This paper proposes a new algorithm that calculates the Available Transfer Capability (ATC) accurately. The accuracy of computing ATC is important to ensure the security of system operation. On the hand, we devise the maximum area concept for the allocation of simultaneous ATC. This concept decides maximum area capability inside the security region by considering the shape of security boundary. The proposed concept is novel because the other allocation methods only use the maximum transfer amount to compute ATC. The proposed concept improves the efficiency of transmission facility use and makes the power interchange more flexible.