The Municipality of San Agustin is a 4th Class Municipality composed of 15 barangays, of which two are coastal barangays; Poblacion and Cabolutan while the rest are coastal-upland barangays; Bachawan, Binonga-an, Buli, Camantaya, Cagbo-aya, Carmen, Cawayan, Doña Juana, Dubduban, Hinugusan, Lusong, Poblacion, Mahabang Baybay and Sugod.
The updated total uncontested land area of the Municipality as certified by Land Management Bureau (LMB)-DENR is 9,512 hectares or 95.12 square kilometers. The largest barangay in terms of area is Barangay Bachawan with 1,094 hectares followed by Dubduban while the smallest barangay is Barangay Poblacion with only 18 hectares. The table below show the area of each barangay in hectares. Previously, the total land area used by the Municipality is 14,048 hectares.