A simple iterative procedure for determination of the statistical parameters of a Weibull distribution is proposed. All experimental results on specimens of different size are considered together as a statistically representative population. The procedure can be used for a population in which each specimen has a unique size. The statistical reliability of the iterative procedure is illustrated by comparison with a minimization analysis and confirmation with existing methods. Experimental confirmation of the analysis is developed using six types of glass and carbon fibres at four gauge lengths each. It is shown that Weibull parameters, obtained separately for populations of fixed length, vary with the fibre length.