Abstract: Routability estimation plays an important role in IC physical design.With the fast feedback on congestion,the estimator serves incremental placement and guides routing to shorten the physical design time.A routability analysis algorithm for IC physical design is proposed.This estimation algorithm firstly extends an existing combinatorial model with the extended bounding box for flat two-pin nets/sections to pre-estimate congestion probabilistically.Then,it routes all nets guided by the pre-estimation to get more exact congestion estimation,which is fast since it does not include any iteration/optimization as a router does.The idea is net-order-independent.This algorithm can obtain a uniform usage of routing resources by probability.