Work out days of supply from on-hand stock and average daily usage. See weeks of cover, days until stockout, and the units needed to reach a target.
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Short answer
Days of supply is on-hand quantity divided by average daily usage. With 4,200 units on hand and usage of 200 units a day, days of supply is 21.0 days, or three weeks of cover. Compare that figure with your replenishment lead time: cover shorter than lead time means a stockout is already in motion.
Days of supply answers the only stock question a planner is asked in a stand-up: how long does this last? It converts a quantity nobody can judge into a number of days everyone can, and it is the figure that sits directly against your lead time.
Enter what is on hand and how fast it moves. The calculator returns days of supply, weeks of cover, the day you reach safety stock, whether the units already on order arrive in time, and how much more you would need to buy to reach a target level of cover.
The result is a day count from now, not a calendar date. Nothing here depends on today's date, so a screenshot of this page still says the same thing next week.
Your numbers
Result
Days of supply
21.0 days
At current usage the stock runs out on day 21 from now, and reaches safety stock on day 17.
Average daily usage200.0 units/day
Weeks of cover3.0 weeks
Days until you reach safety stock17.0 days
Cover including 3,000 units on order36.0 days
Reorder point (usage in lead time + buffer)3,600 units
Value of stock on hand$77,700
Units to buy for 30 days of cover1,800 units · $33,300
Days of supply on hand21.0 days
Lead time to cover14 days
Target cover30 days
Average daily usage = 18,000 units ÷ 90 days = 200.0 units/day
Days of supply = 4,200 ÷ 200.0 = 21.0 days
Weeks of cover = 21.0 ÷ 7 = 3.0 weeks
Days to safety stock = (4,200 − 800) ÷ 200.0 = 17.0 days
Cover − lead time = 21.0 − 14 = 7.0 days of slack
21.0 days of cover against a 14-day lead time leaves 7.0 days of slack. Reorder when on-hand reaches 3,600 units.
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The formula
Days of supply = On-hand quantity ÷ Average daily usage
On-hand quantity
Physically available stock in the base unit of measure. Exclude allocated, quarantined and customer-owned units.
Average daily usage
Issues or shipments per calendar day. Divide period usage by the days in that period, using the same unit of measure as on-hand.
Safety stock
The buffer you do not intend to consume. Subtract it to get days until you breach the buffer rather than days until zero.
Lead time
Days from raising a replenishment to putting it away. Days of supply is only safe when it exceeds this.
Divide by calendar days if you ship seven days a week and by working days if you ship five. Mixing the two overstates cover by about 40%, which is exactly the error that produces a Friday stockout.
Worked example
On-hand quantity
4,200 units
Usage in the period
18,000 units over 90 days
Safety stock
800 units
Lead time
14 days
Units on order
3,000
Result
21.0 days of supply · 17.0 days to safety stock · 7.0 days of slack
Average daily usage is 18,000 ÷ 90 = 200 units a day. On-hand 4,200 ÷ 200 gives 21.0 days of supply, which is 3.0 weeks of cover. Take out the 800-unit buffer and you reach safety stock on day 17. Against a 14-day lead time that leaves 7.0 days of slack before an order has to be placed.
Days of supply against lead time
Cover on its own is neither good nor bad. Twenty-one days is generous for a locally sourced item with a three-day lead time and dangerous for an import with a ten-week one. The comparison that matters is cover minus lead time, which is the slack you have before an order must be placed.
Cover minus lead time
What it means
What to do
Negative
An order placed today arrives after you run out.
Expedite or partial-ship. Then fix the review frequency, not the buffer.
0 to 3 days
You are ordering at the last responsible moment.
Raise the replenishment now and check the reorder point is above lead-time demand.
4 to 15 days
Normal working slack for a reviewed item.
Nothing. Keep the review cadence honest.
Over 60 days
More cover than the supply risk justifies.
Check for a minimum order quantity or a forecast that never got revised down.
Where the number quietly lies
On-hand includes stock you cannot ship. Allocated units, failed QC, customer-owned consignment and damaged pallets all sit in the on-hand field in most ERPs and all inflate cover.
Average daily usage from too short a window. Fourteen days of history on a lumpy item produces a figure that swings by half whenever one order lands.
Calendar days against a five-day operation. If you only ship weekdays, 200 units a working day is 143 a calendar day. Choose one basis and use it in both the numerator and the reorder point.
Forecast demand rather than actual usage. Days of supply is an operational number. Use issues that really happened; keep the forecast for the buying plan.
Averaging across a promotion. A three-day spike inside the usage window raises the daily average permanently and hides the fact that normal demand is much lower.
Turning cover into a reorder trigger
Days of supply is the monitoring view; the reorder point is the acting version of the same maths. Demand during lead time is average daily usage multiplied by lead time — 200 × 14 = 2,800 units — and the reorder point is that plus safety stock, which is 3,600 units here. When on-hand drops to 3,600 you have exactly 18 days of cover left. The reorder point calculator handles the variability case.
Watch cover by value as well as by item. A hundred C-class items sitting on 400 days of supply cost less than one A-class item sitting on 90. Rank them first with the ABC analysis calculator.
Getting usage per item without exporting everything
One item is a two-minute calculation. Every item is a different job: issue history by item and location over a defined window, on-hand net of allocations, open purchase order quantities with dates, and safety stock as configured rather than as remembered.
With ERPray you ask for the list: "items with fewer than 14 days of supply, ranked by annual usage value, showing open PO quantities". It runs on your own data and prints the query it used, so the definition of on-hand is visible instead of assumed.
Frequently asked questions
How do you calculate days of supply?
Divide on-hand quantity by average daily usage. If you hold 4,200 units and use 200 a day, days of supply is 21. Get average daily usage by dividing usage over a recent window by the days in that window — 18,000 units over 90 days is 200 a day. Keep both figures in the same unit of measure.
What is the difference between days of supply and days of inventory?
Days of supply is a unit calculation for one item, based on on-hand quantity and daily usage. Days of inventory is a financial ratio for a whole business: average inventory value divided by COGS, times days in the period. They answer different questions, and they rarely agree because the financial version averages fast and slow lines together.
How many days of supply should I hold?
Enough to cover the replenishment lead time plus a buffer for demand and supply variability. For a 14-day lead time with moderate variability, that is often somewhere near 20 to 25 days. Do not set one target for every item — an A-class item and a cheap consumable justify very different cover.
Should I use calendar days or working days?
Whichever matches how you ship, and then use it consistently. A five-day operation using calendar-day usage understates daily demand by about 40%, so cover looks longer than it is. If you divide usage by working days, your lead time must also be expressed in working days.
Does days of supply include stock on order?
Not in the base calculation. On-hand cover tells you when you run out; adding open purchase orders tells you whether the gap is already covered. Report both, because a comfortable combined figure can still hide a stockout if the receipt is dated after the run-out day.
Why does my days of supply change when demand has not?
Usually because the usage window moved. A rolling average recalculated each week picks up and drops individual orders, so cover shifts even with stable on-hand stock. Lengthen the window for lumpy items, or check whether a one-off promotion or a large single order is still inside the average.
This calculator needs you to find the inputs first. ERPray pulls them from your own ERP account and computes the answer live — with the exact query shown so you can check it.