This technical note outlines the technical background of the data and indicators featured in the Water Security Compass (hereafter referred to as WSC). The WSC is developed by the Research Initiative for Global Hydrologic Cycles, a collaborative research unit established at The University of Tokyo. This note describes the fundamental assumptions and calculation methods to maintain transparency throughout the assessment. Furthermore, it serves as a reference guide to help users understand the WSC's scope of application and limitations when conducting water risk assessments and related studies. The development of the WSC was conducted with financial support from Suntory Holdings Limited and Nippon Koei Co., Ltd. Computational resources were provided by the Data Integration and Analysis System (DIAS), which is developed and operated under the Ministry of Education, Culture, Sports, Science and Technology (MEXT) project (Grant Number: JPMXD0721453504).