Days of inventory calculator
Days of supply is how long the stock you hold will last: units on hand ÷ average units sold per day. Days sales of inventory (DSI) measures the same idea in dollars for the whole business: average inventory value ÷ cost of goods sold × the number of days in the period. Add an inbound shipment and your lead time to see the stockout date and the last day to reorder.
Example values; replace them with yours. Leave the lead time or the inbound shipment empty to skip them.
83days
Out of stock around Thursday, January 21, 2027.
1,800 units on hand last 50 days. The 1,200 inbound units land Nov 20, before you run out, and add 33 days.
Order by Dec 22 so a 30-day lead time lands the next batch before you run out.
How days of supply and DSI are calculated
Days of supply
(1,800 + 1,200 inbound) units ÷ 36 a day = 83 days
Divide the units on hand by the average you sell per day, counted across every sales channel. Shopify’s inventory remaining report works the same way for each variant: ending quantity ÷ average units sold per day over the last 28 days. An inbound shipment that lands before you run out adds its units to the top; one that lands later restarts the count on its arrival day.
Days sales of inventory (DSI)
$84,000 ÷ $420,000 × 365 days = 73 days
The CFA Institute’s financial ratio list defines days of inventory on hand as the number of days in the period ÷ inventory turnover, where turnover is cost of goods sold ÷ average inventory. Multiplied out, that is the formula above. Value inventory at cost, as your balance sheet does, and use the cost of goods sold for the same period. To see the turnover side of the same numbers, try the inventory turnover calculator.
If sales speed up or slow down
A promotion or a quiet month moves the stockout date more than most people expect. The table recalculates your runway, with the same stock and inbound shipment, at faster and slower sales.
A faster pace pulls the reorder date forward too. Set a buffer with the safety stock calculator and a trigger level with the reorder point calculator.
| Sales pace | Units a day | Stock lasts | Out around | Order by |
|---|---|---|---|---|
| −25% | 27 | 111 days | Feb 18 | Jan 19 |
| −10% | 32.4 | 93 days | Jan 31 | Jan 1 |
| Today’s pace | 36 | 83 days | Jan 21 | Dec 22 |
| +10% | 39.6 | 76 days | Jan 14 | Dec 15 |
| +25% | 45 | 67 days | Jan 5 | Dec 6 |
| +50% | 54 | 56 days | Dec 25 | Nov 25 |
Which number to use
Use days of supply for decisions about one SKU: when to reorder, how much to send in, whether a promotion will empty the shelf. It is in units and looks forward from today.
Use DSI for the business as a whole: how much cash sits in stock and whether that is improving from one quarter to the next. It is in dollars at cost and looks back over a period you have already closed. To see which SKUs drive the number, compare them with the sell-through rate calculator.
If your stock is split across warehouses, work out days of supply for each location. A unit sitting in one building can’t fill an order that ships from another until it is transferred. Fulfillment Hub USA runs five U.S. warehouses, in Dover, Miami, Houston, Los Angeles (Commerce) and South San Francisco, plus one in Oroszlány, Hungary, and handles receiving, storage and ecommerce fulfillment from each.
Common questions
How do I calculate days of supply?
Divide the units you have on hand by the average number of units you sell per day. With 1,800 units on hand and sales of 36 units a day, you have 1,800 ÷ 36 = 50 days of supply. Use a recent average across every sales channel; Shopify’s inventory remaining report, for example, averages each variant’s sales over the last 28 days.
What is days sales of inventory (DSI)?
DSI is how many days of cost of goods sold your average inventory covers: average inventory ÷ cost of goods sold × days in the period. With $84,000 of average inventory and $420,000 of cost of goods sold over 365 days, DSI is 84,000 ÷ 420,000 × 365 = 73 days. The CFA Institute’s ratio list calls it days of inventory on hand and defines it as days in the period ÷ inventory turnover, which gives the same number.
What is the difference between days of supply and DSI?
Days of supply looks forward for one SKU, in units: how long the stock on the shelf lasts at the current pace of sales. DSI looks back at the whole business, in dollars at cost: how many days of cost of goods sold your average inventory covered in a past period. Use days of supply to plan reorders and DSI to track working capital over time.
How does an inbound shipment change the stockout date?
If the shipment lands before you run out, its units add to the runway: (units on hand + inbound units) ÷ units sold per day. If it lands after you run out, there is nothing to sell between the two dates, and the runway starts again on the arrival day.
When should I reorder?
Place the next order at least one lead time before the stockout date, so it lands before the shelf is empty. The calculator subtracts your lead time from the end of the runway. To cover late deliveries and sales spikes, add safety stock and reorder at a reorder point instead of on the last possible day.
How many days of inventory should I hold?
There is no single right number. Cover at least your supplier lead time plus a safety buffer, and weigh anything beyond that against storage costs and the cash tied up in stock. Compare your own DSI from period to period rather than against a general rule.
Sources
- CFA Institute, CFA Program financial ratio list: inventory turnover and days of inventory on hand
- Shopify Help Center, Inventory reports: inventory remaining per product
Checked 2026-10-30. Rules change; confirm with the carrier or agency before you rely on a number.
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