Calculate days of supply from inventory and daily usage, or days in inventory from average inventory, COGS, and period length in days.
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Days Of Supply Formula
The basic days of supply calculation estimates how long your current inventory will last at a constant daily usage rate.
DOS = IOH / DUR
- DOS = days of supply
- IOH = inventory on hand
- DUR = daily usage rate
If you leave inventory on hand blank, the calculator rearranges the formula:
IOH = DOS * DUR
If you leave daily usage rate blank, the calculator uses:
DUR = IOH / DOS
For the Days in Inventory mode, the calculator uses average inventory at cost, cost of goods sold, and the period length.
DII = (AI / COGS) * PD
- DII = days in inventory
- AI = average inventory at cost
- COGS = cost of goods sold for the period
- PD = period length in days
The basic mode lets you enter any 2 of the 3 values: days of supply, inventory on hand, and daily usage rate. The missing value is calculated from the same relationship. The financial mode calculates how many days inventory stays on hand on average, based on cost figures rather than physical unit counts.
Common Days of Supply Interpretations
| Days of supply result | What it usually means | Possible action |
|---|---|---|
| Very low | Inventory may run out soon if demand continues. | Review reorder timing, supplier lead time, or usage assumptions. |
| Near target | Inventory is close to the planned coverage level. | Continue monitoring demand and replenishment timing. |
| Very high | Inventory may be moving slowly or overstocked. | Check demand forecasts, holding costs, and slow-moving items. |
Period Lengths Used in the Calculator
| Selected period unit | Days used |
|---|---|
| 1 day | 1 day |
| 1 week | 7 days |
| 1 month | 30 days |
| 1 year | 365 days |
Days Of Supply Example
Example 1: Calculate days of supply.
You have 1,200 units on hand and use 80 units per day.
DOS = 1200 / 80 = 15
The days of supply is 15 days.
Example 2: Calculate days in inventory.
Your average inventory at cost is $50,000, your COGS is $300,000, and the period is 365 days.
DII = (50000 / 300000) * 365 = 60.83
The days in inventory is 60.83 days.
Days Of Supply Calculator FAQ
What is a good days of supply?
A good days of supply depends on your product, lead time, demand variability, and storage cost. A fast-moving item with reliable suppliers may need fewer days of supply. A critical item with long lead times may need more. Compare the result to your reorder policy, safety stock target, and expected demand rather than using one fixed number for every item.
What is the difference between days of supply and days in inventory?
Days of supply usually uses physical units: inventory on hand divided by daily usage. Days in inventory usually uses financial values: average inventory divided by COGS, multiplied by the period length. Both measure inventory coverage, but they are used in different contexts. Use days of supply for operational stock planning and days in inventory for financial or accounting analysis.
Why does the calculator require daily usage to be greater than zero?
Daily usage must be greater than zero because days of supply divides inventory by usage. If usage is zero, inventory is not being consumed, so the formula cannot produce a meaningful finite number of days. If demand is temporarily zero, use an expected average daily usage rate instead.