30 days is the example result. Read it using the units and relationship stated below.
Days of Supply uses inventory units, average daily usage.
Balancing availability against cash tied up in stock
For days of supply, the relationship is Result = (([Inventory units]) ÷ ([Average daily usage])). The amount depends on the supplied inventory units, average daily usage, rather than a live market price or a value imported from another record. An inventory decision must connect demand, lead time and the cost of holding goods. Buying more can reduce ordering frequency but ties up cash and storage. A stock ratio is useful only when its demand period is clear.
What the model includes
Result = (([Inventory units]) ÷ ([Average daily usage])) Use purchase and sales records from a comparable period. Separate regular consumption from a promotion or one-off order; otherwise a reorder assumption can carry a temporary spike into the next purchasing cycle. Check supplier pack sizes, minimum quantities and delivery reliability before turning a calculated amount into an order. The model does not measure deterioration, seasonality or the cost of a stockout unless those inputs are present.
Example with the supplied inputs
Inventory units: 1200; Average daily usage: 40. Result: 30 days. The values are illustrative.
Compare one changed input
Inventory units changes from 1200 to 1320. The result becomes 33 days. All other inputs remain fixed.
Calculation rule
Result = (([Inventory units]) ÷ ([Average daily usage]))
Inputs and output
| Input | Example value | Entry convention |
|---|---|---|
| Inventory units | 1200 | Use the source quantity represented by inventory units. Keep the counted population fixed and avoid mixing a unit value with a period total. |
| Average daily usage | 40 | Use the source quantity represented by average daily usage. Keep the counted population fixed and avoid mixing a unit value with a period total. |
Output: 30 days is the example result. Read it using the units and relationship stated below. Unit: days.
How to use the page
Collect the inputs
Gather Inventory units, Average daily usage. Use one period and the units shown in the form.
Run the calculation
Enter the values and select the action. The calculation rule above explains how the inputs produce the result.
Compare a scenario
Change Inventory units on its own, keeping the other inputs fixed. Read both results before changing another assumption.
Worked example
With the example inputs listed above, the result is 30 days.
Change Inventory units from 1200 to 1320 while keeping every other value fixed. The result becomes 33 days. This comparison isolates that input; it does not forecast how other variables will respond.
Check the stated formula, units and limits before using the result in a decision.
Keep the displayed formula, input units and model scope with the result. Corrections or questions can be sent through the request section on this page.
Content updated: October 11, 2026