Weeks of Supply Calculator

Your supplier lead time in weeks, plus a safety buffer of 1–3 weeks depending on how volatile the SKU’s demand is. Importing on an 8-week cycle means holding 9–11 weeks; a domestic 1-week supplier needs only 2–3. Holding much more than lead time plus buffer is cash parked without purpose; holding less is a scheduled stockout.

A trailing 4-week average is the standard for stable SKUs — long enough to smooth daily noise, short enough to track trend. For fast-trending or promotional products, weight recent weeks more heavily or use the last 2 weeks; for seasonal items, use last year’s same-season rate adjusted for growth. Whatever you pick, exclude stockout weeks — zero sales weeks from being out of stock poison the average and inflate the answer.

Weeks of supply is the per-SKU operational version, computed in units for reorder timing. Days of inventory (DIO) is usually the store-wide financial version, computed in dollars from COGS for cash-flow and health analysis. A buyer decides "reorder SKU-142 this week" from weeks of supply; a founder decides "we are over-invested in inventory" from DIO. Same concept, different altitude.

Set two thresholds per SKU. Reorder when weeks of supply falls to lead time plus your safety buffer — that is the moment to place the PO, not to start thinking about it. Flag overstock when supply exceeds roughly twice that level; those SKUs should skip their next reorder cycle and may need promotion. Reviewing both lists weekly takes minutes and prevents most stockouts and most dead stock.