Hydraulic-fracturing design depends highly on the knowledge about injection-fluid efficiency. Generally, this parameter is derived from interpreting the calibration-test pressure falloff. A new interpretation technique for closure-pressure determination is based on a novel use of Nolte's F-function. Field cases demonstrate the usefulness of the F-function for improving hydraulic-fracturing design, whether for a conventional permeability estimation or for closure pressure and, hence, efficiency determination.