The contribution of body mass and volume of distribution to the estimated uncertainty associated with the Widmark equation

Peter D. Maskell*, Gail A. A. Cooper

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)


The Widmark equation is used forensically for the determination of the amount of ethanol (alcohol) that may have been consumed and also to determine the blood alcohol concentration (BAC) of an individual at a specific time. It is important to be able to estimate the uncertainty associated with Widmark equations. To date, there has been no detailed determination of contribution to the final uncertainty of Widmark calculations of the volume of distribution of ethanol (Vd ), using anthropometric equations, or the contribution of an individual's body mass. Using published data, published literature, and freedom of information data, we determined that the variability (%CV) associated with Vd was ~10% (Watson et al. and Forrest anthropometric equations) and that the %CV associated with estimated body mass was ~15% compared to ~3% when body mass was directly measured. These data allow an estimation of the overall uncertainty of Widmark calculations using general error propagation. The estimated total uncertainty for BAC calculations increased from ~11% (volume consumed) and ~22% (BAC) to ~19% (volume consumed) and ~37% (BAC) when using measured body mass compared to estimated body mass. These results demonstrate that forensic practitioners should be mindful of the increase in estimated uncertainty in calculated Widmark equation results when estimated body mass is used rather than measured body mass. These data further improve the knowledge around the uncertainty of results calculated with the Widmark equation.

Original languageEnglish
Pages (from-to)1676-1684
Number of pages9
JournalJournal of Forensic Sciences
Issue number5
Early online date18 May 2020
Publication statusPublished - 1 Sep 2020


  • Widmark equation
  • Ethanol volume of distribuion
  • Uncertainty
  • Blood alcohol calculations
  • Propagation of error


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