This paper introduces a new method to estimate the total absorption coefficient of uncooled infrared detectors. Current approaches in the literature model the infrared detectors as cascaded transmission lines representing the detector layers, and this model can easily be used to estimate the absorption coefficient if the detector has the same structure at every point. However, the state of the art uncooled infrared detectors do not have same structure at every point, making it not feasible to use this simple model. According to the proposed method, the detector structure is divided into subregions having different layer combinations, and the absorption coefficient of each subregion is calculated separately. Then, the area ratios of the subregions together with these coefficients are used in order to calculate the total absorption coefficient of the detector. As the estimation of the absorption coefficient for complex detector structures can easily be done, the absorption in the required part of the infrared spectrum can be optimized by adjusting the layer properties and layer thicknesses. This approach can be used both for single and double layer uncooled infrared detector structures.