Understanding Inventory Days of Supply
How we calculate. Days of supply = average inventory ÷ daily COGS (COGS/365). See our methodology and accuracy policy .
Real-world scenario: A typical Inventory Days of Supply case uses average inventory 800000 and annual cogs / usage 4800000. Enter the same figures below to reproduce the worked path.
What is Inventory Days of Supply?
The companion to inventory turns (roughly 365 ÷ turns). Useful for buffer targets and cash-tied-in-stock conversations.
- Same cost basis as turns
- Annual COGS (or annualize a shorter window)
- Result in days
The Formula
Worked Example
Common Use Cases
- Working capital: stock cover
- Category planning: overstock flags
- S&OP: turns vs service tradeoffs
Pro Tips
- Annualize partial-year COGS
- Segment slow movers
- Pair with inventory turnover
| Scenario | When to use |
|---|---|
| Percent of a number | Finding a part of a whole (tax, tip, score) |
| Percent change | Comparing old vs new values |
Limitations: Inventory Days of Supply results are educational logistics and supply-chain planning aids—not SLAs, carrier contracts, or inventory valuation advice. Confirm definitions with your WMS/TMS and finance policies.
Common mistakes
- Swapping part and whole: The denominator must be the full total, not a subset.
- Rounding too early: Carry extra decimal places through multi-step work before rounding the final percent.
- Mixing percent and decimal forms: Enter rates in the format the calculator labels expect.
The worked example for inventory days of supply was verified with the calculator form on this page—enter the same values to confirm the result.
When to use this calculator
- Use this page when your wording matches inventory days of supply and the form labels on screen.
- Use percent of a number for “what is X% of Y?” problems.
- Use number is what percent when you know part and whole and need the percent.
Still unsure about inventory days of supply? Start with the quick answer above, then open the linked calculator that matches your wording.
❓ Frequently Asked Questions
How do I use this inventory days of supply calculator?
For inventory days of supply, enter the values labeled on the form, then press Calculate. Compare the result to the worked example on this page.
What formula does this page use?
See the quick answer and formula box above for inventory days of supply—the same percentage calculator expression is applied to your inputs.
What if my result looks wrong?
When checking inventory days of supply, re-check part vs whole (or rate vs base), rounding, and whether percents were entered as 25 rather than 0.25 when the form expects percents.
Authoritative References
For logistics and supply-chain KPI definitions, consult:
- CSCMP — supply chain management resources
- ASCM / APICS — operations and inventory body of knowledge
- ISO 28000 — security management for the supply chain (context)