How to Calculate Blood Alcohol Content (BAC): Widmark Formula

Introduction

Blood alcohol content is estimated with the Widmark formula: BAC % = (A ÷ (W × r)) × 100 − (0.015 × H), where A is the grams of pure alcohol consumed, W is body weight in grams, r is the alcohol distribution ratio (about 0.68 for men and 0.55 for women), and H is the hours since drinking began. In plain English: divide the alcohol dose by the body water it dissolves into, convert to a percentage, then subtract what the liver has already burned off. If you want the answer instantly, the Blood Alcohol Calculator runs the entire calculation for you.

Before we go any further, one thing must be absolutely clear: a calculated BAC is an approximation, never a measurement. Individual variation is enormous, and no formula result — including anything in this article or any online calculator — should ever be used to decide whether you are okay to drive. If you have been drinking, do not drive. Legal limits also vary by country and state; 0.08% and 0.05% are common thresholds in different jurisdictions, and many places apply much stricter or zero-tolerance limits to certain drivers. The value of learning this calculation is understanding how dose, body size, sex, and time interact — not beating a breathalyzer.

Understanding the Widmark Formula

Swedish scientist Erik Widmark developed this equation in the 1930s, and it remains the foundation of forensic alcohol calculations today. Each variable has a specific physical meaning:

Variable	Meaning	Typical Values
A	Grams of pure alcohol consumed	14 g per US standard drink
W	Body weight in grams	pounds × 453.6, or kg × 1,000
r	Widmark distribution ratio	0.68 (men), 0.55 (women)
0.015	Average elimination rate per hour	ranges from 0.010 to 0.025
H	Hours since drinking started	measured from the first drink

Two of these deserve a closer look.

The distribution ratio (r) exists because alcohol dissolves into body water, not fat. Muscle and organs hold a lot of water; fat tissue holds very little. Men average more lean mass, so a larger fraction of their body absorbs the alcohol — hence the higher r value. This is also why two people of identical weight can reach very different BACs from the same drinks: the leaner person has more water to dilute the alcohol.

The elimination rate (0.015 per hour) reflects the liver working at a fixed pace. Unlike most drugs, alcohol is cleared at a roughly constant rate regardless of how much is in your system — the liver processes approximately one standard drink per hour and cannot be hurried. Coffee, cold showers, and exercise change how you feel, not your BAC.

Step 1: Convert Drinks to Grams of Alcohol

The formula needs grams of pure ethanol, not number of glasses. The general conversion is:

Alcohol (grams) = Volume (mL) × ABV (as a decimal) × 0.789

The 0.789 is the density of ethanol in grams per milliliter. For example, a 355 mL (12 oz) beer at 5% ABV contains 355 × 0.05 × 0.789 ≈ 14 grams of alcohol.

Conveniently, a US standard drink is defined as 14 grams of pure alcohol, and common servings line up with it:

Drink	Typical Serving	ABV	Pure Alcohol
Regular beer	12 oz (355 mL)	5%	~14 g
Light beer	12 oz (355 mL)	4.2%	~12 g
Wine	5 oz (148 mL)	12%	~14 g
Fortified wine (port, sherry)	3.5 oz (104 mL)	17%	~14 g
Distilled spirits	1.5 oz (44 mL)	40%	~14 g
Craft IPA	16 oz (473 mL)	7%	~26 g

Note the last row: a pint of 7% IPA is nearly two standard drinks. Undercounting strong beer and generous pours is the single biggest source of error in home BAC estimates. Also be aware that other countries define a standard drink differently — 10 grams in Australia, 8 grams per UK unit — so always work in grams when precision matters.

Worked Example 1: A 180-Pound Man, Four Drinks Over Two Hours

Suppose a 180 lb man drinks four standard US drinks (say, four 12 oz regular beers) over two hours. Let’s walk through every step.

Step 1 — Alcohol dose (A): 4 drinks × 14 g = 56 grams

Step 2 — Body weight in grams (W): 180 lb × 453.6 = 81,648 grams

Step 3 — Apply the distribution ratio (r = 0.68): 81,648 × 0.68 = 55,521 grams of effective distribution volume

Step 4 — Raw BAC before elimination: (56 ÷ 55,521) × 100 = 0.101%

Step 5 — Subtract elimination (H = 2 hours): 0.015 × 2 = 0.030 0.101 − 0.030 = 0.071%

Estimated BAC: about 0.07%. Notice something important: even though this is below the 0.08% limit used in many places, it is above the 0.05% limit used in many others — and impairment of judgment and reaction time begins well below either number. The calculation tells you a concentration, not that you are safe.

Worked Example 2: A 140-Pound Woman, Two Glasses of Wine Over 90 Minutes

Now a 140 lb woman has two 5 oz glasses of 12% wine over an hour and a half.

Step 1 — Alcohol dose: Each glass: 148 mL × 0.12 × 0.789 ≈ 14 g 2 glasses × 14 g = 28 grams

Step 2 — Body weight in grams: 140 lb × 453.6 = 63,504 grams

Step 3 — Apply the distribution ratio (r = 0.55): 63,504 × 0.55 = 34,927 grams

Step 4 — Raw BAC: (28 ÷ 34,927) × 100 = 0.080%

Step 5 — Subtract elimination (H = 1.5 hours): 0.015 × 1.5 = 0.0225 0.080 − 0.0225 = 0.058%

Estimated BAC: about 0.06%. Compare this with Example 1: she drank half the alcohol of the man yet lands at a broadly similar BAC. The combination of lower body weight and a lower r value means the same drinks hit harder — which is exactly what the Widmark formula was designed to show.

Worked Example 3: Metric Version — An 80 kg Man, Three Beers Over Three Hours

For readers who think in metric units, the math is even cleaner. An 80 kg man drinks three 330 mL beers at 5% ABV over three hours.

Step 1 — Alcohol dose: Each beer: 330 × 0.05 × 0.789 = 13.0 g 3 beers × 13.0 g = 39 grams

Step 2 — Body weight in grams: 80 kg × 1,000 = 80,000 grams

Step 3 — Apply the distribution ratio (r = 0.68): 80,000 × 0.68 = 54,400 grams

Step 4 — Raw BAC: (39 ÷ 54,400) × 100 = 0.072%

Step 5 — Subtract elimination (H = 3 hours): 0.015 × 3 = 0.045 0.072 − 0.045 = 0.027%

Estimated BAC: about 0.03%. Spacing the same three beers over three hours instead of one makes a dramatic difference — time is the only thing that genuinely lowers BAC, and this example shows the elimination term doing real work.

How Long Until Your BAC Returns to Zero?

Because elimination runs at a roughly constant 0.015% per hour, estimating time to sober is simple division:

Hours to zero ≈ Peak BAC ÷ 0.015

Peak BAC	Approximate Time to Zero
0.05%	~3.5 hours
0.08%	~5.5 hours
0.10%	~7 hours
0.15%	~10 hours
0.20%	~13.5 hours

This is why a heavy night can leave measurable alcohol in your blood the next morning. Someone who stops drinking at 2 a.m. with a BAC of 0.15% may still be around 0.06% at 8 a.m. — potentially over the legal limit in many jurisdictions on the drive to work, and certainly still impaired. Sleep does not accelerate the process; the liver keeps its own schedule.

BAC Levels and Typical Effects

Effects vary from person to person, but the general progression is well documented:

BAC Range	Typical Effects
0.02–0.03%	Mild relaxation, slight mood elevation
0.04–0.06%	Lowered inhibition, warmth, early impairment of judgment
0.07–0.09%	Reduced coordination, slower reaction time, impaired self-assessment
0.10–0.15%	Clear motor impairment, slurred speech, poor balance
0.16–0.30%	Severe impairment, nausea, memory blackouts possible
Above 0.30%	Risk of loss of consciousness and alcohol poisoning — a medical emergency

Two things stand out. First, measurable impairment begins far below common legal limits — driving skills degrade from around 0.02–0.05%. Second, one of alcohol’s cruelest tricks appears in the 0.07–0.09% band: it impairs the very judgment you would use to assess your own impairment. People at this level routinely believe they are fine. That is precisely why the rule is never “calculate, then decide” — it is if you drank, don’t drive.

Why Your Real BAC Will Differ From the Formula

The Widmark equation uses population averages, and you are not an average. Real-world BAC can deviate substantially from the calculation because of:

Body composition. Two people of identical weight but different muscle-to-fat ratios have different body water, so the fixed r values (0.68/0.55) are only approximations.
Food. A full stomach slows absorption, lowering and delaying the peak. The classic formula assumes an empty stomach and complete absorption.
Elimination rate. The 0.015 average spans a real range of roughly 0.010 to 0.025 per hour. Regular heavy drinkers often metabolize faster; some people metabolize slower.
Drinking speed. The formula treats all alcohol as absorbed instantly. Rapid drinking produces a higher, sharper peak than the same dose sipped slowly.
Age, medications, genetics, and health. Liver function, enzyme variants, and drug interactions all shift the result.

Forensic experts who use Widmark professionally apply uncertainty ranges, not single numbers. Treat any calculated BAC the same way: as a rough midpoint of a wide band.

Common Mistakes to Avoid

Using drink counts instead of grams. A pint of 7% IPA, a generously poured wine, or a double cocktail can each equal two standard drinks. Always convert volume × ABV × 0.789 when servings are non-standard.
Measuring time from the last drink instead of the first. The elimination term H runs from when drinking began, because the liver starts working on the first drink immediately.
Forgetting the weight unit conversion. Weight must be in grams: multiply pounds by 453.6 or kilograms by 1,000. Using pounds directly inflates the result by a factor of 450+.
Using the wrong r value. Swapping 0.68 and 0.55 shifts the result by nearly 20% — enough to change the interpretation completely.
Believing coffee, food, water, or exercise lowers BAC. They may change how you feel, but elimination is time-locked at roughly one drink per hour.
Using any calculation to justify driving. This is the mistake that costs lives. The formula’s error bars are wide, impairment starts below legal limits, and legal limits vary by jurisdiction. A calculated 0.06% is not a green light anywhere.
Ignoring the morning after. Divide your estimated peak by 0.015 — if the answer is longer than your night’s sleep, you may still have alcohol on board at breakfast.

Final Thoughts

The Widmark formula — grams of alcohol divided by body weight times the distribution ratio, minus 0.015 per hour — is a genuinely useful piece of applied chemistry. Working through it by hand shows you exactly why body size, sex, drink strength, and time each matter, and why the same round of drinks affects two people so differently. What it cannot do is certify anyone as safe or legal to drive; only a breath or blood test measures actual BAC, and only abstaining before driving removes the risk entirely.

For instant estimates with all the unit conversions handled automatically, use the free Blood Alcohol Calculator. And if you are interested in how your body processes energy more broadly, the same style of step-by-step math applies in our guides to what BMR is and how to calculate it and how to calculate your TDEE — the resting and total calorie burn numbers that anchor any nutrition plan.

Frequently Asked Questions

How accurate is the Widmark formula? +

The Widmark formula is a population-level estimate, not a measurement. Real BAC can differ from the calculated value by 20 percent or more because of body composition, genetics, food in the stomach, medications, and how fast you drank. It is useful for understanding roughly how alcohol dose, body weight, and time interact, but only a breath or blood test can determine an actual BAC, and no formula result should ever be used to decide whether it is safe or legal to drive.

How many grams of alcohol are in one standard drink? +

In the United States a standard drink contains about 14 grams of pure alcohol, which is what you get from a 12-ounce beer at 5 percent ABV, a 5-ounce glass of wine at 12 percent, or a 1.5-ounce shot of 40 percent spirits. Other countries define it differently: a UK unit is 8 grams and an Australian standard drink is 10 grams. To compute grams directly, multiply the volume in milliliters by the ABV as a decimal, then by 0.789, the density of ethanol.

How long does it take for BAC to return to zero? +

The average person eliminates alcohol at roughly 0.015 percent BAC per hour, so you can estimate the time to zero by dividing your peak BAC by 0.015. A BAC of 0.08 percent takes around 5 to 6 hours to clear, and 0.15 percent takes around 10 hours. Elimination rate varies from about 0.010 to 0.025 per hour between individuals, and nothing speeds it up meaningfully: coffee, cold showers, and exercise do not lower BAC, only time does.

Why do women usually reach a higher BAC than men from the same drinks? +

Alcohol distributes into body water, and women on average carry a smaller proportion of their weight as water than men do. The Widmark formula captures this with the distribution ratio r, typically 0.68 for men and 0.55 for women. The same grams of alcohol therefore dissolve into a smaller effective volume in a female body, producing a higher concentration even when body weight is identical.

Does eating food lower your BAC? +

Food does not remove alcohol from your body, but eating before or while drinking slows the rate at which alcohol is absorbed from the stomach into the bloodstream. That flattens and delays the BAC peak, so the maximum concentration you reach is lower than drinking the same amount on an empty stomach. Total elimination still happens at the same fixed hourly rate, so food changes the shape of the curve, not the total alcohol your liver must process.