The space debris population has increased in the last decades and is still growing. This problem results amongst others in protecting spacecraft against particle impacts, especially for manned missions in low Earth orbits. Depending on the mission, even for smaller spacecraft, it is reasonable to protect the main structure against space debris and meteoroid impacts. For the risk analysis of spacecraft in meteoroid and space debris environment, so called risk analysis and damage prediction codes such as BUMPER or MDPANTO have been developed . They are used in order to quantify the risk for a defined mission resulting from impacts of particles. This analysis can be done based on two idealization concepts of the spacecraft surface. In the first concept, the surface area of the shield is taken for the analysis, in the second concept the surface area of the protected structure is taken. Due to the difference in surface area of the shield and of the protected structure, the analysis results could be described on one hand to be conservative and on the other hand to be optimistic. For large spacecraft this difference in surface area is negligible, but for small spacecraft it becomes more important. In this paper, an approach is developed in order to estimate a surface area, that reflects better the actual impact relations for small spacecraft.