How to use the days of supply and stockout date calculator
- Enter the date of the stock count, the stock on hand and the average sales per day.
- Enter the supplier lead time and the safety buffer in days.
- If an order is already on its way, open the advanced options and enter its quantity and the days until it arrives.
- Read the days of supply, the date you run out and the latest date to place the next order.
Formula
days of supply = stock on hand / sales per day
run-out date = count date + whole days of supply
latest order date = run-out date - lead time - safety buffer
with an order on the way: new days of supply = (stock left at arrival + quantity) / sales per day
If the order arrives after the stock has run out, the tool shows the gap in days and the units of sales that could not be filled, then counts the new supply from the arrival date.
240 units at 12 a day
240 units at 12 a day last exactly 20 days. With a 7 day lead time and a 3 day buffer, the order must go in 10 days after the count, so the page says to order within 10 days.
Add an order of 300 units that arrives 14 days after the count. By then 168 units have been sold and 72 are left, so supply becomes 372 units, or 31 days, from day 14. The stock now runs out on day 45 and the next order is due on day 35. If the same order arrives after 30 days instead, the stock is gone on day 20, so the gap is 10 days and about 120 units of sales are lost. The new supply of 300 units lasts 25 days from day 30.
Days of supply versus a reorder point
A reorder point is a quantity that triggers an order, and days of supply is a time. They carry the same information, but a date is easier to plan around. The Reorder Point Calculator gives the quantity trigger, and the Safety Stock from Demand History Calculator sizes the buffer from real demand swings.
To choose how much to order once the date arrives, use the EOQ Calculator.
When sales are not steady
The calculation assumes level sales. For a seasonal item, run it with the sales rate for the weeks ahead, not the yearly average, and rerun it as the season changes. A promotion that doubles sales halves the days of supply.
Assumptions and limits
- Sales per day stay level until the stock runs out.
- Lead time and the safety buffer are whole days, and orders are placed on the date shown.
- The count date is the start of the first day, and the stock lasts through the whole days of supply.
- Only one order on the way can be included.
- Returns, damage and supplier delays are not modeled.
Frequently asked questions
Why is the run-out date a whole number of days away?
The tool uses the whole days the stock lasts. A part day of stock means you run out during that day.
What does a late order date mean?
The latest date to order has already passed, so even an order placed today will arrive after the stock runs out unless you shorten the lead time or use a faster supplier.
Can I use weeks instead of days?
Enter sales and lead time in days so the dates come out right.
What is the safety buffer for?
It is a cushion of extra days to cover late deliveries and faster sales than expected.