Days of Inventory Calculator: Know How Long Your Stock Will Last
Days of inventory (DOI) tells you one thing that actually matters—how many days your current stock will last at today’s pace.
Fast to calculate. Useful every week. Use it to plan buys, tighten cash, and avoid stockouts.
How To Use This Days of Inventory Calculator
- Enter your average inventory value ($) for the period.
- Enter your cost of goods sold ($) for the same period.
- Click Calculate.
- Review your days on hand and turns per year, plus the performance note.
- Re-run for different periods or categories to spot trends.
Formulas Explained
Days of inventory (days) = (Average inventory ÷ COGS) × 365
Inventory turnover (turns) = COGS ÷ Average inventory
You can also flip it: DOI = 365 ÷ Inventory turnover. Lower days usually mean leaner operations; too low risks stockouts.
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Inputs you’ll need (and what to use)
- Average inventory. Take beginning + ending inventory, then divide by 2. Use the same period you use for COGS.
- COGS. The cost of the items actually sold in that period—not purchases.
Example
You carried $35,000 in average inventory and logged $400,000 in COGS this year.
| Variable | Value |
|---|---|
| Average inventory | $35,000 |
| COGS (annual) | $400,000 |
| Days of inventory | 31.9 days |
| Turns per year | 11 |
About a month of stock on hand and eleven turns. That’s lean for many retailers; confirm it matches your service levels.
What To Do With The Result
Use DOI to tune ordering, not to admire a number.
- Align buys to demand. If DOI is creeping up, cut orders or push promotions until you’re back in range. If it’s crashing, lift buys or speed replenishment.
- Segment by SKU class. Staple items can run low DOI; seasonal or slow movers need a cushion.
- Tighten lead times. Faster suppliers let you run fewer days without risking stockouts.
- Pair with reorder points. DOI shows runway; reorder points tell you when to pull the trigger.
Pro Tips To Improve Inventory Velocity
What would this be without a few fast and direct tactics to help you plan your inventory better and keep more of your cash.
- Forecast with reality, not hope. Use recent sell-through and planned promos.
- Kill dead stock. Bundle, mark down, or liquidate anything that hasn’t moved in 90 days.
- Automate replenishment. Let the system watch thresholds and create POs on time.
- Audit counts monthly. Bad counts = bad DOI. Fix the source, not just the spreadsheet.
Helpful Tools and Guides
Turn this number into action with the right stack:
- Compare options in our best inventory management software.
- Use smarter buying with inventory optimization software.
- Automate POs with inventory replenishment software.
- If DOI looks high, revisit your inventory turnover metrics and clear slow movers.
- For omnichannel setups, consider cloud-based inventory tools or multichannel inventory software.
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