Inventory planning
Days of Supply
By the Supply Chain Manager team · Updated October 16, 2026
Days of supply (or days of stock) is how many days current inventory will last at the recent sales pace before the SKU runs out.
It turns a raw unit count into time, which is what purchasing actually reasons about. 400 units means nothing on its own; 400 units that last 6 days against a 14-day supplier lead time is an emergency.
Retailers often express the same idea in weeks (weeks of supply). The calculation is identical; only the unit changes.
How to calculate days of supply
Days of Supply = Available Inventory ÷ Average Daily Demand
Days of supply example
Illustrative numbers: 84 units available, selling 21 a day. Days of supply = 84 ÷ 21 = 4 days. If the supplier needs 8 days to deliver, an order placed today still leaves roughly 4 days with nothing on the shelf, unless you expedite or transfer stock from another warehouse.
Why days of supply matters for distributors
Comparing days of supply with supplier lead time is the quickest way to triage a long SKU list. Anything with fewer days of supply than lead time is already late; anything with several hundred days ties up cash.
Common days of supply mistakes
- Dividing by zero demand and reporting infinite days. A SKU with no demand needs a different review, not an infinite number.
- Using on-hand instead of available stock, so units already promised to customers are counted twice.
- Looking at one company-wide number when stock sits in the wrong warehouse.
Days of supply in Supply Chain Manager
Supply Chain Manager shows days of stock for every SKU and warehouse, flags 3 days or fewer as stock-out risk, and more than 180 days as slow-moving.
Open the live demoExamples on this page use made-up round numbers to show the calculation. They are not benchmarks or results from real businesses.