This paper presents an approach for calculating a Cumulative Inertia Index (CII) in order to predict high-rise buildings response under lateral loads for cases of medium eccentricity (≥ 5%, and ≤ 10%) in one direction. Different distributions of columns, shear walls, and outriggers are considered. Plan layouts with different aspect ratios are studied. The main aim is to present an index for high-rise building structures subject to lateral loads, which is simplified, and gives results within an acceptable accuracy. Shear walls and tube-in-tube systems with and without outriggers are considered. A set of guide charts and equations for moments, shear, deflection, drift, and period are generated for each case. The utility and accuracy of this approach is be demonstrated by several case study examples.