Methodology

The math behind the estimate

BACtrack models a single ethanol pool that is absorbed from the gut, distributed through total body water, and eliminated by the liver. Each step below shows the equation used and the evidence it rests on. BAL and BAC label the same quantity here: whole-blood ethanol concentration as a percentage (g/100 mL), where 0.08% equals 80 mg/dL.

1 · Ethanol mass per drink

Each drink is converted to grams of pure ethanol from its volume, alcohol by volume, and the density of ethanol (0.789 g/mL at 20 °C).

grams = volume_mL × (ABV% / 100) × 0.789

A U.S. standard drink is defined as 14 g of ethanol, the figure used to display the standard-drink count (NIAAA).

2 · Total body water (Watson)

Ethanol distributes into body water, not body mass, so the volume of distribution is derived from total body water (TBW) rather than a fixed Widmark r factor. TBW is estimated with the sex-specific Watson anthropometric regressions from age, height, and weight.

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

A physiological floor of 0.35 × weight keeps implausible inputs from driving TBW unrealistically low. Source:Watson, Watson & Batt, Am J Clin Nutr 1980, the equations endorsed for blood-alcohol work inANSI/ASB BPR 122 (2024).

3 · From grams to whole-blood concentration

Concentration in body water is converted to whole blood using the mean water content of blood (F_water): 0.825 for males and 0.838 for females. The small difference reflects lower haematocrit in female blood. Dividing ethanol grams by this apparent distribution volume yields the whole-blood percentage.

Vd_L  = TBW_L / F_water          (F_water = 0.825 male, 0.838 female)
BAC%  = (grams_in_body / Vd_L) × 0.1

SeeJones, WIREs Forensic Science 2019for absorption, distribution, and the F_water derivation.

4 · Absorption

Gastric emptying controls how fast ethanol reaches the bloodstream. Each drink is absorbed over a window that widens with stomach contents. Within that window the absorbed fraction follows a Beta(2, 2) curve — absorption starts slowly, peaks near the midpoint, then tapers — rather than entering at a constant rate.

absorbed_fraction(t) = BetaCDF( (t − t_finish) / window ; 2 , 2 )
           = 3x² − 2x³   where x = (t − t_finish) / window

window:  empty 30 min · light meal 75 min · full meal 120 min

Windows followOSAC 2020-S-0003, Guidelines for Performing Alcohol Calculations.

5 · First-pass metabolism (bioavailability)

Food increases pre-systemic (gastric and hepatic) metabolism, so not all ingested ethanol reaches the circulation. The absorbed mass is scaled by a bioavailability factor that lowers peak BAC and the area under the curve on a fuller stomach.

bioavailability:  empty 1.00 · light meal 0.95 · full meal 0.90

6 · Elimination (Michaelis–Menten)

The liver clears ethanol via alcohol dehydrogenase, which saturates at low concentrations. Rather than a pure zero-order rate, BACtrack uses Michaelis–Menten kinetics so clearance is effectively constant through the normal range and tapers smoothly to first-order near zero — removing the artifact where a small drink would clear to exactly 0.000%.

rate(C) = Vmax × C / (Km + C)          Km ≈ 0.002 %BAC (~2 mg/100 mL)

Vmax by weekly intake (U.S. standard drinks):
  rare       < 3 / week        0.012 %/hr
  moderate   3–14 / week        0.015 %/hr
  frequent   14–35 / week       0.018 %/hr
  dependent  problem drinking   0.025 %/hr

Because C is normally far above Km, rate(C) sits within a few percent of Vmax across the usual range, then falls off gradually below ~0.02%. Higher Vmax for frequent and dependent drinkers reflects CYP2E1 (MEOS) induction. All rates lie inside the evidence-based 0.010–0.035 %/hour span (alcoholics 0.025–0.035) reported byJones, Forensic Sci Int 2010;200:1–20.

Profile inputs

Your profile sets the constants used in each step above. Age, height, weight, and sex determine total body water (Watson). Stomach contents set the absorption window and bioavailability. Drinks per week sets the elimination rate (Vmax). Toggle BAL or BAC in the header to display the same estimated whole-blood concentration as mg/dL or as a percentage.

Stomach · empty: 30 min · 100% bioavailability
Stomach · light meal: 75 min · 95% bioavailability
Stomach · full meal: 120 min · 90% bioavailability
Drinks/wk · under 3: 0.012 %/hr clearance
Drinks/wk · 3–14: 0.015 %/hr clearance
Drinks/wk · 14–35: 0.018 %/hr clearance
Drinks/wk · problem drinking: 0.025 %/hr clearance

Limitations

These are population formulas, not measurements. Real individuals vary in body composition, enzyme activity, gastric emptying, and drinking history, and the model assumes drinks are fully consumed at the stated finish time. Estimates are for educational interest only and must never be used to decide whether it is safe or legal to drive.