BAC Calculator

A BAC (Blood Alcohol Concentration) calculator estimates the percentage of alcohol in your bloodstream based on what you have drunk, your body weight, sex, and time elapsed. It is used by people who want to make informed decisions about drinking and driving, or understand how alcohol affects them.

How to Use the BAC Calculator

1. Enter your body weight in kilograms or pounds.
2. Select your biological sex, as this affects alcohol distribution in the body.
3. Add each drink you have consumed, including type and time of consumption.
4. Enter the time since you started drinking.
5. Click Calculate to see your estimated current BAC and a timeline of how it will change.

The Formula

The Widmark formula is the standard method for estimating BAC. BAC = (alcohol consumed in grams) divided by (body weight in grams multiplied by distribution ratio r) multiplied by 100, then subtract (metabolism rate multiplied by hours elapsed). The distribution ratio r is 0.68 for men and 0.55 for women, reflecting different body water percentages. The average metabolism rate is 0.015 percent BAC per hour. Alcohol consumed in grams = volume of drink in ml multiplied by ABV percentage multiplied by 0.789 (the density of ethanol).

Real-World Example

A 75 kg man drinks two standard pints of lager (568 ml each, 4% ABV) over 1 hour. Alcohol per pint = 568 times 0.04 times 0.789 = 17.95 g. Total alcohol = 35.9 g. BAC before metabolism = (35.9 divided by (75,000 times 0.68)) times 100 = 0.070%. After 1 hour of metabolism: 0.070 minus (0.015 times 1) = 0.055%. This is above the UK driving limit of 0.08% BAC in England and Wales, and above the 0.05% limit in Scotland.

Legal Limits and Safety

Drink-driving limits vary by country. In England and Wales the legal limit is 80 mg of alcohol per 100 ml of blood (0.08% BAC). Scotland and most of Europe use 50 mg/100 ml (0.05% BAC). The USA federal limit is 0.08%. At 0.02 to 0.05% BAC, reaction time and judgement are already measurably impaired even if you feel fine. At 0.05 to 0.08%, the risk of a road accident roughly doubles. A BAC calculator provides an estimate only; the only safe approach before driving is not to drink at all.

Frequently Asked Questions

How accurate is a BAC calculator? BAC calculators give a reasonable estimate but cannot account for all individual variables including liver health, food consumption, fatigue, and medication interactions. They should be used as a guide, not a definitive measurement. The only accurate measurement is a breathalyser or blood test.

Does food slow down alcohol absorption? Yes. Eating before or during drinking slows the rate at which alcohol enters the bloodstream, lowering peak BAC. High-fat and high-protein foods are most effective. However, food does not speed up the metabolism of alcohol already in your blood.

How long does alcohol stay in your system? The body metabolises alcohol at roughly 0.015% BAC per hour, regardless of coffee, water, or sleep. A BAC of 0.08% takes approximately 5 to 6 hours to reach zero. There is no way to speed this process up reliably.

Can BAC vary between people of the same weight? Yes. Fitness level, hydration, genetics, tolerance, and liver enzyme activity all affect how alcohol is processed. Two people of identical weight can have meaningfully different BAC readings after the same drinks.

Also try these free tools:

• Calorie Calculator
• BMI Calculator
• Body Fat Calculator

Worked example across a series of drinks

The example above follows a 75 kg man drinking two pints of 568 ml lager at 4 percent ABV in an hour. Applying the same Widmark arithmetic to a longer session gives the table below, with r = 0.68 and elimination at 0.015 percent per hour.

One line of that table in full, for two pints. Grams of ethanol per pint: 568 x 0.04 x 0.789 = 17.93. Grams for two pints: 35.85. Divide by body weight in grams multiplied by the distribution ratio: 35.85 / (75,000 x 0.68) = 0.000703, and multiply by 100 to read it as a percentage: 0.070. Subtract one hour of metabolism at 0.015 per hour and the estimate lands at 0.055.

The peak figure is the one at the moment drinking stops, because elimination starts the clock the moment alcohol reaches the blood in this model. Three pints take the estimate to 0.090, close to double the 0.050 figure used in Scotland, and four pints reach 0.126.

The same two drinks on three different bodies

Weight and the distribution ratio both sit in the denominator, so both push the answer around.

The 60 kg woman reaches 0.109 on the same two pints, which is more than half again the 0.070 recorded for the 75 kg man. Two factors do that work: less body mass in the denominator, and the smaller ratio, because a higher proportion of a woman's body mass is fat and fat holds less water for the alcohol to dissolve into. A 90 kg man on the same drinks reaches 0.059, below the 0.080 limit but not by much, and still well above the 0.050 figure used in Scotland.

How long the estimate takes to return to zero

Elimination is treated as a straight line at 0.015 percent per hour, so the arithmetic is a division.

Worked line for 0.070: 0.070 / 0.015 = 4.687, and 0.687 of an hour is 41 minutes. Coffee, a cold shower, food and sleep do not change the division, because none of them speeds up the liver. They change how alert you feel, which is a different measurement from the one this tool reports.

What the standard drinks actually contain

A UK pint at 4 percent carries 17.93 g of ethanol, which is 2.24 UK units at 8 g each. A 175 ml glass of 13 percent wine lands within 0.02 g of the same figure, so the two drinks behave almost identically inside this model despite looking nothing alike. A 25 ml spirit measure carries 7.89 g, less than half a pint, and a US standard drink at 14 g sits below a UK pint as well. Anyone comparing guidance from two countries needs those figures in front of them before drawing a conclusion from a drink count.

Assumptions the formula makes

• Elimination runs at a constant 0.015 percent per hour. Real elimination is enzyme-limited and roughly linear at low concentrations, then slower at high ones, and the rate differs between people.
• Every gram of alcohol reaches the blood at once. Absorption normally takes 30 to 90 minutes and continues while drinking continues, so a real peak usually arrives later and lower than the model predicts.
• A single distribution ratio stands in for body composition. Two people of the same weight and sex can differ by enough to move the estimate by several hundredths.
• Ethanol density is taken as 0.789 g per millilitre at room temperature.
• Food, medication, hydration, fatigue, liver health and the pace of drinking sit outside the formula, and each of them can shift a reading by more than the difference between two consecutive lines in the table above.

What this estimate cannot do

A calculated figure is not a measurement. The only ways to know a blood alcohol concentration are a breath test or a blood test, and both come with their own margins. The Widmark equation is most useful for understanding how weight, sex, drink strength and elapsed time move the number, which is exactly the reasoning that helps someone plan a safe route home rather than guess at one. Treat any figure here as an upper or lower bound arrived at from estimates, not as a defence.

A note on the numbers behind the formula

The US National Institute on Alcohol Abuse and Alcoholism defines a standard drink as any beverage containing 0.6 fluid ounces or 14 grams of pure alcohol. The UK unit is 10 millilitres or 8 grams. GOV.UK publishes the driving limits used above: 80 milligrams of alcohol per 100 millilitres of blood and 35 micrograms per 100 millilitres of breath for England, Wales and Northern Ireland, and 50 milligrams per 100 millilitres of blood and 22 micrograms per 100 millilitres of breath for Scotland. The distribution ratios of 0.68 and 0.55 come from work on body water content rather than from the driving limits, and different laboratories publish slightly different values for both.