Forensic Science

Widmark’s formula : How to Calculate Blood Alcohol Content (BAC)

Widmark’s formula, Blood Alcohol Content calculator, Blood Alcohol Content formula, BAC Alcohol, BAC calculator, Widmark formula calculator

Measurement of Alcohol

1. Blood Alcohol Concentration (BAC)

  • Alcohol can be detected in blood within 2-3 minutes of a few sips of beer or whisky. After single dose, usually within half to one hour, the blood-alcohol concentration reaches the maximum and gets proportional to the amount consumed.
  • The BAC is the most useful measure, as there is rapid equilibration across the blood-brain barrier, therefore BAC reflects the concentration of alcohol currently affecting the brain.
  • BAC is a direct method for the determination of a blood alcohol level.

2. Urine Analysis

  • Urine alcohol analysis is an indirect method. The pool of urine in the bladder at any given time is an accumulation of secreted urine since the last emptying of the bladder.

3. Breath alcohol, unlike urine, is in equilibrium with blood, even though in a very small concentration of about 1: 2300.

Widmark’s Formula

Widmark’s formula is used to estimate blood alcohol level.

According to this:

Formula for estimating alcohol in blood is

a = prc

Where in

  • a – the total amount of alcohol (in grams)absorbed in the body
  • p – the weight of the person (in kg)
  • r – Widmark’s constant (0.68 for men and 0.5 for women)
  • c – the concentration of alcohol in blood (in mg/kg)

Formula for estimating alcohol in urine is

a = ¾ prq

Where in

  • p – the weight of person (in kg)
  • r – Widmark’s constant (0.68 for men and 0.5 for women)
  • q – concentration of alcohol in urine (in mg/kg).

Any method to determine alcohol (ethanol) in blood and urine can be classified into:

  • Macro (requiring 1–2 ml of specimen)
  • Micro (requiring 0.1–0.2 ml of specimen or less)

The methods are:

  • Chemical: Widmark method, Cavett method, Nickolls method, Southgate and Carter method, Kozelka and Hine method
  • Biochemical: Alcohol Dehydrogenase (ADH) method
  • Physical: GLC
  • Breathalyzer (or breath analyzer) is a device for estimating BAC from a breath sample.
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