Days Sales of Inventory Calculator

Written out

Days sales of inventory is how long current stock would last at the current rate of sale: (average inventory ÷ COGS) × 365. Equivalently it is 365 ÷ inventory turnover, and the two are the same measurement expressed in different units.

Days are more useful than turns for operational decisions because they compare directly against lead time. Thirty days of stock against a twenty-one day lead time is a different situation from thirty days against a sixty-day one, and the turnover figure makes that comparison harder than it needs to be.

How that looks in practice

Average inventory $3,132 and annual COGS of $37,440. DSI = (3,132 ÷ 37,440) × 365 = 30.5 days.

The lead time is three weeks, 21 days. So the business holds about 9.5 days more stock than the lead time requires, which is the safety stock expressed in time rather than units.

If the supplier moved to a six-week lead time, 42 days, the same 30.5 days of stock would guarantee a stockout on every cycle. The days view makes that immediately visible in a way the turnover figure of 11.95 does not.

Where this breaks down

It is backward-looking. DSI uses historical COGS to project how long current stock will last, which is fine for a stable product and badly wrong going into or out of a season. Thirty days of stock in October may be five days of stock in December.

Average inventory also smooths out the peaks. A business that receives a large delivery and works it down to nothing has an average that describes neither the full nor the empty state, and both matter more than the middle.

Turning it into a decision

Compare it against lead time plus review period rather than against a target. Stock has to cover the time until the next order arrives, and DSI below that figure means stockouts are structural rather than bad luck.

For seasonal products, calculate forward DSI using the forecast rather than history: current units ÷ forecast daily demand. That is the number that answers whether you will make it through the season, and it is the only version worth acting on in November.

DSI in the cash conversion cycle

DSI is one of three terms in the cash conversion cycle: DSI + days sales outstanding − days payable outstanding. It measures how long cash sits in stock, and it is usually the largest of the three for a product business.

With DSI at 30.5 days, receivables at 4 days for a consumer business taking card payment, and supplier terms of 30 days, the cycle is 30.5 + 4 − 30 = 4.5 days. That is close to self-funding, which is why consumer ecommerce can grow with less working capital than wholesale.

Shift supplier terms to payment in advance and the cycle becomes 34.5 days, which means every dollar of growth needs about five weeks of funding ahead of it. That single change is the difference between growth funding itself and growth requiring a facility, and it is a negotiation rather than an operational improvement, which makes it one of the cheapest wins available.