Inventory Days on Hand Calculator

How the figure is built

Days on hand is current stock expressed as days of supply: units on hand ÷ average daily usage. At the SKU level it is the operational twin of days sales of inventory, which is normally calculated at business level from accounting figures.

The distinction is worth keeping. DSI answers a finance question about capital tied up over a period; days on hand answers an operations question about whether this product will last until the next delivery.

The same thing with real figures

174 units on hand with daily usage of 40 ÷ 7 = 5.71 units. Days on hand is 30.5.

The lead time is 21 days, so there are 9.5 days of cushion. Comfortable, until usage rises. At 65 units a week, daily usage is 9.29 and days on hand falls to 18.7, which is below the lead time.

The order that was not urgent yesterday is late today, and nothing about the stock level changed. That is the value of expressing stock in days rather than units: the unit count is meaningless without the rate beside it.

Where the figure deceives

Average daily usage over a long window smooths away exactly the variation that matters. A product averaging 5.71 a day but selling 20 on Saturdays has a weekend exposure the average hides.

It also uses shipped units rather than demand, so any period spent out of stock lowers the usage figure and inflates days on hand, making the product look better supplied than it is, precisely because it was not supplied.

Acting on it

Compare it against lead time plus review interval, not against a fixed target. That comparison is the whole point of the measure and it is the version that produces an ordering decision.

Use a usage window that matches the product's rhythm, four weeks for steady lines, and a forecast rather than history for anything seasonal. The default 90-day window in most software is wrong for both.

Days on hand across a whole catalogue

Plotting days on hand for every SKU produces a distribution that is far more informative than any average. A healthy catalogue clusters in a band somewhere above lead time and below a few months, with short tails at either end.

The left tail is the ordering list and the right tail is where capital is trapped. In most businesses the right tail is longer than anyone expects, a handful of SKUs at several hundred days, often bought on a volume discount or a minimum order that nobody has revisited since.

Reviewing the two tails weekly and ignoring the middle is a complete inventory management routine for a small business. It takes twenty minutes, it catches both failure modes, and it is considerably more effective than a monthly report that averages the two tails into a number describing neither.

It is worth setting the alert threshold above lead time rather than at it. A product hitting exactly its lead time in days on hand has no margin for a delivery running a day late, and the alert arrives too late to do anything about it.

Lead time plus the review interval plus a few days is a workable trigger, and it gives whoever receives the alert enough time to place the order without treating it as an emergency.