Lead Time Planner

Lead time is the elapsed clock time between placing an order and having the goods usable, and it is modelled here as a sum because the phases run in sequence: order processing, manufacturing, inspection, transit, and customs clearance each hand off to the next. Its practical importance is inventory. The reorder point formula, average daily demand multiplied by lead time plus safety stock, means every extra day of lead time forces another day of demand to be carried on the shelf. Safety stock scales with the square root of lead time, typically written as Z times the standard deviation of daily demand times the square root of lead time in days, where Z is 1.65 for a 95 percent service level and 2.33 for 99 percent. That square root is why a supplier who is slow but consistent can require less inventory than a supplier who is faster on average but erratic. Realistic phase values differ sharply by sourcing geography. Domestic manufacture plus ground freight often totals two to four weeks, while an Asian import runs 15 to 25 days of ocean transit to the US West Coast and 30 to 40 days to the East Coast, with customs clearance adding two to seven days depending on classification and whether an inspection is triggered. Buffers of 10 to 15 percent are conventional for established domestic suppliers and 20 to 30 percent for new or international ones. The main weakness of a flat percentage buffer is that it spreads protection evenly across phases whose variability is not even. Order processing and inspection are fairly predictable; ocean transit and customs are where delays actually concentrate, so a statistically derived buffer built from historical variance per phase outperforms one percentage applied to the total. Seasonality compounds this, with factory shutdowns around Lunar New Year and peak-season port congestion adding weeks that no average captures.

Quantitative inventory planning began with Ford W. Harris, who published the economic order quantity model in 1913 while working at Westinghouse, balancing ordering cost against carrying cost; R. H. Wilson popularised and extended the approach in the 1930s, and the reorder point with safety stock followed as the companion rule for when rather than how much to order. Lead time entered the calculation as the parameter linking the two. The Toyota Production System developed by Taiichi Ohno after 1945 pushed the logic in the opposite direction, treating lead time itself as the variable to attack so that inventory could be reduced rather than merely optimised. Computing made multi-stage lead time explosion practical. Joseph Orlicky at IBM formalised Material Requirements Planning, publishing his book on it in 1975, and with Oliver Wight and George Plossl promoted it through APICS, the American Production and Inventory Control Society founded in 1957, whose MRP Crusade began in 1972; MRP II and then Enterprise Resource Planning, a term coined by Gartner in 1990, extended the same time-phased logic across the whole business. Physical transit time was transformed by containerisation after Malcom McLean sent the converted tanker Ideal X from Newark to Houston in April 1956, which cut both the cost and the unpredictability of ocean freight and made long-distance sourcing viable. Jay Forrester described the amplification of demand swings upstream in Industrial Dynamics in 1961, and Hau Lee with V. Padmanabhan and Seungjin Whang analysed it as the bullwhip effect in 1997, showing lead time to be a principal driver. The Supply Chain Council introduced the SCOR reference model in 1996, standardising how these phases are named and measured.