The complexometric method is usually applied to quantitative calcium determination in different materials; however the application of this method to calcium determination in molluscs shells infers significant interferences to the results. The snail Bradybaena similaris, a terrestrial gastropod, was used as experimental model to the improvement of this method. The shells were calcinated and dissolved in nitric acid, the hydrogen peroxide was also used to clarify the medium after the acid addition. The calcination procedure and the use of nitric acid reduced the significantly the interferences, allowing a major degree of destruction of the organic substances of the shell. The improvement of the calcium determination technique usually employed showed calcium content of 874.24 ± 56.617 mg of CaCO3/g of ash in comparison to the conventional technique that allowed the determination of 607.79 ± 67.751 mg of CaCO3/g of shell, wet weight.