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What does the Inventory days of supply calculate?
How many selling days will current inventory cover? This calculator uses sellable units on hand, average units sold per day, and units reserved or unavailable to estimate selling days covered by stock immediately in your browser.
With the values currently entered, the result is 22.4 days — inventory supply. It also shows available units, and weeks of supply.
How to use the Inventory days of supply
- Replace the example values with your own numbers.
- Review the result and supporting figures as they update automatically.
- Check the formula and assumptions before using the estimate for a decision.
Inputs used
- Sellable units on hand — entered in units
- Average units sold per day — entered in units/day
- Units reserved or unavailable — entered in units
Inventory days of supply formula
(Inventory on hand − unavailable units) ÷ average daily sales
Assumptions
- Daily demand stays constant.
- Inbound purchase orders are excluded.
Practical guide
Inventory days of supply example and edge cases
How many selling days will current inventory cover? Let's use a concrete example, then look at the assumptions that can move the answer.
Example: A practical inventory days of supply scenario
For this example, use sellable units on hand of 1,800 units, average units sold per day of 75 units/day, and units reserved or unavailable of 120 units. These are starting values, so replace them with numbers that match your situation.
- Sellable units on hand
- 1,800 units
- Average units sold per day
- 75 units/day
- Units reserved or unavailable
- 120 units
Calculated result22.4 daysinventory supply
Start with inventory supply. Then check available units, and weeks of supply to understand what sits behind the main result.
Example results use the default display profile. The calculator above follows your selected country and units.
How to read the result
- Read the main result first. The supporting figures for available units, and weeks of supply explain how the estimate is built.
- The method is (Inventory on hand − unavailable units) ÷ average daily sales. Keep the units consistent and use values from the same time period.
Edge cases worth checking
When sellable units on hand is unusual
Daily demand stays constant. Double-check this input before relying on the result.
When units reserved or unavailable is uncertain
Inbound purchase orders are excluded. Run a lower and higher value to see a useful range.
What changes the result most
Sellable units on hand
Use the count you expect in real life. Round up when a partial units cannot be purchased or used.
Average units sold per day
Keep average units sold per day on the same time basis as the other inputs. Monthly and annual values are easy to mix up.
Units reserved or unavailable
Use the count you expect in real life. Round up when a partial units cannot be purchased or used.
Try a different scenario
Small changes show whether the answer is stable or sensitive.
Sellable units on hand: 10% lower
1,620 units20 daysinventory supply
Sellable units on hand: 10% higher
1,980 units24.8 daysinventory supply
Average units sold per day: 10% higher
83 units/day20.2 daysinventory supply
Common mistakes
Check sellable units on hand
Daily demand stays constant. Make sure this matches the number you enter.
Keep units reserved or unavailable consistent
Inbound purchase orders are excluded. Use the same units and time period throughout the calculation.
Do not rely on one inventory days of supply scenario
Run a cautious case and an optimistic case. The range is often more useful than one exact-looking number.
Use this result well
How many selling days will current inventory cover?
Carrier rules, dimensional divisors, customs decisions, and live quotes take priority.