Coating formulations for paper and paperboard are continuously evolving as new materials, new application processes and new end uses are developed. Even basic formulations (in terms of the simplicity of the formulation or application) can have complex rheological and physical properties that must be understood in order for the coating color to be able to run and perform its intended function. This method describes a procedure for measuring the Euler number of coating colors using a High-Shear Viscometer with an orifice configuration. In this method, the extensional viscosity is evaluated by calculating the Euler number when the coating color flows through an orifice at a velocity of 15 m/s. The method is applicable for water-based coating colors used in various coating processes such as in blade, rod, and curtain coating. Typically, suitable coating colors have a solids content between 30 % and 80 %, and they contain mineral pigments, synthetic and / or natural polymers and coating additives. An Euler number of 1-2 signifies a lack of extensional viscosity in the sample. Euler numbers of 2-5 signify moderate amounts of extensional viscosity, while numbers over 5 signify high extensional viscosity. Typical extensional viscosity values can be seen in Appendix E.