In manfacturing systems, to overcome the inherent complexities and uncertainties, due-dates are usually decided by some simple rules. This study proposes a computational procedure for estimating production lead time for computerized manufacturing systems. Based on the current system status stored in the computers, the procedure estimates production lead time for an incoming order. The result is more accurate than the ones produced by old rules, and yet the procedure is fast in computation speed enough to use in real time. Simulations are used to prove these.