Days of Supply Calculator
Days of Supply Calculator
How Days of Supply Works
Days of Supply is an operational inventory metric that answers a forward-looking question: given what you have right now, how long will it last? Unlike financial inventory metrics that look backwards at historical averages and COGS, DOS uses current on-hand stock and current demand rates to produce an actionable number you can use today.
The core insight is that your DOS should always exceed your supplier lead time. If you have 10 days of supply and your supplier takes 14 days to deliver, you will stockout before the replenishment arrives. The moment DOS falls to your lead time, that is your reorder point. This is why DOS connects directly to the Reorder Point Calculator — both answer the same question: when do I need to order?
DOS is commonly used alongside other names: Days on Hand (DOH), Days of Inventory on Hand (DIOH), and Days of Coverage all refer to the same metric. The formula is always the same.
The Days of Supply Formula
The standard operational formula is:
If you only know annual demand, convert it first:
| Variable | Definition | Notes |
|---|---|---|
On-Hand Inventory |
Current stock available for sale or use | Can be in units, cases, or $ value — must match Daily Demand units |
Daily Demand |
Average units (or value) consumed per day | Use a rolling average (e.g. 30-day or 90-day) to smooth spikes |
Annual Demand ÷ 365 |
Alternative way to derive daily demand | Useful if you only have annual sales figures available |
For a full breakdown of formula variants (including the financial DIO formula), see the Days of Supply Formula guide.
Reading Your Result: The Lead Time Rule
Your Days of Supply result is only meaningful when compared to your supplier lead time. The relationship between DOS and lead time drives every replenishment decision:
- You have enough stock to cover replenishment
- No immediate action required
- Monitor regularly as demand consumes stock
- Stock will run out before next order arrives
- Reorder immediately to minimise stockout
- Consider expediting if possible
The target formula is: Target DOS = Lead Time (days) + Safety Stock (days). Safety stock days equals your safety stock quantity divided by your daily demand. If your lead time is 14 days and you hold 7 days of safety stock, your target DOS is 21 days. Order when DOS approaches 21 days.
Days of Supply Calculation Example
A consumer electronics distributor holds 1,200 units of a particular SKU. Average daily sales run at 40 units/day. Their supplier takes 21 days to deliver and they want 7 days of safety stock.
Days of Supply vs Days Inventory Outstanding (DIO)
These two metrics sound identical but serve completely different purposes. Understanding the distinction prevents confusion when finance and operations teams discuss inventory health:
| Dimension | DOS Days of Supply | DIO Days Inventory Outstanding |
|---|---|---|
| Formula | On-Hand Inventory ÷ Daily Demand | (Average Inventory ÷ COGS) × 365 |
| Perspective | Forward-looking (current stock) | Backward-looking (historical averages) |
| Question answered | "How long will this stock last?" | "How long did inventory sit on average?" |
| Used by | Operations, planners, buyers | Finance, analysts, investors |
| Update frequency | Daily or weekly | Quarterly or annually |
| Related to | Reorder Point, Lead Time, Safety Stock | Inventory Turnover (DIO = 365 ÷ Turnover) |
DIO is the financial version, calculated from your income statement and balance sheet. It tells you how efficiently you managed inventory over the past year. DOS is the operational version, calculated from your current WMS or ERP data. It tells you what to do right now. For the financial metric, see the Inventory Turnover Calculator — DIO is one of its outputs.
Common Days of Supply Mistakes
Yesterday's demand figure may be a spike or an anomaly. Use a 30-day or 90-day rolling average to smooth out noise, especially for products with volatile or seasonal demand. A single day's demand can cause your DOS to swing dramatically and trigger unnecessary orders — or miss genuine depletion signals.
On-Hand Inventory and Daily Demand must use the same unit. If your on-hand is in units but your demand is in dollars (or vice versa), the result is meaningless. Be consistent: either track both in units or both in $ value.
An average DOS across all SKUs hides the products that actually matter. A warehouse average of 30 days could include 10 fast-movers at 5 days (about to stock out) and 50 slow-movers at 200+ days (tying up capital). Calculate DOS at the SKU or category level for decisions that have teeth.
If you have a purchase order already on its way, your effective DOS is higher than what the on-hand figure shows. Some teams calculate DOS using (On-Hand + In-Transit) ÷ Daily Demand to get a fuller picture of coverage. Just make sure your lead time assumption aligns — if in-transit stock is already en route, use the remaining transit time as your effective lead time for that order.
Frequently Asked Questions
What is the Days of Supply formula?
Days of Supply = On-Hand Inventory ÷ Average Daily Demand. If you have annual demand, convert it first: Daily Demand = Annual Demand ÷ 365. For example, 500 units on hand with a daily demand of 20 units/day gives 500 ÷ 20 = 25 days of supply.
What is a good Days of Supply value?
The right number depends on your lead time and safety stock policy, not a fixed industry benchmark. Target DOS = Lead Time (days) + Safety Stock (days). If your supplier lead time is 14 days and you want 7 days of safety buffer, your target DOS is 21 days. Anything well above this ties up unnecessary capital; anything below your lead time is a stockout risk.
What is the difference between Days of Supply and Days Inventory Outstanding (DIO)?
Both measure how many days of inventory you hold, but from different perspectives. DOS is operational and forward-looking: On-Hand ÷ Daily Demand — it uses current figures to answer "how long will this stock last?" DIO is a financial metric: (Average Inventory ÷ COGS) × 365 — it uses historical averages for benchmarking and financial reporting. For replenishment decisions, use DOS. For investor and financial reporting, use DIO (available in the Inventory Turnover Calculator).
What happens if Days of Supply is less than my lead time?
You will run out of stock before the next replenishment arrives — a stockout. Reorder immediately. Your reorder point is the inventory level at which DOS exactly equals your lead time plus safety stock. Use the Reorder Point Calculator to find this level so you can set an automatic trigger for future orders.
Is Days of Supply the same as Days on Hand?
Yes — Days of Supply, Days on Hand (DOH), Days of Inventory on Hand (DIOH), and Days of Coverage are all names for the same operational metric. The formula is always On-Hand Inventory ÷ Daily Demand. The naming varies by industry and company convention.