IP-reuse can enhance the design productivity only if the design methodology treats the IPs in a proper way. Especially in the floor-planning phase, sensitive IPs should be treated as routing obstacles, which is impossible when the conventional HPWL-based method is used for routing estimation. This paper proposes an obstacle-aware minimum wiring length (MWL) estimation algorithm, based on the theoretical result in [5], through algorithmic improvements and practical approximation. The experimental results suggests that MWL-based estimation is now possible with only few times larger computational cost comparing to the HPWL-based estimation.