Weeks of Supply Calculator
Know how long your stock will last, and how much to order to reach your target cover.
- Days of supply
- 56
- Units to order to reach target
- 300
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How to use the weeks of supply calculator
- Enter the units you have now, including stock at a 3PL or marketplace warehouse.
- Enter your average weekly sales over the last 4 to 12 weeks.
- Set the weeks of cover you want to hold to see how many units to order.
Weeks of supply formula
Your target should at least cover your supplier's lead time plus a safety buffer.
Example
You have 1,200 units and sell 150 a week, so you have 8 weeks (56 days) of supply. To hold 10 weeks of cover you need 1,500 units, so order 300.
Why weeks of supply matters
It turns a stock count into time, which is easier to plan with. If your supplier takes 6 weeks and you have 5 weeks of supply, you are already late.
Choosing a target
Add your lead time, the time until your next order and a safety buffer. Fast-moving products with reliable suppliers can run lean; imported or seasonal products need more cover.
Frequently asked questions
How do I calculate weeks of supply?
Divide stock on hand by average weekly sales.
How many weeks of supply should I keep?
Enough to cover lead time plus a buffer; many ecommerce brands aim for 8 to 12 weeks for imported goods.
Should I include stock in transit?
Include it if it will arrive before you run out; otherwise count only stock you can sell.
Is weeks of supply the same as days of inventory?
They measure the same thing in different units. Days of supply is weeks × 7.
Which sales period should I average?
Use the last 4 to 12 weeks, and adjust for known promotions or seasonality.
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