<p>A class of carbonaceous material called “activated carbon” (AC), gained a remarkable attention in the field of renewable energy storage technology such as supercapacitor (SC) due to the enhanced performance than the other commercial carbons. In particular, activated carbon derived from biomass gained a significant potential owing to its tunable physical/chemical properties, low-cost raw material and abundance around us. Herein, sweet flag (Acorus <i>Calamus</i>) derived hierarchical porous carbons with different weight percentage of activating agent (Potassium Hydroxide—KOH) and heteroatoms (thiourea) such as 30, 40 and 50&#xa0;wt% are prepared through a simple hydrothermal technique. This work also aims to study the synergistic effect of undoped and nitrogen/sulfur-doped porous activated carbon for supercapacitor applications. From the obtained results, the high surface area, well-developed pores and doping of nitrogen and sulfur into AC are confirmed from Brunauer–Emmett–Teller (BET) and Field Emission Scanning Electron Microscopy (FESEM) analysis. X-ray Photoelectron spectroscopy (XPS) study reveals the chemical bond that exist between different elements in the prepared activated carbon. The electrochemical characterization of all the samples exhibits electrochemical double layer behavior and among all, the undoped activated carbon derived out of 30&#xa0;wt% KOH shows higher specific capacitance value of 420&#xa0;F/g at 1&#xa0;A/g in H₂SO₄ electrolyte in three electrodes set up. Also, the stability test reveals good capacitive retention of 94% even after 5000 charge and discharge cycles at a current density of 5&#xa0;A/g and disclose the potential of sweet flag derived AC for SC applications.</p>