Forensic practitioners regularly use the Widmark equation to determine theoretical blood alcohol concentrations for use in cases involving alcohol. It is important in these calculations to determine the uncertainty associated with any result. Previous work has investigated the uncertainty in %ABV from beers produced by small independent breweries in the UK but did not study the top selling beers. The top selling lagers and ales/bitters in the UK were identified by sales volume and the alcohol by volume determined. This data was then used to determine the percent coefficient of variation (%CV) that should be used by forensic practitioners when constructing alcohol technical defence reports for use in forensic cases. These samples, from what may be described as ‘big’ brewers, were determined to have a smaller root mean square error (RMSE) (±0.1 %v/v, n = 35), and %CV than those previously reported for beers produced by small, independent breweries in the UK. The results from this study shows that different RMSE’s should be used for %ABV when determining the uncertainty of results from Widmark calculations when drinks have been consumed from either ‘big’ brewers or small, independent breweries.