The Early Strength App for underground construction calculates the early strength of sprayed concrete up to a level of 1 MPa based on penetration needle results. The calculation background is the Austrian Guideline on Sprayed Concrete and the EN 14488-2 “Compressive Strength of young Sprayed Concrete. The input for the calculation is the penetration force of a 3 mm diameter needle, which is pushed 15 mm into the sprayed concrete. The App allows to control the timing, store the readings of the penetration force and convert it to compressive strength. The results are displayed in a graphical way by showing the strength development over time with reference to the J curves. All data may be transferred as text files for further evaluation.