Free tool
Inventory Days Calculator
Express historical inventory efficiency in days.
This inventory days calculator uses average inventory at cost and cost of goods sold to describe the period you have already measured. It brings days sales in inventory, days inventory outstanding and historical days on hand into one clearly defined calculation.
The result helps you compare how much inventory the business carried relative to the cost of goods it sold. It does not say when a particular SKU will run out. For available units at an expected demand rate, use the Weeks of Supply Calculator. The Inventory Planning Calculator puts that historical measure alongside the wider stock decision.
Historical period · Inventory at cost · Matched cost of goods sold
Historical reporting period
Use a consistent scope and cost basis for every figure.
The example is illustrative. Replace it with your own figures; results update as you type. Your entries stay in this page until you choose to copy them.
Need help finding your figures in Shopify?
Copy an input checklist for Sidekick. Store reports may not contain every cost or scenario assumption; verify the source and mark missing figures rather than guessing.
How this tool works
How inventory days are calculated
Divide average inventory at cost by cost of goods sold for the period, then multiply by the number of days in that period. The equivalent calculation divides period days by inventory turnover measured on the same basis.
Select the actual reporting dates so the derived day count matches the inventory and COGS figures. Average inventory should represent that same period; a beginning-and-ending average can miss seasonal peaks. Inventory and cost of goods sold need the same cost basis and business scope. Revenue is not a substitute for cost of goods sold.
Inventory days = average inventory at cost ÷ period COGS × days in period.
A 90-day trading window
A business carries average inventory of $30,000 at cost and records $90,000 of cost of goods sold over 90 days. Inventory turnover for the period is 3×, and historical inventory days are 30. This does not establish 30 days of current SKU cover.
Your full calculation
| Measure | Result · USD |
|---|---|
| Historical inventory days | 91.3 days |
| Inventory turns in entered period | 4× |
| Average inventory at cost | $50,000.0 |
| Period cost of goods sold | $200,000.0 |
| Days in reporting period | 365 days |
| Average daily COGS | $547.9 |
Questions & answers
Understanding your result
How do I calculate days sales in inventory?
Divide average inventory at cost by cost of goods sold for the same period, then multiply by the period’s days. With $20,000 of average inventory and $80,000 of COGS across 120 days, the result is 30 days.
Are DSI, DIO and days on hand the same?
They can describe this historical cost-based inventory-days measure. Some teams also use days on hand for forward unit stock cover, so check the formula and inputs. This calculator uses historical average inventory and COGS.
Should I use sales revenue or cost of goods sold?
Use cost of goods sold. Average inventory is measured at cost, so a revenue denominator would mix cost and selling-price bases. Keep the inventory and COGS scope matched as well.
Does lower inventory days always mean a better position?
A lower figure can reflect faster movement or a leaner inventory base, but it can also coexist with stockouts or missing sizes. Review availability, margin, seasonality and buying commitments alongside the historical ratio.