10 calculations every supply chain planner should know Most planners learned these numbers the hard way — standing at a dock door explaining to a buyer why their replenishment didn't land in time. Nobody hands you a cheat sheet on day one. You pick it up in pieces, usually after a miss. So here's the list, written the way I wish someone had handed it to me. Days of Supply: on-hand inventory divided by average daily demand. Simple, but I've seen teams carry 90 days of a slow-m