Blood Alcohol Calculator (BAC) — Widmark Formula

Alcohol and Workplace Safety: Why BAC Matters for WHS

Alcohol-related impairment is a significant workplace safety risk across many industries in Australia, particularly in construction, mining, transport, and hospitality. Safe Work Australia and state WHS regulators require that workers not be impaired by alcohol or other substances at work. Understanding how alcohol behaves in the body is a core component of nationally recognised training units such as 11369NAT Course in Alcohol and Other Drugs Awareness — a qualification widely incorporated into workplace Drug and Alcohol (D&A) management programs.

Blood Alcohol Concentration (BAC) is the mass of ethanol per unit volume of blood, expressed as a percentage. A BAC of 0.05% means 0.05 grams of ethanol per 100 mL of blood. Even at levels below the legal driving limit, alcohol measurably impairs reaction time, divided attention, risk perception, and fine motor coordination — all critical to workplace safety. Research consistently shows impairment begins at BAC levels as low as 0.02%.

The Widmark Formula

BAC% = (A ÷ (BW × r)) × 100 − β × t

Where:
  A   = total ethanol consumed (grams)
      = Σ [volume_mL × (ABV/100) × 0.789]
  BW  = body weight (grams)
  r   = Widmark distribution factor
        Male: 0.68 | Female: 0.55 | Average: 0.60
  β   = elimination rate (%/hour) — typically 0.010–0.020,
        commonly 0.015%/hr
  t   = hours elapsed since drinking began

Note: Result must be clamped at 0 — BAC cannot be negative.

The Widmark factor r reflects the proportion of body weight that distributes alcohol. Lean muscle tissue has high water content and distributes alcohol well, while adipose (fat) tissue has low water content and does not. This is why body composition — not just body weight — affects BAC. A lean 70 kg person will have a meaningfully different BAC than an obese 70 kg person after the same drinks.

Factors That Affect BAC

The Widmark formula is a mathematical model, not a physiological certainty. Actual BAC is influenced by many factors that the formula cannot capture:

• Food: Eating before or during drinking slows gastric emptying and alcohol absorption dramatically. A full meal can reduce peak BAC by up to 50% compared to drinking on an empty stomach.
• Drink type: Carbonated beverages (beer, sparkling wine, mixers) increase the rate of alcohol absorption. Spirits mixed with non-carbonated mixers absorb more slowly.
• Drinking rate: Consuming drinks rapidly increases peak BAC significantly — the liver eliminates at a constant rate regardless of intake rate.
• Genetics: Genetic variation in alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) enzymes causes substantial individual variation in elimination rate (β). Some individuals metabolise alcohol at 0.010%/hr; others at 0.020%/hr or higher.
• Medications: Many medications interact with alcohol metabolism. Metronidazole, disulfiram, and some antihistamines potentiate alcohol effects significantly.
• Liver health: Liver disease reduces alcohol metabolism capacity. Regular heavy drinkers may develop tolerance, appearing less impaired at a given BAC, but are not actually less affected in terms of reaction time and cognition.

Legal BAC Limits by Jurisdiction

• Australia: 0.05% for full-licence holders; 0.00% for learners, provisional/probationary licences, heavy vehicle drivers, and taxi/bus drivers.
• United States: 0.08% for drivers aged 21+; 0.00–0.02% for under-21 drivers; 0.04% for commercial vehicle operators.
• United Kingdom (England & Wales): 80 mg/100 mL blood = 0.08%; or 35 μg/100 mL breath.
• Scotland: 50 mg/100 mL blood = 0.05%; stricter than England and Wales since December 2014.

Many workplaces impose a zero-BAC policy regardless of legal driving limits, particularly in safety-critical industries. Workers subject to a workplace D&A policy must comply with its specific limits, which may be more restrictive than road traffic law.

Why BAC Estimates Are Unreliable for Decision-Making

Studies comparing Widmark formula estimates to actual measured BAC show error ranges of ±0.02–0.03% in typical subjects, with outliers showing even greater deviation. This means a person who estimates they are at 0.04% BAC (safely below the 0.05% limit) may in fact be at 0.06–0.07%. Given this uncertainty, using a calculated estimate to make go/no-go decisions about driving or working is fundamentally unsafe. Additionally, individual cognitive and motor impairment varies substantially at any given BAC — tolerance does not protect against impairment, it merely masks the subjective feeling of intoxication.

Workplace Drug and Alcohol Policy Essentials

Effective workplace D&A programs include: a written policy with clear BAC limits and consequences; pre-employment and random testing programs using calibrated devices; for-cause testing following incidents; supervisor training to recognise impairment signs; an employee assistance program (EAP) for workers with substance use issues; and return-to-work protocols including follow-up testing. In Australia, the model WHS Act and state equivalents impose a duty of care on both employers and workers regarding alcohol and drug impairment. Consulting Australian Standard AS/NZS 4308 (urine drug screening) and AS 4760 (saliva testing) ensures testing programs are legally defensible.