Inventory turnover is the ratio every operator has heard of and almost nobody acts on, and the reason is not laziness. Calculated the usual way — one number, whole catalogue, once a quarter — it is genuinely hard to do anything with. Calculated per line, it points straight at the stock you should stop reordering.
This page covers the formula and which figures belong in it, why there is no universal good ratio to aim at, and what a catalogue-wide average conceals about the tail of your range.
What is inventory turnover?
Inventory turnover is the rate at which stock cycles: how many times you sold and replaced your average holding over a period. It equals cost of goods sold divided by average inventory at cost. Six turns means you cycled the average holding six times, and what it does show is speed, not profit.
The useful way to read it is as speed rather than as a score. Stock is cash in a temporary shape, and turnover measures how quickly that cash comes back to you. Two businesses with identical revenue and identical margin can have very different cash positions purely because one of them cycles its holding twice as fast, and that difference shows up nowhere in a profit figure.
What is the inventory turnover formula?
Turnover equals cost of goods sold divided by average inventory at cost. Average inventory means opening plus closing divided by two, not a single snapshot. Both figures must be at cost: using sales in the numerator inflates the ratio by your markup and breaks comparability with your own history.
| Input | Value | Where it comes from |
|---|---|---|
| Cost of goods sold | 1,200,000 | Twelve months, at cost, not at sales value |
| Opening inventory | 180,000 | Stock at cost on day one |
| Closing inventory | 220,000 | Stock at cost on the last day |
| Average inventory | 200,000 | Opening plus closing, divided by two |
| Inventory turnover | 6.0 | 1,200,000 divided by 200,000 — about 61 days of stock |
Two figures decide whether the answer means anything. The first is the averaging: a single snapshot taken at your quietest moment of the year will flatter the ratio, and one taken at your busiest will punish it, so opening plus closing halved is the minimum honest treatment. The second is that both sides must be at cost. Dividing sales by inventory at cost is a common shortcut and it silently bakes your gross margin into the ratio, which means the number moves when you reprice even though nothing about your stock behaviour changed.
What counts as a good inventory turnover ratio?
There is no universal figure, and any single benchmark you are offered is describing somebody else’s category, margin and shelf life. The honest comparisons are your own prior periods and your own lines against each other. A ratio is a trend, not a grade.
The reason a cross-industry benchmark cannot help is that turnover trades against two things you have already chosen. Higher turnover means holding less, which means less cash tied up and less risk of stock ageing out — and also a thinner buffer against a demand spike or a late delivery. So the ratio you want is a consequence of your service level and your shelf life, not an independent target to chase. Chasing the number directly is how a business talks itself into the stockouts that cost more than the holding did.
For anything with a date on it there is a firmer ceiling. Turnover has to be fast enough that stock clears well inside its sellable life, and that constraint binds long before any industry average would. If a line turns four times a year and its goods keep for sixty days, the arithmetic has already failed and no benchmark is needed to see it.
Why does one turnover number hide your slow movers?
Because it is an average weighted by volume, and your fast lines dominate it. A catalogue turning six times can hold a long tail turning once, and the ratio will not move enough to notice. The stock that is actually costing you is invisible at the level most people measure.
The mechanism is worth being concrete about. Cost of goods sold is dominated by the lines that sell, so those lines set the numerator. The slow tail contributes almost nothing to the top of the fraction while sitting in the bottom of it as inventory. A range where a handful of products do most of the volume — which is most ranges — produces a healthy-looking headline number regardless of how badly the tail behaves. Improving the headline ratio and clearing the tail are close to unrelated activities.
The fix is to stop computing turnover for the catalogue and compute it per line, then sort ascending and read the bottom. That ordering is the actionable artefact: it names the reorders to stop, the lines to mark down, and the ones already past saving. It is the same ranking that separates a slow line from stock that is genuinely dead, and it pairs with days on hand, which asks the same question in units of time instead of turns.
What should you do next?
One catalogue pass, then read the bottom of the list.
Fix the inputs
- Use cost of goods sold, never sales, in the numerator.
- Average opening and closing inventory rather than taking a snapshot.
- Hold both sides at cost over the same period.
Break it down
- Recompute per SKU instead of per catalogue.
- Sort ascending and read the slowest lines first.
- Compare each line against its own sellable window.
Act on the tail
- Stop reordering the lines that turn slowest.
- Mark down while the goods still have a window left.
- Track the trend monthly, not the absolute figure.
Frequently asked questions
Should I use cost of goods sold or sales in the turnover formula?
Cost of goods sold. Average inventory is held at cost, so using sales on top puts margin into the numerator and inflates the ratio by whatever your markup happens to be. That version of the number is not comparable to your own history if your pricing moved.
Is a higher inventory turnover always better?
No. Turnover rises when you hold less, and holding less is exactly what causes stockouts. A ratio climbing while your stockout rate climbs with it is not an efficiency gain, it is the same stock shortage measured two ways.
How often should I calculate it?
Monthly is enough for a trend, and the trend is the part that carries information. A single figure has no meaning without either your own prior periods or a per-SKU breakdown to compare against.