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BAC

General

Blood Alcohol Content

The concentration of alcohol in the bloodstream, expressed as a percentage by weight (g/dL), used to assess intoxication level and determine legal driving limits.

Definition

Blood Alcohol Content (BAC), also called Blood Alcohol Concentration, is a measure of the amount of alcohol in a person's bloodstream. It is expressed as a percentage by weight — the grams of alcohol per 100 millilitres (or per decilitre) of blood.

BAC is the scientific standard used globally for:

  • Determining legal impairment for driving (legal limits range from 0.00% to 0.08% depending on country)
  • Medical assessment of intoxication in emergency settings
  • Forensic and legal proceedings
  • Personal awareness of alcohol consumption and its effects

BAC rises as alcohol is absorbed into the bloodstream and falls as the liver metabolises it. Many variables affect the rate of rise and fall, which is why a BAC calculator gives only an estimate — not a medically precise value.

Formula

Widmark Formula:

BAC (%) = (A × 5.14) / (W × r) − (0.015 × T)

Where:

  • A = Number of standard drinks consumed (1 standard drink = 14g pure alcohol in the US)
  • W = Body weight in pounds
  • r = Body water constant (0.73 for men, 0.66 for women)
  • T = Hours since drinking began
  • 5.14 = Conversion factor (grams per standard drink / dL conversion)

For metric units:

BAC (%) = (A × 0.806 × 1.2) / (BW × r) − 0.015 × T

Where BW is body weight in kg.

Worked Example

A 75kg (165 lb) male drinks 4 standard drinks over 2 hours.

  • A = 4, W = 165 lbs, r = 0.73, T = 2
  • BAC = (4 × 5.14) / (165 × 0.73) − (0.015 × 2)
  • BAC = 20.56 / 120.45 − 0.03
  • BAC = 0.1707 − 0.03 = ~0.14%

At 0.14%, this person is significantly above the 0.08% legal driving limit in the US. They would need approximately 4 more hours for their BAC to return to below 0.08% [(0.14 − 0.08) ÷ 0.015 = 4 hours]. Use the BAC calculator for your own estimate.

Key Things to Know

  • Calculator estimates only: BAC calculators provide approximations. Real BAC varies with individual metabolism, health conditions, medications, food in the stomach, and hydration. Do not rely on a BAC estimate to decide whether to drive.
  • Food slows absorption: Eating before or while drinking slows alcohol absorption into the bloodstream, resulting in a lower peak BAC and more gradual rise. On an empty stomach, peak BAC is reached in 30–60 minutes; on a full stomach, 1–2 hours.
  • Legal limits by country: US/Canada/UK: 0.08%; India: 0.03%; Germany/France: 0.05%; Japan: 0.03%; Sweden: 0.02%; Czech Republic/Hungary: 0.00%.
  • Tolerance vs impairment: People with high alcohol tolerance may feel less impaired at 0.08% BAC, but their actual driving performance is equally compromised. Tolerance affects subjective feelings, not blood alcohol levels or physiological impairment.
  • India-specific context: India's 0.03% limit is stricter than the global standard. A single standard drink can potentially push an average person close to this limit. The Motor Vehicles Amendment Act 2019 significantly increased penalties for drunk driving — awareness is important.

Related Calculators

BAC Calculator →Estimate your blood alcohol concentration (BAC) in mg/100ml using the Widmark formula, and time to reach zero. Legal driving limits vary by country (e.g. 80 mg/100ml in the US/UK, 30 mg/100ml in India). For educational use only.

Related Terms

BMRBasal Metabolic RateTDEETotal Daily Energy Expenditure

Frequently Asked Questions

A BAC of 0.08% means there are 0.08 grams of alcohol per 100 millilitres of blood, or equivalently, 0.8 grams per litre. This is the legal driving limit in the US, UK, Australia, and many other countries. At 0.08%, most people experience impaired coordination, slower reaction times, and reduced ability to track moving objects — all critical for safe driving.
The liver eliminates alcohol at a fixed rate of approximately 0.015% BAC per hour — roughly one standard drink per hour for most adults. There is no way to speed this up: coffee, cold showers, food, and exercise do not accelerate alcohol metabolism. If your BAC is 0.12%, it will take approximately 8 hours to reach 0.00% (0.12 ÷ 0.015). The only solution is time.
Women typically reach a higher BAC than men after drinking the same amount for two reasons: (1) women have a lower average percentage of body water (52% vs 61%), so alcohol is more concentrated in a smaller volume; and (2) women produce less of the stomach enzyme alcohol dehydrogenase, which breaks down some alcohol before it reaches the bloodstream. The Widmark formula uses different r-factors (0.73 for men, 0.66 for women) to account for this.
A standard drink contains 14 grams (0.6 fluid ounces) of pure alcohol in the US. This equals: 12 oz (355ml) regular beer at ~5% ABV, 5 oz (148ml) wine at ~12% ABV, or 1.5 oz (44ml) spirits at ~40% ABV. Standard drink sizes vary by country — the UK unit is 8g, Australia uses 10g. When using a BAC calculator, verify the standard drink definition being used.
Under the Motor Vehicles Act, the legal BAC limit for driving in India is 30mg per 100ml of blood, equivalent to 0.03% — stricter than the 0.08% limit in the US and UK. Some Indian states have stricter enforcement or have declared themselves dry. The penalty for drunk driving in India includes fines up to ₹10,000 and/or up to 6 months imprisonment under the 2019 Motor Vehicles Amendment Act.