What Is Days of Supply?
Days of supply (DOS) tells you how many days your current inventory will last if sales continue at their current rate. It is the most direct answer to the question every inventory manager asks: "When will I run out?"
The Formula
The basic calculation is straightforward:
Days of Supply = Current Inventory / Average Daily Sales
If you have 500 units on hand and sell 15 per day, you have about 33 days of supply. When you factor in inventory that's already on order, the formula expands to:
DOS (with incoming) = (Current Inventory + Incoming Inventory) / Average Daily Sales
Days of Supply vs. Reorder Point
Days of supply and reorder point are two sides of the same coin. Your reorder point is the inventory level at which you should place a new order. Days of supply tells you how far away you are from that threshold in calendar days.
When your days of supply drops below your supplier lead time, you are already late. Any new order placed at that point will arrive after you have stocked out. That is why this calculator also shows the surplus or deficit at your next order arrival: a negative number means you should have ordered already.
How Safety Stock Fits In
In practice, you do not want to cut it right to the wire. Safety stock acts as a buffer against demand spikes and supplier delays. A healthy days-of-supply target accounts for both your lead time and your safety stock buffer. If your lead time is 14 days and your safety stock covers 7 days of demand, you want at least 21 days of supply before you start worrying.
Ideal Days of Supply by Business Model
| Model | Target DOS | Why |
|---|---|---|
| Dropship | 0 days | You don't hold inventory; your supplier ships directly |
| Just-in-Time (JIT) | 7–14 days | Tight turns with reliable local suppliers |
| Standard E-commerce | 30–60 days | Balances carrying cost with stockout risk for most sellers |
| Seasonal / Overseas Sourcing | 60–120 days | Long ocean freight lead times and seasonal demand spikes |
Tips for Improving Days of Supply Accuracy
- Use a rolling average — A 30-day or 60-day rolling average of daily sales smooths out daily noise while still reflecting recent trends.
- Adjust for seasonality — If Q4 sales double your annual average, using a yearly average will overstate your days of supply heading into peak season.
- Count only sellable inventory — Damaged, reserved, or in-transit units that can't be sold today should not be included in your current stock figure.
- Track per-SKU, not just overall — An aggregate DOS of 45 days can hide the fact that your top seller has 8 days left while slow movers have 200+.
Related Tools
- Demand Forecasting — Predict future sales velocity instead of relying on past averages
- Safety Stock Calculator — Determine the right buffer to hold above your reorder point
- Reorder Point Calculator — Find the exact inventory level that triggers a new purchase order