Results of experiments of friction losses in conduits with large roughnessformed by spheres and hemispheres in different concentrations are used as abasis for the determination of the reference datum. This enables the computationof the friction loss for this type of roughness without further experimentalwork. A mathematical model for the computation of friction coefficient, applicableto conduits with roughness elements of any shape and based on the coefficientof drag, is presented. The model has been successfully tested by experimentsin channels both with uniform and with random roughness size and shapeincluding experiments with natural river stones up to 150 mm high.