The most widely used approach to calculate a material’s gravimetric surface area, i.e. surface area per unit mass, is the Brunauer-Emmett-Teller (BET) method. The BET method computes the surface area of a material given the adsorption isotherm of a probe gas (i.e., N2 or Ar) in that material. Many researchers either obtain the BET area from commercial software that comes with measurement equipment, or perform the analyses manually on a spreadsheet, which is time-consuming and nearly impossible for some types of isotherms. Furthermore, these two approaches lead to large variability in BET-calculated areas. These challenges have motivated the development of programs for the automated and standardized calculation of BET areas. The SESAMI web interface allows a user to make surface area calculations on their web browser simply by uploading isotherm data. The motivation for this interface is to lower the barrier of entry for research groups seeking to use SESAMI code, which was previously packaged in Python and Jupyter Notebook scripts.