What it means
The measure can be run in units or in money. Operations teams usually work in units for a specific product, while finance teams work in dollars across the whole business using cost of goods sold as the consumption figure, and both versions answer the same underlying question.
It matters because inventory is cash in a different shape. Every extra day of supply is another day's worth of cash that cannot be used to pay suppliers, fund marketing or reduce borrowing, so the metric is one of the most direct links between operations and the cash flow statement.
In practice you compare days of supply against supplier lead time. If a component takes 30 days to arrive and you hold 12 days of supply, you have a structural problem that no amount of expediting will fix; if you hold 120 days for the same component, you are carrying four months of unnecessary risk and cost.
The figure is closely related to days inventory outstanding and to inventory turnover, and the three are simply different views of the same relationship. Turnover of 8 times a year is the same statement as about 45 days of supply, since 365 divided by 8 is roughly 46.
In practice
Real-world examples.
Example
A car parts distributor sets a target of 40 days of supply across the range but finds one supplier's lines sitting at 130 days. Reviewing the orders shows the buyer had been ordering full pallets on a product that sells three units a week.
Example
A brewery holds 18 days of supply on packaging materials while its can supplier quotes a 25-day lead time. The production manager raises the target to 40 days after a delayed delivery forces a shutdown that costs a full day of output.
Example
A hospital pharmacy reports days of supply weekly for critical medicines. When one line drops below seven days, the system flags it automatically and a purchaser contacts the alternative supplier before the shortage becomes clinical.
Think of it
“Days of supply shows how long your inventory will last-your stock runway.
Formula
Calculation
Inventory days of supply = inventory on hand / average daily usage
where average daily usage = annual cost of goods sold / 365
A speciality food wholesaler has inventory valued at cost of $900,000. Its annual cost of goods sold is $7,300,000.
Step 1: average daily usage = $7,300,000 / 365 = $20,000 per day
Step 2: days of supply = $900,000 / $20,000 = 45 days
So the business is holding about six and a half weeks of stock. If management wanted to cut this to 35 days, the target inventory would be 35 x $20,000 = $700,000, releasing $900,000 - $700,000 = $200,000 of cash. That $200,000 is the prize the operations team is being asked to chase, and stating it in cash terms usually gets more attention than stating it in days.Case study
Seen in the real world.
Ashcombe Instruments is an invented maker of laboratory equipment, used here as an illustrative case only. It had grown quickly and financed the growth with an overdraft that was permanently near its limit, which made the bank uncomfortable.
The finance director calculated days of supply by product family rather than as one company average. Finished goods sat at a reasonable 38 days, but raw components averaged 96 days, driven by a purchasing policy of ordering a year's worth of anything with a long lead time.
Ashcombe renegotiated call-off arrangements with its three largest component suppliers, agreeing to commit to annual volumes while taking delivery quarterly. Component days of supply fell to 45 and roughly $1.1 million of cash came out of stock over a year, most of which went straight against the overdraft. This fictional example illustrates how a single averaged number can hide the part of the business where the cash is actually trapped.
Watch out
Common mistakes.
- Calculating average daily usage from selling price rather than cost, which mismatches the numerator and denominator and understates the days figure.
- Using a single company-wide average that hides both the overstocked lines and the ones about to run out.
- Basing usage on last year's average when demand is strongly seasonal, so the figure looks healthy in one month and dangerously wrong in another.
Questions
People also ask.
What is a good number of days of supply?
It depends entirely on lead time, shelf life and demand volatility, so the useful comparison is against your own supplier lead times and your industry peers rather than a universal target.
How does this differ from inventory turnover?
They are the same relationship expressed differently: divide 365 by turnover to get approximate days of supply, and divide 365 by days of supply to get turnover.
Should I use 365 days or working days?
Either works provided you are consistent, though using working days gives a more realistic picture for businesses that only consume stock on production days.
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