Why anyone bothers with this number

If you sell physical products, your stock is money you have already spent and cannot spend again. It is money that does not show up as a problem in any obvious place. Nobody sends you an invoice for it. It just quietly sits on shelves, and the only symptom is that you feel poorer than your profit and loss statement says you are.

Inventory turnover puts a number on how long that money stays still. Once you have it, two questions that usually get answered by instinct become answerable with arithmetic: how much stock is too much, and which products are quietly eating the cash you need somewhere else.

The formula, with a worked example

Take one product, and one period. Say you sold 2,500 drinking glasses last year, and your average stock across the year was 420 glasses.

The ratio, 5.95, is the answer to “how many times did I sell through my shelf last year”. It is correct and it is nearly useless in conversation. The second number is the one to keep: on average, a glass sits in your warehouse for about two months before somebody buys it. That is a sentence you can take to a meeting.

The comparison that turns it into a decision

Sixty one days is neither good nor bad. It becomes one only next to a second number: how long your supplier takes to deliver. That is the whole trick, and it is the part most explanations of inventory turnover leave out.

If those glasses arrive 15 days after you order them, and they are reliably there in 15 days, then you are holding roughly four times the stock you need. You could halve it and still never run out. If the same glasses come from the other side of the world with a lead time of 70 days, 61 days of stock is not comfortable at all, it is thin.

Four ways the number comes out wrong

• Average stock that is not an average. Taking the closing balance, or the level on the day you happened to look, gives you a number shaped by that one day. Use several readings across the period, monthly is usually enough.
• Mixing quantities and money. Either divide units sold by average units, or cost of goods sold by average stock value. Units sold divided by stock value is not a ratio of anything.
• One number for the whole warehouse. A single company-wide figure averages your fastest product together with the pallet nobody has touched since 2022, and hides both. The per product figure is where the decisions are.
• Ignoring the season. A full year of sales divided by average stock says nothing about whether you are correctly stocked in November. If your demand has a shape, calculate per season and read it in that context.

What to do once you have the number

Sort your products by days of inventory and read the list from both ends. The top and the bottom are two different problems with two different answers.

• The slowest lines are where your cash is. Before discounting, check whether the problem is the product or the quantity you buy at a time. Often the item sells perfectly well and you are simply ordering a year of it.
• The fastest lines are where you lose sales you never see. If days of stock is below your lead time, you are running out between deliveries, and a stockout leaves no trace in your sales figures. It looks like demand that was never there.
• The lines with no movement at all are not a stock problem, they are a decision you have been postponing. Turnover just makes it visible enough to act on.

Getting the two inputs without a spreadsheet evening

The arithmetic is trivial. The work is getting quantity sold per product per period, and stock levels over time, out of your own records and lined up next to each other. That is exactly what a system that keeps sales and stock in the same place is for: the movements are already recorded, so the two inputs are a question you ask rather than a file you rebuild every quarter. If your stock lives in one place and your sales in another, that reconciliation is the actual cost of not knowing this number.