Power-gating is a technique for efficiently reducing leakage power by shutting off the idle blocks. However, the presence of powergating may also introduce negative effects, which are not considered in the earlier design stages. Ignoring those effects may result in suboptimal designs and potentially even nullify the intended power savings. In this paper, we propose a novel measure to efficiently capture the power-gating effects. We apply this measure in a floorplanner for power-gated chips. Experimental results show that the power-gating aware floorplanner can achieve 50% decap saving compared to a floorplanner unaware of power gating. Leakage power can be saved by inserting less decap, especially when thin-oxide decap are used due to the area constraint. Our approach can reduce leakage power consumed by decap from 60mW to 39mW when area overhead is limited to about 17% of the total chip area.