BAC & Alcohol Pharmacokinetics Glossary

Most BAC calculators online get the math close enough but treat the underlying vocabulary as if it does not matter. It does. Knowing the difference between the absorption phase and the elimination phase is what separates an estimate that is useful at 1 AM from one that puts you back behind the wheel an hour too early. Knowing why the Widmark constant is 0.68 for men and 0.55 for women is what tells you whether the result on screen is honest about your own physiology, or borrowed from somebody else's.

Each entry below is short on purpose — definition, the number that matters, and how it shows up inside our BAC calculator, our methodology page, or one of our legal reference pages. Sources are linked where the definition comes from a primary authority (NIAAA, NHTSA, peer-reviewed pharmacology). For educational reference only — this is not medical, legal, or pharmacological advice.

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The Metabolic Pathway, Visualized

A–Z Definitions

Absorption Phase

The window between the first sip and peak BAC. On an empty stomach, ethanol clears the stomach and small intestine into the bloodstream within 30–90 minutes (average 45–60). Food slows it down by delaying gastric emptying, which is why stomach state is an input on our calculator.

Acetaldehyde

The toxic intermediate the liver produces from ethanol via ADH. Chemical formula CH₃CHO. The IARC classifies it as a Group 1 carcinogen in the context of alcoholic beverages. It is the molecule responsible for hangover symptoms and the alcohol flush reaction.

ADH (Alcohol Dehydrogenase)

The liver enzyme that catalyzes the first step of alcohol metabolism — converting ethanol to acetaldehyde, with NAD⁺ as cofactor. About 90% of clearance happens in the liver. ADH variants explain part of why metabolism rate differs between individuals, even at the same body weight.

ALDH2 & the Flush Reaction

ALDH2 is the enzyme that clears acetaldehyde to harmless acetate. The rs671 variant (ALDH2*2 allele) cripples this step and is carried by 20–30% of people with Chinese, Japanese, or Korean ancestry. Carriers can experience facial flushing, nausea, and tachycardia from small amounts of alcohol — covered in detail on our alcohol tolerance page.

Alcohol Use Disorder (AUD)

The DSM-5 medical diagnosis for problem drinking. NIAAA describes 11 criteria graded by severity: mild (2–3 met), moderate (4–5), severe (6+). Tolerance and withdrawal both count as criteria, which is one reason rising tolerance matters.

BAC (Blood Alcohol Concentration)

The mass of ethanol per unit of blood. The US reports it in grams per deciliter (g/dL) and writes 0.08% to mean 0.08 g/dL. Europe uses per mille (‰) and writes the same threshold as 0.8‰. Our entire calculator estimates this single number across time.

Binge Drinking

NIAAA defines it as a pattern that pushes BAC to ≥0.08 g/dL — typically 5+ drinks for men or 4+ for women in about 2 hours. Our party calculator shows how easily that threshold gets crossed during normal social drinking.

Bioavailability

The fraction of a consumed drug that reaches systemic circulation unchanged. Oral ethanol bioavailability runs 80% or higher — high by drug standards, which is why even moderate drinking moves the BAC needle so quickly. The remainder is lost to first-pass metabolism in the stomach and liver.

Breath-to-Blood Partition Ratio

The conversion factor breathalyzers use to translate breath alcohol into a blood alcohol estimate. The US standard is 2,100:1 (Henry's Law applied to a population average). Real ratios vary from roughly 1,500:1 to 2,400:1 between people, which is the foundation of the partition ratio defense — see our breathalyzer accuracy guide.

DUI / DWI

"Driving Under the Influence" and "Driving While Intoxicated" — the legal charges for operating a vehicle above the BAC limit, or impaired regardless of BAC. Terminology varies by state. Our legal limits page maps every state's threshold and our DUI calculator estimates penalty exposure.

Elimination Phase

The period after peak BAC during which the body clears ethanol at a roughly constant rate. The decline is linear, not exponential — a defining feature of zero-order kinetics. Our sober calculator projects when the line will cross zero.

Elimination Rate (β)

Symbolized β in the Widmark equation. Population mean is 0.015 g/dL per hour, with documented individual range from roughly 0.012 to 0.020. Heavy drinkers can metabolize slightly faster due to enzyme upregulation. Our calculator uses the conservative 0.015 default.

Ethanol

The two-carbon alcohol (C₂H₅OH) found in beer, wine, and spirits. Pharmacologically the same molecule across every drink — only the concentration and total dose differ. When pharmacology papers say "alcohol," they mean ethanol unless otherwise noted.

First-Pass Metabolism

Ethanol metabolized in the gastric mucosa and during liver transit before reaching systemic circulation. Women have less gastric ADH on average than men, which contributes to higher peak BAC from identical doses — a sex difference the Widmark constant only partially captures.

Heavy Drinking

NIAAA defines it as 5+ drinks on any day or 15+ per week for men, and 4+ on any day or 8+ per week for women. SAMHSA uses a different yardstick — binge drinking on 5+ days in the past month — so published numbers depend heavily on which definition is in play.

Implied Consent

The legal doctrine that operating a vehicle on public roads constitutes consent to a chemical test if lawfully arrested for DUI. All 50 states have implied consent laws, and refusing the test typically triggers automatic license suspension on top of the underlying DUI exposure. See our legal limits page for the per-state breakdown.

Legal BAC Limit

The statutory threshold for a per se DUI offense. Forty-nine US states set it at 0.08 g/dL for adult non-commercial drivers; Utah is the lone outlier at 0.05. CDL drivers face 0.04 federally. Drivers under 21 face zero-tolerance limits of 0.00–0.02 depending on the state.

Mouth Alcohol

Residual alcohol in the mouth, throat, or esophagus that inflates a breathalyzer reading because the device cannot distinguish it from deep-lung air. Sources include mouthwash, cough syrup, recent drinks, and GERD reflux. Standard police protocol is a 15–20 minute observation period before testing.

NAD⁺ / NADH

The cofactor pair that ADH and ALDH2 require to oxidize ethanol and acetaldehyde. NAD⁺ accepts the electrons removed during oxidation and becomes NADH. The shifted NAD⁺/NADH ratio during heavy drinking partially explains why prolonged alcohol exposure disrupts liver lipid metabolism.

Partition Ratio Defense

A DUI defense that argues the defendant's individual breath-to-blood ratio differs materially from the 2,100:1 statutory assumption — meaning the breath test overstates true BAC. Forensic toxicologists can sometimes estimate the individual ratio from blood test data and physiology.

Peak BAC

The highest BAC reached during a drinking session, hit at the end of the absorption phase. Time to peak runs 30–90 minutes after the last drink, with food, sex, and body weight all shifting the curve. Our calculator projects peak BAC and the timeline back down to zero.

Pharmacokinetics

The branch of pharmacology that describes how a drug moves through the body — absorption, distribution, metabolism, and elimination (ADME). Ethanol is unusual in being one of the few common substances cleared by zero-order rather than first-order kinetics.

Per Mille (‰)

"Parts per thousand," the unit Europe and most non-US jurisdictions use to report BAC. 0.08% (US notation) equals 0.8‰ (European notation) — the same physiological number, different decimal point. Both translate to 80 mg of ethanol per 100 mL of blood.

Retrograde Extrapolation

The forensic calculation that estimates BAC at the time of driving from a sample taken later. The formula is BAC₀ = BAC_measured + β × hours elapsed. It assumes the subject was past peak absorption and metabolizing at a constant rate — both assumptions defense attorneys regularly attack.

Standard Drink

NIAAA defines the US standard drink as 14 grams of pure ethanol — 12 oz of 5% beer, 5 oz of 12% wine, or 1.5 oz of 80-proof spirits. The UK unit is smaller at 8 grams. Our standard drinks calculator converts any beverage to standard drink equivalents.

Tolerance

A reduced subjective response to the same dose of alcohol after repeated exposure. Pharmacologically real, but it does not reduce actual impairment — reaction time and coordination decline at the same BAC regardless of how sober the drinker feels. NIAAA flags rising tolerance as an AUD diagnostic criterion.

Volume of Distribution (Vd)

The theoretical volume the body would need to dilute a given dose to the observed plasma concentration. For ethanol, Vd ≈ total body water — typically 60% of body mass for men and 50–55% for women, which is exactly why the Widmark constant differs by sex.

Watson Equation

The 1980 sex-specific formulas Watson and colleagues published for total body water (TBW). Male TBW (L) = 2.447 − 0.09516 × age + 0.1074 × height(cm) + 0.3362 × weight(kg). Female TBW (L) = −2.097 + 0.1069 × height(cm) + 0.2466 × weight(kg). Used to derive the body water constants documented on our methodology page.

Widmark Constant (r)

The body water ratio in the Widmark BAC equation — 0.68 for men, 0.55 for women on average. Sometimes called the "distribution coefficient." It captures the proportion of body mass available to dilute ethanol. Our calculator uses these standard values per the WIDMARK_R constants in src/lib/constants.ts.

Widmark Formula

Erik Widmark's 1932 equation: BAC = (A × absorption_factor) / (W × r) − β × t, where A is grams of alcohol, W is body mass in grams, r is the Widmark constant, β is the elimination rate, and t is hours since the first drink. Still the foundation of every credible BAC calculator nearly a century later.

Zero-Order Kinetics

An elimination pattern where a fixed amount of substance is cleared per unit time, regardless of plasma concentration. Ethanol follows zero-order kinetics at typical recreational doses because ADH is enzyme-saturated. That is why doubling your BAC doubles your sober time, rather than extending the half-life by a fixed percentage.

Sources & Further Reading

• NIAAA — Drinking Levels Defined: Binge & Heavy (definitional source for binge, heavy, AUD)
• NIAAA — Understanding Alcohol Use Disorder (DSM-5 criteria, severity)
• Watson PE et al. (1980) — Total body water volumes for adult males and females, American Journal of Clinical Nutrition
• Widmark EMP (1932) — Die theoretischen Grundlagen und die praktische Verwendbarkeit der gerichtlich-medizinischen Alkoholbestimmung
• IARC Monograph Vol. 100E — Acetaldehyde associated with consumption of alcoholic beverages (Group 1)
• NHTSA — Drunk Driving (legal limits, crash risk by BAC)

For educational purposes only. Definitions on this page summarize widely-accepted clinical pharmacology references but should not be used as medical or legal advice. If you have questions about your own drinking, SAMHSA's free helpline is open 24/7: 1-800-662-4357.