According to the sign convention, the focal length of a concave mirror is taken as

1. Distances are measured from the pole; those along the incident light are positive, opposite ones negative. 2. A concave mirror focus lies in front, on the incident-light side, i.e. the negative direction. 3. So its focal length is taken as negative. _Source: AP/TS SCERT Class 10 Physical Science, "Reflection of Light at Curved Surfaces"_