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Using Days of Supply

Use days-of-supply as a simple coverage metric without ignoring demand changes.

Days of supply translates inventory into time at an assumed consumption rate.

What this topic is trying to solve

Use days-of-supply as a simple coverage metric without ignoring demand changes. The useful question is usually not whether one number looks high or low, but what created it, how variable it is, and which part can actually be changed.

A practical approach

  1. Use available stock.. Treat this as a visible planning assumption and update it when current evidence shows the assumption is no longer representative.
  2. Choose a representative demand rate.. Treat this as a visible planning assumption and update it when current evidence shows the assumption is no longer representative.
  3. Separate normal and peak periods.. Treat this as a visible planning assumption and update it when current evidence shows the assumption is no longer representative.
  4. Compare by item class rather than blindly across all SKUs.. Treat this as a visible planning assumption and update it when current evidence shows the assumption is no longer representative.

Inputs worth checking

  • The exact scope included.
  • The time period and unit of measure.
  • Known constraints such as contracts, suppliers, access windows or local requirements.
  • Recent actual data that can be compared with the planning assumption.
  • Whether a peak, exception or one-time event is distorting the average.

Common ways plans go wrong

  • Using an average while ignoring predictable peaks.
  • Double-counting a buffer, allowance or cost.
  • Comparing options with different scope or service levels.
  • Treating a maximum or best case as normal performance.
  • Leaving an old policy unchanged after operations change.

Review the result

Compare the plan with actual outcomes after a representative period. Investigate material differences, then improve the next estimate rather than forcing reality to match the original one.

Useful habit Keep the assumptions beside the result so the next person can understand and revise them.

Where generic guidance stops

These tools are general inventory-planning aids, not accounting, tax, regulated-product, hazardous-material, food, pharmaceutical or safety-stock policy advice. Requirements for controlled, perishable or safety-critical inventory can be materially different; use applicable regulations, contracts, manufacturer requirements and qualified professional guidance.