In this paper, assurance of proper downlink outage probability, which is a design criterion based on feasible femto base station (BS) density, is analyzed. Considering femto BS deployment, a 3-D Poisson model of random spatial distribution and stochastic geometry are used. From the study, a closed form of feasible femto BS density will be identified. The analysis results not only can be used to predict the performance of various femto BS deployment scenarios but also can be used as a design criterion for resource control mechanism designs.