- The Widmark Equation: Theoretical BAC (%) = Alcohol Grams ÷ [Body Mass (kg) × r × 10] − [0.015 × Hours Elapsed].
- Distribution Factors (r): Models body water ratio: 0.68 for average males and 0.55 for average females.
- Ethanol Density: Volume (ml) × (ABV% ÷ 100) × 0.789 g/ml = exact grams of pure ethanol consumed.
- Model Assumptions: Online formulas assume instantaneous absorption and linear elimination; they cannot measure real-time gastric uptake or individual liver kinetics.
What this BAC estimation page covers
Here, BAC means blood alcohol concentration. This page explains the alcohol-estimation formula used by our calculator; it does not refer to school examination scores or other uses of the same abbreviation. The goal is to make a simple model understandable and reproducible.
A Widmark-based estimate connects reported ethanol intake with body mass and a distribution factor. An elapsed-time term represents assumed elimination. This is a useful educational simplification, but it cannot reproduce every person's absorption and clearance curve or replace a breath or blood measurement.
Step 1: convert beverages to ethanol grams
For each drink, multiply the volume in milliliters by ABV divided by 100, then by the approximate ethanol density of 0.789 g/ml and the number of servings. Add the results for all drink rows. If the input uses US fluid ounces, the tool first converts them using approximately 29.5735 ml per US fl oz.
For example, 355 ml × 0.05 × 0.789 gives 14.00475 g. Displaying this as 14.0 g is sensible for an educational interface because a real pour and label are not exact to all those decimal places. The alcohol-volume tool performs this step without estimating BAC.
Step 2: apply body mass and a distribution factor
The distribution factor r represents a simplified relationship between body mass and the space into which alcohol distributes. This calculator offers 0.55 and 0.68 as broad assumptions. They are not measurements of an individual's body water and should not be treated as certain values based on sex or gender.
Because the factor is in the denominator, selecting the lower factor produces a higher modeled concentration for the same mass and dose. Body composition and physiology are more complicated than two choices. The tool exposes the assumption instead of pretending to know a user's personal value.
Step 3: apply the elapsed-time assumption
The central elimination assumption is 0.015 percentage points per hour. The result panel also shows the values obtained using 0.010 and 0.020. This is sensitivity analysis: it illustrates how changing one input changes the answer. It is not a validated confidence interval, personal clearance test or guarantee that actual BAC lies within those bounds.
Hours are measured from the first drink in the recorded session. The formula does not separately track each drink's timing or ongoing absorption. During or soon after a session, that simplification can be particularly misleading. BAC can still rise after the last drink.
A worked example you can reproduce
Consider a hypothetical 75 kg person, a factor of 0.68, two 500 ml drinks at 5% ABV and two hours of elapsed time. The alcohol amount is 500 × 0.05 × 0.789 × 2 = 39.45 g. The theoretical concentration is 39.45 ÷ (75 × 0.68 × 10), or approximately 0.07735%.
Subtracting 0.015 × 2 gives approximately 0.04735%, displayed as 0.047%. Changing the elimination assumption to 0.020 produces approximately 0.037%; using 0.010 produces approximately 0.057%. These are outcomes of the selected equation, not evidence about an actual person's concentration or ability to drive.
The example is deliberately not a recommended drinking pattern. Do not reverse the formula to choose a dose that reaches a target or stays below a legal threshold. Uncertainty in both the inputs and physiology makes that use inappropriate.
Uncertainty that the displayed range does not include
The sensitivity range changes elimination only. It does not cover every possible distribution factor, unrecorded drinks, serving-size error, ABV error, food effects or absorption pattern. Actual results can fall outside it. Showing a range is more transparent than hiding one assumption, but does not solve all limitations.
The equation floors negative results to zero because a negative concentration is not meaningful. A zero display therefore means the mathematical subtraction exceeded the modeled concentration. It does not establish a zero breath or blood test and says nothing about fatigue or a hangover.
Read the absorption guide and the sobering-up explanation for the time-course distinctions. Formal testing and clinical interpretation require their own procedures and cannot be inferred from this page.
How the implementation is checked
The calculation code is tested against independently calculated ethanol and Widmark examples. Unit-conversion checks compare kilograms with pounds and milliliters with US fluid ounces. Boundary checks reject invalid weights, non-finite numbers, negative time and impossible ABV entries, while allowing zero-alcohol beverages.
These checks verify that the software follows its stated equation. They do not clinically validate the equation for every individual. Software correctness and medical accuracy are different claims. The site does not claim a professional medical review, regulatory approval or superior accuracy over properly obtained measurements.
Using the estimate responsibly
Use the model to learn how quantities and assumptions relate. Use the BAC chart to understand concentration units and the legal reference to see why a threshold is not a personal drinking allowance. For driving, arrange transport that does not depend on an estimate.
If you find a calculation problem, send the page address and a non-sensitive reproducible example to info@bac-calculator-us.com. Questions about symptoms, medicines or individual risk should go to a qualified health professional rather than a website support inbox.