Days of Supply Calculator

About the Days of Supply Calculator

Days of supply (DOS) measures how many days your current inventory will last at the current rate of consumption. It is a straightforward yet powerful metric used by warehouse managers, supply chain planners, and purchasing teams to gauge whether stock levels are adequate, excessive, or dangerously low.

The calculation is simple: divide the average inventory on hand by the average daily usage. The result tells you exactly how many days of demand coverage you currently have. When days of supply drops below your lead time plus safety days, it is time to reorder.

This calculator lets you enter your average inventory (units or value) and average daily usage to compute days of supply quickly.

Use the result to compare operating scenarios, pressure-test assumptions, and rerun the model when volumes, rates, or service targets change.

When This Page Helps

Days of supply translates abstract inventory quantities into a time-based metric that is intuitive for decision-making. Saying you have 1,200 units is less actionable than saying you have 24 days of supply. DOS makes it easy to compare coverage across items with vastly different volume levels and to set universal thresholds for replenishment triggers.

How to Use the Inputs

1. Enter the average inventory on hand (units or dollar value).
2. Enter the average daily usage or demand (in the same unit).
3. Review the calculated days of supply.
4. Compare DOS against your lead time to assess replenishment urgency.
5. Use DOS to identify overstocked items (high DOS) and understocked items (low DOS).
6. Track DOS over time to monitor inventory health trends.

Example Calculation

Tips & Best Practices

• Use the same unit of measure (units or dollars) for both numerator and denominator.
• For seasonal products, calculate DOS using seasonal-adjusted daily demand.
• Compare DOS to lead time — if DOS < lead time, you need to order immediately.
• Target DOS should be lead time + safety days for optimal coverage.
• Investigate any SKU with DOS > 90 days for potential overstock issues.
• Plot DOS trends weekly to catch deteriorating inventory positions early.

DOS in Supply Chain Planning

Days of supply is a lingua franca across supply chain functions. Procurement uses it to prioritize purchase orders. Warehouse teams use it to manage storage allocation. Finance uses it to assess working capital tied up in stock. A single, simple metric aligns cross-functional conversations.

Segmenting by DOS Bands

Create DOS bands to segment inventory: Critical (< lead time), Healthy (1-2× lead time), Overstocked (> 3× lead time), and Dead (> 180 days). This bucketing makes exception-based management practical and highlights where action is needed.

DOS and Demand Sensing

Static DOS calculations use historical averages. Advanced supply chains feed real-time POS or order data into DOS calculations, giving a forward-looking view. If a demand spike occurs, DOS drops immediately, triggering earlier replenishment.

Limitations of DOS

DOS assumes a constant consumption rate. In reality, demand fluctuates daily. Combine DOS with safety stock analysis and reorder point logic for a complete replenishment framework.

Frequently Asked Questions

• What is days of supply?

  Days of supply is the number of days your current inventory will last at the current rate of demand. It converts a quantity metric into a time metric, making it easier to plan and prioritize.

• How does DOS differ from inventory turnover?

  They are inversely related. If annual turnover is 12x, DOS ≈ 30 days (365/12). Turnover is a financial efficiency metric; DOS is an operational planning metric that is more intuitive for warehouse teams.

• What is a good days of supply target?

  Target DOS depends on lead time, demand variability, and service level goals. A common target is 1.5× to 2× lead time. For a 10-day lead time, target DOS of 15-20 days provides adequate coverage.

• Should I use on-hand or on-hand plus on-order?

  On-hand gives you the immediate coverage view. On-hand plus on-order (scheduled receipts) gives a projected coverage that accounts for incoming replenishment. Both are useful depending on the decision context.

• Can DOS be calculated in dollar terms?

  Yes. Divide average inventory value ($) by average daily COGS ($). This gives a value-weighted DOS that is useful for aggregate inventory analysis and financial reporting.

• How do I calculate average daily usage?

  Take total units consumed over a period and divide by the number of days. Use 30-90 days of recent data for a representative average. For seasonal items, use the corresponding seasonal period.

• What if daily usage is zero?

  If average daily usage is zero, DOS is effectively infinite — the item is dead stock. Flag these items for review and consider liquidation, return to supplier, or discontinuation.

• Can I use DOS for raw materials?

  Absolutely. DOS is widely used for raw materials, work-in-process, and finished goods. Use the appropriate consumption rate for each stage — production schedule for raw materials, shipment rate for finished goods.