Days Of Supply Calculator
How long before the shelves are empty? Days of supply answers that in the units planners think in: days. It is also a term with more than one legitimate meaning, and this calculator keeps all three apart instead of blending them into one vague number.
In inventory mode it divides stock on hand by average daily usage and, if you add a supplier lead time, tells you how long you can wait before ordering. The COGS mode is the finance version, built from average inventory value and annual cost of goods sold. The prescription mode is the pharmacy calculation, quantity dispensed divided by daily dose, the figure recorded on insurance claims.
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Results show two decimals. The prescription result is an estimate from the directions you enter; the prescription label and your pharmacist govern the actual refill date.
Quick Answer
Key takeaways
- Days of supply is always a stock figure divided by a daily rate. What changes between modes is where the daily rate comes from: counted usage, annual COGS, or prescription directions.
- The lead time field changes nothing about the days figure. It converts it into a deadline: days of supply minus lead time is how long you can wait before ordering.
- The COGS mode equals 365 divided by inventory turnover, so a finance team and a warehouse team quoting different numbers may be describing the same stock.
- Pharmacy days supply is a recorded, claim-facing number, not a rough estimate. Small differences from rounding or from a misread direction change refill eligibility.
- Quick Answer
- How to use the days of supply calculator
- The three formulas
- Worked examples in all three modes
- Where days of supply sits: the cash conversion cycle
- Days of supply vs days of coverage
- Reading each mode without misusing it
- What competing calculators miss
- Frequently Asked Questions
- Add This Calculator to Your Website
- References
How to use the days of supply calculator
Pick the mode that matches the data you actually have. Inventory suits a stock count and a usage rate: enter both, and add the supplier lead time if you want the reorder warning. From annual COGS suits accounting figures: enter average inventory value for the year and annual cost of goods sold for the same year. Prescription suits a dispensed fill: enter the quantity and the total daily dose from the directions, meaning dose size multiplied by doses per day, not the dose size alone.
Switching tabs keeps each mode separate and hides the result, because the three answers are not interchangeable. A pharmacy figure of 33.33 days and an inventory figure of 33.33 days look identical and mean entirely different things, one about a patient's medication and one about a warehouse shelf. Keep the mode label attached to any number you copy into a report or a claim note.
Two inventory-mode details decide whether the result can be trusted. Count only stock that is on hand and available to use: leave out units on order, damaged stock, and stock reserved for another job. Then choose the averaging window deliberately. A 7-day window reacts fast but jumps around; a 90-day window is calm but slow to notice a genuine change in demand, and 30 days suits most steady items. For seasonal stock, average the same season last year rather than the quiet weeks just gone, or the figure will flatter you going into your busiest period. If a low result worries you, the cost of understocking calculator prices the lost sales and penalties that running out actually causes.
The three formulas
The finance formula deserves one pause. Dividing annual COGS by 365 gives cost flowing out per day, and average inventory divided by that flow gives days. Nothing in it counts units, which is why it can describe a whole business, including mixed stock that could never share a unit count. The inventory formula is the opposite: pure units, one item at a time, blind to what the stock is worth. Neither is a refinement of the other; they answer for different audiences.
Worked examples in all three modes
The calculator is pre-filled with these cases, one per tab, and every figure below is what it returns.
| Mode | Calculation | Result |
|---|---|---|
| Inventory | 4,800 units / 120 per day | 40.00 days, reorder within 30.0 days at a 10-day lead time |
| From annual COGS | $50,000 / ($730,000 / 365) = $50,000 / $2,000 per day | 25.00 days |
| Prescription | 200 inhalations / 6 per day | 33.33 days |
Cross-check the COGS example against turnover: $730,000 divided by $50,000 is a turnover of 14.6 times a year, and 365 divided by 14.6 is exactly 25.00 days. If your turnover figure and your days of supply ever disagree with that relationship, one of the inputs covers a different period from the other, which is the most common error in the finance version.
Where days of supply sits: the cash conversion cycle
In finance, the COGS version of days of supply is the inventory leg of the cash conversion cycle, the days sales in inventory or DIO figure. The full cycle adds the days a business waits to be paid and subtracts the days it takes to pay its own suppliers. The days outstanding calculator covers the receivables leg, the days payable calculator covers the payables leg, and the days working capital calculator works with the combined cycle. Cutting days of supply shortens the whole cycle and frees cash, which is why finance teams watch a warehouse number.
That context also marks the limit of the COGS mode. It is an average across everything the business holds, so a healthy 25 days can hide one product at 200 days and another perpetually out of stock. Run inventory mode per item for anything you actually reorder; reserve COGS mode for reporting, benchmarking, and spotting drift over time.
Days of supply vs days of coverage
The inventory mode on this page performs the same core division as the old days of coverage calculator, stock divided by daily usage, and some supply chain glossaries list coverage as another name for days of supply. This site used to run coverage as its own single-purpose page; that page has been merged into this one, so both readings now live here. What differs is scope and presentation:
| Feature | Days of supply (this page) | Days of coverage |
|---|---|---|
| Modes | Inventory, annual COGS, prescription | Inventory only |
| Result format | Decimal days, to two places | Whole days, rounded down |
| Lead time reorder warning | Yes | No |
| Best for | Reporting, finance review, pharmacy fills | A fast, conservative count of full days for one item |
The rounding gap is small but real: stock of 2,475 units at 85 a day reads 29.12 days as a decimal, or 29 full days on the whole-day reading. For a claim or a finance report, the decimal is the record. For deciding whether a storeroom survives a full working day, the whole-day figure is the safer promise.
The whole-day reading: full days of cover
This calculator reports the decimal and never rounds it down for you. Planners who need a promise they can staff and schedule against apply the whole-day reading themselves: drop the fraction and count only the days that end with the item still available for the whole day. It answers a narrower question than the decimal does: on which morning do we open the store, ward, or production line and find nothing left? Run the coverage example from the table above through that reading:
| Step | Calculation | Result |
|---|---|---|
| Exact division (what this calculator shows) | 2,475 units / 85 units per day | 29.12 days |
| Full days of cover (drop the fraction) | 29.12 read down to whole days | 29 days |
| Stock left after 29 full days | 2,475 minus (29 x 85) | 10 units |
| Day the stock ends | 10 units cover part of day 30 | During day 30 |
So read 29.12 days as 29 full days of cover, with the stock running out partway through day 30. The same reading works for anything consumed at a roughly steady daily rate, not just shelf stock: fuel left in a tank, cleaning chemicals, packaging, or cash reserves divided by daily spend, which is the figure the days of cash on hand calculator works with directly. The units cancel out, so the answer is always in days.
The whole-day reading also exposes how fast cover collapses when usage rises, because usage sits on the bottom of the division. The same 2,475 units at four usage rates:
| Average daily usage | Result here (decimal) | Full days of cover |
|---|---|---|
| 70 units | 35.36 days | 35 days |
| 85 units (the example) | 29.12 days | 29 days |
| 100 units | 24.75 days | 24 days |
| 120 units | 20.63 days | 20 days |
A rise from 85 to 120 units a day costs 9 full days of cover. Check that sensitivity before a promotion or a seasonal peak, not after: a single decimal result hides how quickly the position weakens.
Reading each mode without misusing it
Inventory mode. The reorder note assumes usage stays at the average until the new stock arrives. A promotion or a seasonal peak inside the lead time breaks that assumption, so treat the deadline as the latest safe date under normal demand, not a guarantee. If the note says order now, it means supply ends before a normal order can land; expediting or a partial local purchase becomes the real decision.
COGS mode. Match the periods. Annual COGS against a single month-end inventory balance produces a number that is neither the annual figure nor a monthly one. Average inventory exists precisely to smooth the spikes a single balance carries, and seasonal businesses should average monthly balances rather than two year-end points.
Prescription mode. Build the daily dose from the directions exactly: 2 tablets three times a day is 6 per day, and an as-needed direction is usually calculated at its maximum permitted daily amount for claim purposes, which makes the days supply shorter than typical use. Inhalers count inhalations, insulin counts units, and drops count drops per day across both eyes where directed. When a direction is genuinely ambiguous, the dispensing pharmacy's recorded figure governs, not a calculator's.
High cover is not free. Stock sitting on a shelf is cash that has been spent and not yet recovered, and it occupies space, needs insurance, and can expire or go obsolete. Perishables make the point sharply: cover beyond shelf life is not cover at all, it is future waste. A comfortable figure held across many items at once is one way working capital quietly stretches.
Low cover is a service risk, not just a number. At 3 or 4 days of cover, one delayed truck or one unusually busy day empties the shelf. The right response is rarely panic ordering of everything; it is checking which items are both low on cover and hard to substitute, and protecting those first.
What competing calculators miss
Search results for this term split into camps that rarely acknowledge each other, and each camp's tools inherit the blind spot.
Miss 1: one meaning only. Inventory KPI pages explain the COGS formula and stop; pharmacy tools do the dispensing division and stop. A pharmacy technician searching the same term as a warehouse manager gets sent to the wrong tool half the time. This page separates the three meanings into labeled modes with their own inputs, so nobody applies an inventory average to a prescription.
Miss 2: days with no deadline. Tools that return days of supply without touching lead time leave the actionable half undone. Forty days sounds comfortable until a 35-day lead time is set against it. The lead time warning here converts the figure into the decision it exists to support.
Miss 3: no turnover cross-check. Finance users hold turnover figures already, yet most COGS-mode tools never mention that days of supply is 365 divided by turnover. Showing the relationship lets a reader validate both numbers against each other and catch a period mismatch, as the worked example demonstrates.
One honest limit: this calculator does not forecast. Every mode divides by a current or historical rate, so a demand shift, a discontinued line, or a changed prescription makes the result stale the moment the rate changes.
Frequently Asked Questions
Days of supply is how long current stock lasts at the current rate of use. In inventory terms it is stock on hand divided by average daily usage. The same name covers two relatives: a finance version, average inventory divided by cost of goods sold per day, and the pharmacy days supply, quantity dispensed divided by daily dose. This calculator handles all three.
Turnover counts how many times inventory is sold and replaced in a year: annual COGS divided by average inventory. Days of supply is the same relationship expressed as time: 365 divided by turnover. A business turning inventory 14.6 times a year holds 25.00 days of supply. Turnover suits year-end comparisons; days of supply suits operational decisions, because days are easier to act on than turns.
It depends on lead time, perishability, and how costly a stockout is. Supply must at least cover the supplier lead time plus a safety margin, or normal reordering cannot keep up. Grocery and fast-moving goods often run at a few weeks; slow industrial parts can run at several months. Judge your figure against your own lead times and history, not a universal benchmark.
Total the units used or sold over a representative period and divide by the days in that period. Thirty days is a common window. Stretch the window for seasonal goods, and leave out abnormal days such as a one-off bulk order, or the average will misstate the normal rate and every result built on it will drift.
Before the supply runs out, by at least the supplier lead time. In inventory mode this calculator subtracts the lead time for you: 40.00 days of supply with a 10-day lead time means reorder within 30.0 days. If the lead time is longer than the days of supply, the warning says to order now, because even an order placed today arrives after the stock ends.
Quantity dispensed divided by the total daily dose from the prescription directions. An inhaler with 200 inhalations used at 6 per day is a 33.33 days supply, and 90 tablets at 2 per day is 45.00 days. The figure is recorded on the claim and sets refill timing, so pharmacies work it out from the directions exactly as written, including doses per day rather than dose size alone.
To turn an annual cost flow into a daily one. Average inventory divided by COGS per day gives the number of days the average stock would take to sell through at that cost rate, which equals average inventory divided by annual COGS, times 365. Use matching periods: annual average inventory with annual COGS. A quarter's COGS needs the quarter's day count instead.
The inventory mode here and days of coverage share the same core division, stock divided by daily usage, and some glossaries treat the names as one metric. They differed in scope: the old days of coverage page on this site was the single-purpose version that reported whole days rounded down. That page has been merged into this one, which reports decimals and adds the lead time warning, the COGS mode, and the prescription mode.
Because a fraction of a day is not a covered day. This calculator reports decimals and does not round down, so apply the whole-day reading yourself when the decision needs full days: stock of 2,475 units used at 85 a day returns 29.12 days here, which reads as 29 full days of cover, because only 29 of those days end with the item still available for the whole day and the stock runs out during day 30. The conservative reading stops you promising stock you will not have late on the final day.
Yes, in inventory mode, for anything consumed at a roughly steady daily rate: fuel in a tank, cleaning chemicals, packaging, or cash reserves divided by daily spend. The units cancel out, so the answer is always in days. It fails when use is lumpy rather than steady, because an average then hides the day a single large draw empties the stock.
Recalculate whenever the stock count or the usage rate changes materially, and on a fixed weekly cycle for fast-moving items. Every mode is a snapshot built on a current or historical rate, so a promotion, a lost customer, or a production change makes yesterday's number stale quickly.
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References
- NetSuite, "Inventory Turnover Ratio Defined" - turnover as COGS divided by average inventory, and days sales of inventory as 365 divided by turnover. NetSuite: Inventory Turnover Ratio. Accessed October 2026.
- NetSuite, "Average Inventory Defined" - average inventory from beginning and ending balances, and days sales of inventory as average inventory divided by COGS times 365. NetSuite: Average Inventory. Accessed October 2026.
- NetSuite, "What Is Inventory Management?" - days sales of inventory as a planning measure for reorders and selling pace. NetSuite: Inventory Management. Accessed October 2026.
- Penske Logistics glossary, "What are Inventory Days of Supply?" - days of supply as average inventory divided by daily COGS, across raw materials, work in process, and finished goods. Penske: Inventory Days of Supply. Accessed October 2026.
- Supply Chain Math, "Inventory Management Glossary" - days of supply as inventory on hand divided by average daily usage, and inventory coverage as the same division. Supply Chain Math: Inventory Glossary. Accessed October 2026.
- ShipBob, "Reorder Point Formula" - reorder point as demand during lead time plus safety stock, the basis of the lead time warning in inventory mode. ShipBob: Reorder Point Formula. Accessed October 2026.
- Cleverence, "How to Calculate Stock Turnover Days" - inventory days directly from average inventory and COGS, or as 365 divided by turnover, with period day counts for shorter windows. Cleverence: Stock Turnover Days. Accessed October 2026.
- eCQI Resource Center, "Medication, Dispensed" data element - daysSupplied defined as the number of days of medication supplied per dispense, in the CMS quality data model. eCQI: Medication, Dispensed. Accessed October 2026.
- New Medicare Part D Rule Affects Long-Term-Care Pharmacies - the CMS short-cycle dispensing rule built on 14-day or shorter days supply increments for brand-name drugs in long-term care. PMC: Medicare Part D Short-Cycle Dispensing. Accessed October 2026.
- Varsity Tutors, "Days Supply Calculation" for pharmacy technicians - quantity dispensed, directions for use, and days supply as three linked values, with daily usage taken from the prescription directions. Varsity Tutors: Days Supply Calculation. Accessed October 2026.