The formula
Inventory turnover measures how many times you sell and replace your entire inventory over a period. It is calculated as:
Turnover = Cost of goods sold ÷ Average inventory
Both figures are stated at cost, not at selling price. That is the most common calculation error: putting revenue in the numerator and cost in the denominator inflates the result and makes a slow operation look healthy.
Average inventory is the opening plus closing inventory for the period, divided by two. If your business is seasonal, that two-point average lies quite a bit — averaging monthly closes is better.
From times to days: the version people actually understand
"We turn 6 times a year" tells nobody anything. Converted to days, it does:
Days of inventory = 365 ÷ Turnover
A turnover of 6 is about 61 days of inventory: on average, each product sits in the warehouse for two months before selling. That is the number you can discuss with a team, because it translates directly into cash tied up and space occupied.
And that is where it meets the operation: days of inventory are, literally, what you are paying your warehouse for. If storage is billed by occupied position, lowering days of inventory lowers the invoice without negotiating the rate.
Why there is no "good number"
It is the question that always follows: so what should it be? The honest answer is that no universal figure exists, and you should be skeptical of anyone who hands you one. Healthy turnover depends on margin, shelf life, replenishment lead time, and the cost of running out:
- A high-margin, slow-to-replenish product — imported, three months in transit — tolerates low turnover: stocking out costs more than storing it.
- A low-margin, fast-to-replenish product with low turnover is dead money: every day on the rack eats the margin.
- A product with an expiry date has a non-negotiable floor: if turnover is slower than shelf life, that is not a financial problem, it is guaranteed write-off. There, what decides is the picking method — worth reviewing FIFO vs FEFO.
That is why the useful comparison is not against an industry average: it is against yourself, per SKU, over time. Catalog-wide turnover is a nearly useless number — it averages your best sellers with the items that have not moved in a year and produces something that looks reasonable.
The three mistakes that distort the calculation
1. Mixing cost and selling price. Mentioned above, and still the most frequent. Both terms go at cost.
2. Calculating it only at catalog level. The average hides exactly what you need to see. A catalog turning 6 times might be 20% of SKUs turning 20 times and 30% that never turn at all. The decision — liquidate, stop replenishing, renegotiate supplier minimums — lives in that 30%, and the average buries it.
3. Measuring across a seasonal period. Calculating turnover from a December closing inventory, right after peak season emptied the shelves, produces a beautiful number that never repeats.
What to do with the result
Turnover is not a metric to report; it is a metric to act on. Three concrete moves:
- Sort your SKUs by turnover and look at the tail. Anything that has not moved in months is paying rent. Deciding to liquidate hurts less than continuing to store it.
- Cross turnover with margin. Low turnover plus high margin can be fine. Low turnover plus low margin almost never is.
- Review your purchase minimums. Low turnover often is not a sales problem: it is a supplier forcing you to buy six months of inventory to get the price. That discount carries a storage cost that rarely gets calculated.
Frequently asked questions
What is the inventory turnover formula? Cost of goods sold divided by average inventory for the period, both valued at cost.
How do I convert turnover into days? Divide 365 by the turnover. A turnover of 4 equals roughly 91 days of inventory.
Should I calculate it monthly or annually? Monthly if your business is strongly seasonal, because the annual average hides it. What matters most is using the same basis consistently so the numbers stay comparable.
Is higher turnover always better? No. Very high turnover can mean you are running out of stock and losing sales that never show up in your numbers, because a customer who found nothing available leaves no trace.
Turnover is the metric that translates purchasing decisions into warehouse cost. If you want to know what your non-moving inventory is costing you today — and what would change if you only paid for the space you actually use — take a look at our warehousing service and how fulfillment connects to it.