Antecedent Soil moisture (ASM) is one of the most important factors affecting the rainfall-runoff modelling process. Its existence and influence in computing runoff using Soil Conservation Service Curve Number (SCS-CN) method have been a point of discussion for many decades. In this study, a novel procedure has been proposed to estimate the ASM by modifying the SCS-CN method and verify its applicability by comparing the ASM with the observed soil moisture. Natural rainfall, runoff and soil moisture data from eight small experimental farms with similar land use and varying slopes viz. 1%, 3% , 5% and 8%, located at Roorkee, India have been utilized. The ASM is computed by optimizing two parameters i.e., absolute maximum potential retention (Sabs) and initial abstraction coefficient (λ), and the optimization is carried out by minimizing Root mean square error (RMSE). Results show that there exists a good agreement between observed and computed ASM for slopes 1%, 3% and 5% (R2 > 0.5). A fair correlation for slope of 8% (R2 of 0.31) is observed, which may be due to the inadequacy of data. This study will be helpful in calculating ASM for ungauged catchments and will further broaden the applications of SCS-CN method.