ABSTRACT: The coccolithophores are among the few organisms of the phytoplankton to calcify. Although together with the planktonic foraminifera they dominate calcite production in the open ocean, the amount of calcite they secrete is difficult to determine. The amount of calcite in the form of coccoliths that is dissolved during sedimentation is also difficult to estimate as is the amount of calcite represented by sedimented coccoliths. Because of their minuteness, coccoliths cannot be directly weighed. Instead, indirect methods must be used to estimate their weight. I describe here a method that allows quick estimations of the weight of individual coccoliths, based on the measure of their brightness when viewed in cross-polarized light. This birefringence method can only be applied to the group Heliolithae (excluding some of the coccoliths produced in the Family Coccolithaceae). For Coccolithaceae and some Ortholithae an estimate of the weight can be made using a correcting factor. The birefringence method is highly reproducible (+/- 0.13pg). However, a significant systematic error of 13.5% may occur when coccoliths with asymmetrical appearance in cross polarized light (e.g., Helicospheara carteri) are not measured in specific orientations.