How to Calculate BAC Manually: The Widmark Formula Step-by-Step
Alex Harmon · Updated June 2026 · Reviewed for accuracy
Want to understand the math behind blood alcohol content — not just plug numbers into a calculator? This guide walks you through the Widmark formula with real worked examples, so you can do it yourself on paper or on your phone. It's simpler than it looks, and understanding it helps you read your calculator results with more context.
The Widmark Formula
The formula most BAC calculators use comes in two versions — one using metric units and one using US customary units. Both give the same result; pick whichever matches how you think about weight and volume.
Metric Version
US Units Version
The variables are:
- Alcohol — the amount of pure ethanol you consumed, in grams (metric) or fluid ounces (US version)
- Weight — your body weight in kg or lbs
- r — the Widmark distribution factor:
0.68for men,0.55for women on average - Hours — how long ago you started drinking (or time elapsed since your last drink if you want the current value)
- 0.015 — the average rate at which the liver clears alcohol from blood per hour
The result is your estimated BAC as a percentage. A BAC of 0.08% means there is 0.08 grams of alcohol per 100 millilitres of blood.
Step 1: Calculate How Much Alcohol You Drank
First, you need to convert your drinks into grams of pure alcohol. Alcohol has a density of 0.789 g/ml, so the formula is:
For reference, here's what a "standard drink" contains in different countries:
- United States: 14 grams of pure alcohol (e.g. 355ml beer at 5%, or 44ml spirit at 40%)
- United Kingdom: 8 grams of pure alcohol (one "unit")
- Australia: 10 grams of pure alcohol
These are standardised definitions — actual drinks vary. A 568ml UK pint of 5% beer contains about 22 grams of alcohol, which is 2.75 UK units, not one. Always calculate from the actual volume and ABV rather than assuming "one drink = one standard drink."
Worked Example 1 — Metric (80kg Male, 2 Beers, 1 Hour)
Scenario: An 80kg man drinks two 355ml cans of 5% beer and we want to estimate his BAC one hour after he started drinking.
Step 1: Calculate alcohol consumed
Step 2: Apply the Widmark formula
At 0.037%, this man is below most legal driving limits (typically 0.05–0.08%) but is not unaffected by alcohol. Many people feel mild effects — slightly relaxed, minor reduction in reaction time — at this level.
Worked Example 2 — Metric (60kg Female, 3 Glasses Wine, 2 Hours)
Scenario: A 60kg woman drinks three 150ml glasses of 12% wine over two hours.
Step 1: Calculate alcohol consumed
Step 2: Apply the Widmark formula
At 0.099%, this is above the legal driving limit in most countries. At this level, most people feel noticeably impaired — slowed reaction time, reduced coordination, impaired judgment. Three glasses of wine in two hours is more alcohol than many people realise.
Worked Example 3 — US Units (175lb Male, 3 Beers, 2 Hours)
Scenario: A 175lb man drinks three standard 12 oz beers at 5% ABV over 2 hours. Each beer contains 0.6 fluid ounces of pure alcohol, so total = 1.8 oz.
Note: The US formula uses r = 0.73 for men and r = 0.66 for women — slightly different averages than the metric version uses, reflecting different published reference values. Both are valid approximations; results may differ slightly between versions.
When the Formula Is Less Reliable
The Widmark formula gives a population average estimate. It's most accurate for people who are reasonably close to the average in terms of body composition, liver health, and metabolism. It becomes less reliable in these situations:
- Very high body fat percentage — less body water means alcohol is less diluted than the formula assumes, so actual BAC may be higher
- Medications — certain antibiotics, antihistamines, and other drugs interact with alcohol metabolism
- Gastric bypass surgery — alcohol absorbs much faster, leading to higher and quicker peak BAC
- Liver disease — affected liver function means slower alcohol clearance
- Drinking on a full stomach — slows absorption significantly, so your actual peak BAC may be lower than calculated
For the average healthy adult in a typical drinking situation, expect results to be within about 15–20% of actual BAC. That's good enough for educational purposes — but not for deciding whether you're safe to drive.
Skip the Math — Use the Calculator
Now that you understand how it works, let the calculator do the arithmetic. Enter your drinks and weight for an instant BAC estimate.
CALCULATE MY BACFrequently Asked Questions
How many grams of alcohol are in a standard drink?
It depends on the country. A US standard drink contains 14 grams of pure alcohol. A UK unit contains 8 grams. An Australian standard drink contains 10 grams. To calculate for any drink, use: volume (ml) × ABV% × 0.789. For example, a 330ml can of 5% beer contains 330 × 0.05 × 0.789 = 13 grams of alcohol — almost exactly one US standard drink.
What is the r value in the Widmark formula?
The r value (Widmark distribution factor) represents the proportion of your body that distributes alcohol. Since alcohol is water-soluble, it's essentially a measure of how much of your body is water-containing tissue. The commonly used averages are 0.68 for men and 0.55 for women, though actual values range from about 0.44 to 0.85 depending on body composition. Leaner, more muscular people tend to have higher r values.
Can I do this calculation on a phone calculator?
Yes, easily. The math is just multiplication, division, and subtraction. For the metric version: multiply your weight in kg by 1000, then by r (0.68 male, 0.55 female). Divide your alcohol in grams by that number. Multiply by 100. Subtract 0.015 × hours. That's your BAC. Most phone calculators handle this in about 30 seconds if you know your inputs.
Why do the metric and US unit versions of the formula look different?
They use different reference values for r and different unit conversions built into the constants. The US version uses r = 0.73 for men and r = 0.66 for women (slightly different averages from the metric convention), and the "5.14" constant converts fluid ounces of alcohol at the US measurement standard. Both versions produce similar but not always identical results. Neither is definitively more accurate — the underlying biology is the same.
When should I start counting the hours — first drink or last drink?
In the standard Widmark formula, hours (H) typically refers to time elapsed since you started drinking, assuming all the alcohol is absorbed. If you're calculating your current BAC mid-session, use the time since your first drink. If you've stopped drinking and want to know your current BAC, you can use time since your last drink but remember that any alcohol consumed earlier is still in your system. The most accurate approach for mid-session estimates is to calculate peak BAC (assuming all drinks are absorbed) then subtract the metabolised amount based on time elapsed since the first drink.