Inventory Planning Calculations: 10 Essential Formulas to 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-moving SKU while stocking out on a fast one because nobody was checking it weekly. Reorder Point: average daily usage multiplied by lead time, plus your safety stock buffer. If you're not adding the safety stock, you're gambling. Safety Stock itself: take your maximum daily usage minus your average daily usage, multiply by your max lead time. It sounds like overkill until a supplier misses a ship date in Q4. Cycle Stock is just your average demand during a replenishment cycle — it tells you what you actually need on hand versus what you're holding as insurance. Economic Order Quantity balances ordering costs against holding costs; the formula is ugly but the concept saves real money once you run the numbers on a high-velocity item. Carrying Cost Rate is your annual holding cost divided by average inventory value — most operations land somewhere between 20 and 30 percent when you factor in space, handling, and capital. Inventory Turnover is cost of goods sold divided by average inventory; a distributor running 4 turns a year in a category that benchmarks at 10 has a conversation to have. Fill Rate: units shipped on time divided by units ordered — not the same as in-stock rate, and confusing them will give you a false sense of how you're actually performing. GMROI, gross margin return on investment, is gross margin divided by average inventory cost; useful when you're deciding which slots deserve prime real estate near the dock. And Forecast Accuracy, usually measured as one minus the mean absolute percentage error, because if your input numbers are off, every calculation above is just organized guessing.
None of these are exotic. They're the arithmetic that keeps a warehouse from quietly bleeding out while everyone's focused on the fire drill of the week. The hard part isn't the math — it's building the habit of pulling the numbers before the problem surfaces, not after.
Yuneva builds tools for exactly that kind of visibility. Poke around at www.yuneva.com or, if physical inventory counts are your pain point right now, www.count-inventory.com is worth a look.
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