The popularity of the atria after its revival since the 1970's has caught the attention of designers and opened ways for new strategies of energy conservation for large buildings. Early atria were visual statements rather than integrated energy systems. But a growing recognition of the contribution of daylight for energy conservation in atria requires study in greater depth to exploit the potential of the atrium in the use of daylight for energy conservation. Present methods for calculating daylight distribution in conventionally designed buildings are not presently configured to deal with atria. This study takes advantage of scale model simulation process to develop a mathematical model which will predict daylight distribution in a square atrium under an overcast sky. Data generated from twelve model studies representing thirty six cases were analyzed using statistical methods as a measure to develop the mathematical model. The mathematical model developed has the ability to predict illumination level on the vertical surface at different floor locations in a square type atrium within the specified limitations. This model is reliable, as the predicted illumination levels have been found to have strong correlation with the values obtained from scale model studies. The mathematical model can be effectively used to assist designers in estimating illumination levels in an atrium and to provide opportunity to test design alternatives while the design is in the preliminary design stage. / Master of Science