1.2 BAC, Individual Differences, and Service Decisions

Understanding the measurement

Blood alcohol concentration (BAC) describes the amount of alcohol in a volume of blood. A BAC of 0.08% is 0.08 grams per 100 milliliters of blood. Breath testing uses a different specimen and its own legally defined units; a server should not confuse a breath reading with a direct blood measurement.

BAC is a concentration, not a count of beverages. Knowing that a customer ordered three glasses does not establish their BAC. You may not know each pour's strength, whether drinks were shared, what was consumed before arrival, or the customer's individual response. A service decision should therefore combine observed consumption with behavior and the applicable law and house policy.

Factors that affect risk

Amount and pace are practical observations. Two measured drinks over a long meal and several doubles consumed rapidly are different situations. However, a slow pace is not proof of safety. Do not turn a rough hourly guideline into an entitlement to another drink.

Body water and composition help explain why equal amounts can produce different concentrations. On average, women have less body water than men of the same weight, but individual variation is substantial. Body size or appearance does not let a bartender determine a customer's BAC. Do not rank guests as safe or unsafe by sex or build, and do not ask intrusive questions about body composition.

Food changes the timing of absorption. It does not remove alcohol already absorbed, and it does not make a large dose harmless. A guest who ate may still need immediate refusal. Conversely, knowing that someone has not eaten is a reason to offer food and monitor carefully, not a reason to calculate a precise multiplier for their BAC.

BAC and impairment are related but different

A person can be impaired without obvious stumbling. Someone else may have speech or movement differences unrelated to alcohol. Fatigue, illness, disability, or a medical emergency may affect appearance. Observe changes and respond respectfully; do not diagnose a person from a single cue.

Some medicines intensify sedation or other risks. Alcohol with opioids, sleep medicines, or certain antihistamines can be dangerous. Other interactions can involve bleeding or liver injury rather than the same sedation mechanism. Avoid the false rule that every medication raises BAC or that all combinations multiply effects by a known number. Servers should not recommend doses, treatment, or a waiting period for medication interactions.

If a guest reports medication use, that information can support a cautious service decision. If the guest is confused, hard to wake, or breathing abnormally, obtain emergency assistance. A discussion about whether their measured BAC would be high enough is unnecessary and can delay care.

Driving rules do not define a serving allowance

NHTSA identifies 0.08 g/dL as the general adult per se driving limit in the United States, with Utah using 0.05 g/dL. Impairment can occur below those figures, and additional rules apply to younger drivers and particular driving activities. A per se limit concerns an offense established by a specified alcohol concentration; it does not mean driving below that number is safe or always lawful.

Alcohol-service laws often use terms such as obviously or visibly intoxicated. Those standards depend on jurisdiction and circumstances. A server should not continue serving until someone seems likely to reach a driving limit. You are not administering an evidentiary test, and an estimated number cannot override observable risk or a required refusal.

Practice interpreting incomplete information

Same order, different responses. Two colleagues each order two gin and tonics. One becomes slow to respond and drops a glass. You do not know the exact pours or individual BACs. Stop further alcohol for the guest showing concerning signs and seek assistance. Do not conclude that the other guest must have the same concentration or can safely drive.

The guest quotes a calculator. A customer says an app estimates their BAC below the legal limit. Explain that you cannot serve more alcohol given the observed behavior. An app depends on uncertain inputs and cannot decide compliance with your venue's service obligations.

The confident regular. A familiar customer says their size protects them. Continue applying the same observation and refusal standards used for everyone. Large size does not remove alcohol's effects, and tolerance does not create permission to ignore risk.

Sources checked October 16, 2026: NHTSA drunk-driving guidance, NIAAA harmful interactions, and NIAAA alcohol-medication interactions.