Days of Supply (Inventory Cover) Calculator

How it works

Days of supply = on-hand ÷ daily usage; stock-out date = today + (on-hand + on-order) ÷ daily usage; order by = stock-out date − lead time; turns = 365 ÷ days of supply

Days of supply (or weeks of cover) answers the planner’s daily question: how long will what we have last at the rate we expect to sell or consume it? Unlike inventory turns, which look backward at cost of goods sold, cover is forward-looking and uses the forecast. Adding inbound purchase orders gives the projected stock-out date, and subtracting the supplier lead time from that date gives the latest day to place the next order without a gap — the time-phased version of a reorder point. Cover targets vary by item: fast, reliable items may run on 10–15 days, imported or long-lead items on 60–90, and the gap to target tells you what to order. Very high cover ties up cash and risks obsolescence; very low cover invites expediting and stock-outs.

Worked example

4,500 on hand and 2,000 inbound at 150 a day: 30 days of cover on hand, 43 with the inbound, so stock-out is about 43 days out; with a 14-day lead time the next order must go in within 29 days. A 30-day target (4,500 units) is already exceeded by the inbound.

Frequently asked questions