inhibition. Judgments regarding risk become impaired.
• 0.060% to 0.079% (Noticeable Impairment): Pronounced blunting of motor coordination, reduced reaction speed, and diminished depth perception. Self-control and reasoning weaken.
• 0.080% and Above (Legally Intoxicated in the US): Muscle coordination is poor, speech may slur, reaction times are significantly degraded, and cognitive hazard processing is heavily compromised. Driving is dangerous and illegal.
• 0.150% and Above (Severe Intoxication): Significant loss of balance, gross motor impairment, nausea, and high risk of blackouts.

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Practical Tips for Managing Alcohol Consumption

1. Pace Drinks to One Per Hour: The average adult liver clears roughly one standard drink (about 0.015% BAC) every 60 to 90 minutes. Sipping slowly allows metabolism to keep pace with absorption.
2. Eat Substantial Meals Before Drinking: Having food in the stomach delays gastric emptying into the small intestine, where the vast majority of alcohol is absorbed. This flattens peak BAC levels.
3. Watch Alcohol by Volume (ABV): A 16-ounce pint of an 8% ABV India Pale Ale contains over 2.4 standard drinks, not one. Always check serving sizes against the 14-gram standard.
4. Alternate with Water: Drinking non-alcoholic beverages between alcoholic drinks slows consumption speed and prevents dehydration.

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When Not to Rely on This Calculator

Mathematical formulas calculate statistical medians, not biological certainties. Do not rely on this calculator under any of the following conditions:

• Deciding to Drive: Never use an estimated BAC to justify getting behind the wheel. Alcohol begins degrading split-second decision-making at BAC levels well below 0.05%.
• Medication Interactions: Many prescription and over-the-counter medications—including antihistamines, sedatives, antidepressants, and pain relievers—amplify impairment without changing your blood alcohol reading.
• Underlying Health Conditions: Impaired liver function, metabolic disorders, and recent bariatric surgeries significantly alter absorption and elimination rates.
• Legal Proceedings: Law enforcement and courts rely on certified evidential breathalyzers or gas chromatography blood tests. Mathematical self-estimations carry zero legal standing.

Comparison

Scenario	Input	Result
Lower input	Standard drinks (14 g alcohol each): 2.4	Estimated BAC: 0.036%
Base example	Standard drinks (14 g alcohol each): 3	Estimated BAC: 0.052%
Higher input	Standard drinks (14 g alcohol each): 3.6	Estimated BAC: 0.069%

Common mistakes

• Equating one specialty cocktail, heavy pour, or high-ABV craft beer to a single standard drink.
• Measuring elapsed time from the last drink rather than the first drink, underestimating total metabolism time.
• Assuming coffee, cold showers, exercise, or drinking water accelerates the body's rate of alcohol elimination.
• Confusing zero alcohol absorption with peak absorption; BAC often continues rising after drinking stops.
• Using theoretical BAC formulas as legal justification or proof of sobriety during law enforcement interactions.

Checklist

• Count drinks using standard units (12 oz regular beer, 5 oz wine, or 1.5 oz 80-proof spirits = 14 g pure alcohol).
• Track time starting from the moment you take your first sip, not when you finish drinking.
• Convert your body weight accurately to kilograms (pounds divided by 2.205).
• Select the biological sex factor to reflect average body water differences.
• Factor in whether alcohol was consumed with food, which slows absorption but does not speed metabolism.
• Never rely on a calculated BAC to decide whether you are safe or legal to drive.

Calculators Percentage Editorial Team

Calculator guides from the Calculators Percentage team. These introductory examples are educational, not professional advice.

Last reviewed: November 12, 2026

This calculator provides a rough mathematical estimate based on average population metrics. It should never be used to determine legal sobriety or your ability to safely operate a motor vehicle.