In this paper, we present a statistical delay fault estimation technique. The basic method is an extension of STAFAN to include delay faults. A strategy to calculate the transition observabilities of fanout stems is proposed. Correlation within each fanout free region is considered in calculating gate line transition controllabilities. Results show this is a practical method of calculating detectabilities of delay faults. When compared with transition delay fault simulations, the estimations of fault coverage are within 2.3% for the benchmark circuits. Finally, the estimation technique is used to grade delay faults, with comparison to fault simulation results used to validate the method.