Why does a full warehouse matter?
For manufacturers and retailers, inventory is often one of the largest assets on the balance sheet. It is not cash until it sells, it costs money to store, and it can lose value as fashions change or technology moves on. How quickly inventory sells is therefore a useful window into a business.
This guide shows how to calculate inventory turnover and days of inventory, what it means when turnover slows or becomes unusually fast, and how to look beneath the total at inventory mix and write-downs, using hypothetical numbers.
Turnover and days of inventory
Inventory turnover is usually calculated as cost of goods sold divided by average inventory. Because inventory is carried at cost, using cost of sales rather than revenue keeps the comparison consistent. Average inventory is often the mean of the opening and closing balances.
Dividing 365 by turnover gives days of inventory. A turnover of 6 means inventory sits for about 61 days on average. Days are often more intuitive and fit neatly into working-capital analysis.
| Item | Year 1 | Year 2 | Change |
|---|---|---|---|
| Cost of goods sold | 1,200 | 1,260 | +5% |
| Average inventory | 200 | 280 | +40% |
| Inventory turnover | 6.0× | 4.5× | -1.5× |
| Days of inventory | ≈ 61 | ≈ 81 | +20 days |
Cost of sales rose 5% while inventory rose 40%, so goods now sit in the warehouse about 20 days longer.
When turnover slows
Falling turnover can mean sales are weaker than planned or that the company over-ordered against optimistic forecasts. When inventory grows far faster than sales, as in the example, discounting or production cuts often follow in the next quarters.
Not every increase is bad. A company may be building stock ahead of a product launch or holding extra materials to guard against supply disruptions. Check how management explains the increase and whether later quarters' sales bear that explanation out.
The trap of very fast turnover
Higher turnover looks better, but very fast turnover can also mean the company is running short. Stock-outs and late deliveries cost sales and can send customers to competitors.
Turnover also uses average inventory, so a company that trims stock just before year-end can make the ratio look better. Comparing quarter-end balances through the year shows whether inventory dips only at the reporting date.
Look at the inventory mix
Inventory is usually split into raw materials, work in progress, and finished goods. The same total can tell different stories depending on which part grew. Rising finished goods suggest weak sales; rising raw materials may reflect buying ahead of price increases or a change in production plans.
IAS 2 requires inventories to be measured at the lower of cost and net realizable value; U.S. GAAP applies a similar lower-of-cost test for most inventories. When expected selling prices fall below cost, the difference is written down. A growing inventory allowance means the company itself has judged that some stock is worth less.
Industry and seasonality
Companies selling fast-moving goods such as food or household products turn inventory quickly, while makers of aircraft or heavy equipment with long production cycles turn it slowly. Comparing turnover across very different industries is not very informative.
Seasonality matters too. Many retailers build stock at the end of the third quarter ahead of the holiday season. Comparing a quarter with the same quarter a year earlier is usually more accurate than comparing it with the previous quarter.
Checking inventory in a 10-K
Investor.gov's guide to the 10-K points to the financial statements and the MD&A. A practical order for inventory:
- Use cost of goods sold and opening and closing inventory to calculate turnover and days for three to five years.
- In the inventory note, check the split between raw materials, work in progress, and finished goods.
- Look for changes in inventory reserves or write-downs in the same note or in the MD&A.
- Compare quarter-end inventory with the same quarter a year earlier to filter out seasonality and year-end effects.
- Find management's explanation for any build-up or drawdown and check it against subsequent quarters' sales.
What turnover hides
Turnover is an average, so it can hide a mix of fast-selling and stale products. If product-level data is not disclosed, write-downs and changes in mix are the closest clues to that difference.
The company in the table is hypothetical. For real analysis, use the latest filings with their dates and compare turnover with peers in the same industry over the same periods.