In recent years, indoor positioning system using existing Wi-Fi infrastructure and MEMS sensors have received significant attention. A key requirement for a complete 3-dimensional indoor positioning algorithm is to provide an accurate and robust user floor location estimate in a multi-floor building. In this paper, we propose novel techniques to estimate user floor location based on the received radio signal strengths from Wi-Fi access points in the building. We illustrate its performance advantages through simulations and multiple field test results across typical use case scenarios. We further discuss Wi-Fi outage scenarios during which the performance of the Wi-Fi based floor determination algorithm is limited. We then propose a method to blend the Wi-Fi access points based floor estimate with pressure sensor measurements to enhance the overall performance of the algorithm. Finally, we present performance results from field tests to validate the efficacy and robustness of our proposed techniques.