Blood Alcohol Concentration (BAC) Explained: What Every Server Should Know

Blood Alcohol Concentration (BAC) is the measurement of alcohol in a person's bloodstream, expressed as a percentage. A BAC of 0.08 means that 0.08% of the person's blood is alcohol. While servers cannot measure a patron's BAC in real time, understanding how BAC works is essential for recognizing signs of intoxication, pacing service responsibly, and knowing the legal thresholds that matter in Ontario.

Ontario's Legal BAC Limits

Ontario uses a tiered system of BAC limits for drivers, and these thresholds are important for servers to understand because they define the legal context in which your patrons are operating when they leave your establishment.

Every licence suspension also comes with a licence reinstatement fee. The figures above were checked against the Ontario government's impaired driving page in October 2026: Ontario.ca: Impaired driving.

Why this matters for servers: When a patron with a BAC of 0.05 gets behind the wheel, they already face provincial penalties. By the time someone is visibly intoxicated (typically around 0.08 or higher), they are in criminal territory. Your responsibility is to stop serving well before a patron reaches these levels.

How the Body Processes Alcohol

When someone drinks alcohol, the process follows a predictable pattern:

1. Absorption: Alcohol enters the bloodstream primarily through the stomach (about 20%) and the small intestine (about 80%). Absorption begins within minutes of the first sip.
2. Distribution: Once in the bloodstream, alcohol is distributed throughout the body's water content. This is why body composition matters — more water means more dilution.
3. Metabolism: The liver metabolizes approximately 90-95% of consumed alcohol using an enzyme called alcohol dehydrogenase (ADH). The remaining 5-10% is eliminated through breath, sweat, and urine.
4. Elimination: The average person metabolizes alcohol at a rate of approximately 0.015 BAC per hour. This rate is essentially fixed — you cannot speed it up with coffee, food, cold showers, or exercise.

Key fact for the SmartServe exam: The body eliminates alcohol at a fixed rate of approximately one standard drink per hour. Nothing can speed up this process. Coffee makes a drunk person more awake, not less drunk.

Factors That Affect BAC

Two people can drink the same amount of alcohol and have very different BAC levels. Understanding these factors helps you assess risk, even when you cannot measure BAC directly.

Body Weight and Composition

A person who weighs more generally has more water in their body to dilute the alcohol, resulting in a lower BAC for the same amount consumed. However, body composition matters too — muscle tissue contains more water than fat tissue, so a muscular person and a person of the same weight with more body fat will have different BAC levels after the same number of drinks.

Biological Sex

On average, women tend to reach higher BAC levels than men of the same weight after consuming the same amount of alcohol. This is due to differences in body water percentage (women typically have proportionally less body water), differences in the enzyme ADH (women generally produce less of it in the stomach lining), and differences in average body composition.

Food in the Stomach

Eating before or while drinking slows alcohol absorption significantly. Food, especially food high in protein and fat, keeps the pyloric valve (the opening between the stomach and small intestine) closed longer, meaning alcohol stays in the stomach where it is absorbed more slowly. A person drinking on an empty stomach can absorb alcohol up to three times faster than someone who has eaten a full meal.

Practical implication: This is why offering food to patrons is a core responsible service practice. It is not just good hospitality — it directly slows intoxication.

Rate of Consumption

Drinking quickly overwhelms the liver's ability to metabolize alcohol, causing BAC to spike. Two drinks consumed in 15 minutes will produce a much higher peak BAC than two drinks consumed over two hours, even though the total alcohol consumed is identical. This is why pacing drinks — limiting service to approximately one standard drink per hour — is a fundamental service guideline.

Medications and Health Conditions

Many common medications interact with alcohol to increase its effects. Antihistamines, pain medications, antidepressants, and anti-anxiety medications can all amplify alcohol's impact on the brain, meaning a person on medication may become impaired at a much lower BAC. Liver conditions, diabetes, and other health issues also affect metabolism. You cannot know a patron's medical history, which is another reason to monitor behaviour rather than try to count drinks.

Carbonation

Carbonated alcoholic drinks (champagne, sparkling wine, mixed drinks with soda) are absorbed faster than non-carbonated equivalents. The carbon dioxide increases the rate at which alcohol passes from the stomach to the small intestine.

Tolerance

Regular drinkers develop functional tolerance — they may appear less impaired at a given BAC than an infrequent drinker. However, tolerance only masks the behavioural signs. The actual impairment to motor skills, reaction time, and judgment is the same regardless of tolerance. A high-tolerance drinker at 0.08 BAC is just as dangerous behind the wheel as a low-tolerance drinker at 0.08.

This is critical: A patron who "holds their liquor well" is not less impaired — they are just better at hiding it. Do not use apparent sobriety as your only measure.

Standard Drinks: The Measurement That Matters

In Canada, one standard drink contains 13.45 grams (17.05 mL) of pure alcohol. This is equivalent to:

The problem is that many real-world drinks contain more than one standard drink:

• A pint of beer (568 mL) at 5% is approximately 1.7 standard drinks
• A craft beer at 7-9% ABV can be 2 or more standard drinks per pint
• A large glass of wine (250 mL) at 14% is about 2 standard drinks
• A Long Island Iced Tea or other multi-spirit cocktail can contain 3-4 standard drinks
• A double of any spirit is, by definition, 2 standard drinks

When you are tracking a patron's consumption, count in standard drinks, not in "number of orders." A patron who has had "only three drinks" might have consumed the equivalent of six or more standard drinks depending on what they ordered.

BAC Estimation: A Rough Guide

While you cannot measure BAC behind the bar, understanding general estimates helps you assess risk:

• After 1 standard drink: Most people reach approximately 0.02 BAC. Minimal noticeable effects.
• After 2-3 standard drinks in one hour: BAC of approximately 0.04-0.06 for an average-weight person. Relaxation, lowered inhibitions, slight impairment of judgment.
• After 4-5 standard drinks in two hours: BAC of approximately 0.08-0.10. This is at or above the criminal limit. Clear impairment of motor skills, reaction time, and judgment. Visible signs of intoxication.
• After 6+ standard drinks in two hours: BAC above 0.10. Significant impairment. Slurred speech, poor coordination, emotional instability.

These are rough estimates only. Individual variation based on the factors discussed above means these numbers can shift dramatically. Always rely on observable signs of intoxication rather than drink counts alone.

Common Myths About BAC

• "Coffee sobers you up" — False. Caffeine masks drowsiness but does not reduce BAC or impairment. The person is now a wide-awake drunk.
• "Eating after drinking absorbs the alcohol" — Mostly false. Eating before drinking slows absorption, but eating after the alcohol is already in the bloodstream has minimal effect on BAC.
• "A cold shower or fresh air will help" — False. Neither reduces BAC. They may make the person feel more alert temporarily, but impairment remains.
• "You can tell how drunk someone is by counting their drinks" — Unreliable. As we have seen, body weight, food intake, medications, drink strength, and tolerance all create massive variation.
• "Beer before liquor, never been sicker" — Myth. The order of drinks does not matter. Total alcohol consumed and the rate of consumption are what determine BAC.

Applying BAC Knowledge on the Job

You will never know a patron's exact BAC. But understanding the science helps you make better decisions:

• Pace service to approximately one standard drink per hour
• Encourage food throughout the visit, not just at the beginning
• Monitor behaviour changes from the patron's baseline when they arrived
• Account for drink strength — a double counts as two, a pint counts as almost two
• Be extra cautious with small-framed patrons, those who have not eaten, and those who arrived already showing signs of consumption

For more on standard drink measurements and practical service guidelines, review our standard drinks and study guide sections.