FREE INVENTORY CALCULATOR
Days of Supply Calculator
Estimate how many days and weeks your current inventory can support demand, then compare stock coverage with safety stock and supplier lead time.
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- No sign-up required
- Built for real inventory decisions
Calculator
Results
CalculatedDays of Supply24 days
At the current average demand rate, your on-hand inventory would support approximately 24 days of demand if no replenishment arrives.
Decision metrics
Decision Insight
Your current inventory covers approximately 24 days of demand. With a 14-day replenishment lead time, you have about 10 days of coverage beyond lead time and are expected to remain approximately 300 units above safety stock when replenishment arrives.
Covered Through Lead Time
Inventory Coverage Timeline
Markers update from the current demand, safety-stock, and lead-time inputs.
Now0Lead Time14Safety Stock20Stockout24
Key Takeaways
- Days of supply estimates how long current on-hand inventory can support average demand if no replenishment arrives.
- Weeks of supply is the same coverage expressed in weeks.
- Compare days of supply with supplier lead time to understand whether current stock can cover the replenishment period.
- Safety stock adds context by showing when inventory may enter the buffer range before replenishment arrives.
How It Works
Determine current on-hand inventory.
Estimate average daily demand directly or from sales over a selected period.
Divide stock by average daily demand.
Compare stock coverage with safety stock and supplier lead time. Days of Supply is a simple coverage measure based on average demand, not an exact stockout forecast.
Formula
Formula 1
Average Daily DemandUnits Sold in Period ÷ Sales PeriodFormula 2
Days of SupplyCurrent Inventory ÷ Average Daily DemandFormula 3
Weeks of SupplyDays of Supply ÷ 7Formula 4
Days Until Safety Stockmax(Current Inventory − Safety Stock, 0) ÷ Average Daily DemandFormula 5
Lead Time DemandAverage Daily Demand × Lead TimeFormula 6
Projected Stock at Lead-Time Endmax(Current Inventory − Lead Time Demand, 0)Formula 7
Lead-Time Coverage BufferDays of Supply − Lead TimeWhere:
- Average Daily Demand: the demand rate entered directly or calculated from units sold over a period.
- Safety Stock: an optional buffer used to interpret when stock may enter the protected range.
- Lead Time: the optional expected time between placing and receiving a replenishment order.
Example Calculation
Let’s say:
- Current Inventory: 1,200 units
- Average Daily Demand: 50 units/day
- Safety Stock: 200 units
- Lead Time: 14 days
Current stock is expected to cover the 14-day lead time and remain above the entered safety-stock level under the current average-demand assumption.
How to Use the Result
- Compare Days of Supply with replenishment lead time before placing or delaying purchase orders.
- Use Days Until Safety Stock to see when your inventory buffer may begin to be consumed.
- Review low coverage together with Reorder Point and Safety Stock rather than relying on one metric alone.
- Recalculate when demand, stock levels, lead times, or safety-stock assumptions change.
- Use short-term sales history carefully when demand is seasonal or promotional.
Limitations
- Days of Supply assumes demand continues at the entered average rate.
- The calculation does not automatically model seasonality, promotions, demand growth, or sudden demand drops.
- The basic calculation uses current on-hand inventory and does not automatically include future purchase orders or transfer arrivals.
- Lead Time is treated as a single expected value and does not model lead-time variability.
- Safety Stock is optional context and does not replace a full safety-stock analysis.
- A high Days of Supply value is not automatically good because excess inventory can increase carrying cost and obsolescence risk.
- A low Days of Supply value is not automatically bad if replenishment is very fast and reliable.
- Coverage Status is a planning signal based on entered assumptions, not a guaranteed stockout prediction.
Frequently Asked Questions
What is Days of Supply?
Days of Supply estimates how many days current on-hand inventory can support average demand if no replenishment arrives.
How do you calculate Days of Supply?
Divide current on-hand inventory by average daily demand.
What is Weeks of Supply?
It is Days of Supply divided by seven.
What is the difference between Days of Supply and inventory turnover?
Days of Supply estimates forward coverage from current stock and demand. Turnover measures how efficiently inventory moved over a past period.
What is a good Days of Supply?
It depends on demand variability, lead time, service levels, product lifecycle, and the cost of holding stock.
How does lead time affect Days of Supply?
Comparing coverage with lead time indicates whether stock is expected to last until a replenishment order arrives.
How does safety stock affect inventory coverage?
It shows when the remaining stock may enter the buffer range, providing context beyond the projected stockout date.
What if I have no recent sales?
Use a realistic forecast or demand estimate. A zero demand input cannot produce a useful coverage calculation.
Should I use forecast demand or historical average demand?
Use the demand measure that best represents the near-term planning decision, especially when demand is seasonal or changing.
Can Days of Supply be too high?
Yes. More coverage can also mean excess capital, carrying cost, or obsolescence exposure.
What is the difference between Days of Supply and reorder point?
Days of Supply measures current coverage. Reorder Point estimates the inventory level at which a replenishment order should be placed.
How often should Days of Supply be recalculated?
Recalculate whenever current stock, demand, lead time, or safety-stock assumptions materially change.