Your best seller sold out in week 6. What did it cost you?

Here is a conversation that happens every year, in every planning meeting, in every apparel business I have worked with.

A style did well. It sold 4,000 units and finished the season at a 94% sell-through. Everyone agrees it was a winner. So next year it gets planned at 4,400 — the same style, up ten percent, because the business is growing ten percent.

Nobody in the room mentions that it was out of stock from week six onward.

The number you actually want

The question worth asking is not "what did it sell?" but "what would it have sold if it had been on the floor the whole time?" That number — call it potential — is recoverable from data you already have.

The method is unglamorous. For each style, in each week, you know whether it had sellable inventory. Take the weeks it was genuinely in stock and calculate a rate of sale from those weeks only. Then apply that rate across every week it could have been selling — the weeks between its launch and the end of its intended life on the floor.

The gap between that figure and what actually rang through the register is the demand you never captured. In the example above, a style selling roughly 310 units a week in its in-stock weeks, dark for the back half of a twenty-week window, didn't have 4,000 units of demand. It had closer to 6,200.

Planning it at 4,400 next year isn't a ten percent increase. It's a thirty percent cut, dressed up as growth.

Three gaps, and they chain

Once you can calculate potential, three comparisons fall out of it, and they should be read together rather than separately.

They chain in that order. The week shortfall explains the unit gap; the unit gap drives the revenue gap. Reading them as a sequence tells you something a single variance number never will — whether you had a demand problem, a buying problem or a flow problem.

A style with no stock miss and a large unit gap didn't sell as well as you hoped, and that is a merchandising conversation. A style with a large stock miss and a proportional unit gap sold everything you gave it, and that is a buying conversation. They look identical on a sales report.

Where this bites hardest: size curves

If potential matters anywhere, it matters in sizing — and this is where I see the most expensive version of the mistake.

Size curves are almost always built from historical unit sales by size. But sizes do not sell out evenly. The middle of the curve goes first, and once mediums are gone, the only sizes left to sell are the ones at the ends. Your sales history therefore under-represents exactly the sizes that were most in demand and over-represents the ones that lingered.

Build next season's curve from that history and you will buy proportionally fewer mediums than you needed — and then be surprised, again, when mediums go early. The error compounds every year you repeat it.

Curves built on potential units, computed only from the weeks each size was genuinely available, do not have this problem. It is the single highest-return change most planning processes can make, and it costs nothing but the analysis.

The obvious objection

"If we buy to potential, we'll be over-inventoried."

Sometimes, yes — and potential is an upper bound, not a target. It tells you the ceiling of what demand supported, and the merchant still decides how close to that ceiling is prudent given margin, cash and confidence in the read.

But that is a decision you can now make deliberately. Today most businesses buy well under potential without knowing it, because the number was never in front of them. Choosing to buy 80% of demand is planning. Accidentally buying 65% of it because your history was capped by your own inventory is not.

Where to start

You do not need new software for this. You need weekly inventory positions alongside weekly sales at SKU level, and most ERPs will give you both. From there it is a rate-of-sale calculation, an in-stock flag, and the discipline to use the result instead of the raw history.

Run it on last season before you plan the next one. The styles where potential and actual diverge most are, reliably, the ones you have been under-buying for years.

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