Thank you very much, Steve! From your reply I learned: - to think twice; not in physical values but actually in units - that supplements on existing systems are not bad - errors related to conversion factors indicate that I should check my own units (formulas) The body of the main function simplifies to: bu::quantity<SI::frequency> WSR_x(100*SI::hertz); bu::quantity<SI::dynamic_viscosity> eta(4.6*(SI::kilograms/(SI::meter*SI::second))); bu::quantity<SI::stress> WSS_x(WSR_x*eta); Best regards, Andre