Size-out personalization: what to show when their size is gone
When a shopper's size sells out, most stores show a dead end: a grayed-out button and a suggestion to pick another size. That moment is actually high-intent traffic telling you exactly what it wants. Size-out personalization means treating the out-of-stock size as a signal, not a failure: notifying the shopper on restock, offering the closest fitting alternative, and adjusting what you show them next so the visit still converts.
The short answer
Treat the sold-out size as the strongest signal on the page. The shopper has selected a product, chosen a size, and hit a wall; that is more intent than most of your traffic will ever show. Capture it with a restock alert signup, show alternatives ranked by fit similarity rather than generic bestsellers, and follow up when the size returns.
The stores that do this well convert size-out visits at rates that surprise them, because the follow-up lands when desire is highest. The ones that show a grayed-out button get nothing but a bounce.
Why the size selector is a data goldmine
Every interaction with a size selector is a fit data point, including the failures. A shopper who clicks a sold-out medium has told you their size in this product, which is more precise than any quiz answer. Aggregate those clicks and you get a demand map: which products are understocked in which sizes, where your buy was wrong, and which sizes deserve deeper inventory next season.
Most personalization systems ignore this data completely. They track product views and purchases but not size-level intent, so the most specific signal a shopper can give disappears into a bounce. Start logging size selector interactions, including sold-out clicks, as first-class events in your personalization data. It costs nothing and it is immediately useful.
Restock alerts that actually convert
The mechanics are simple: one click on the sold-out size, email or SMS captured, alert sent the moment the size is back. The details decide the conversion rate. Send the alert fast, within minutes of the restock hitting the site, because the shopper's intent decays quickly and your competitors are one search away.
Personalize the alert beyond the product. Include the shopper's size in the subject line so it reads as meant for them, and link directly to the size-selected product page rather than the generic listing. If you have fit data, mention it: the fit they bought last time is back in stock. Alerts written this way routinely outperform generic back-in-stock blasts by multiples.
Alternatives ranked by fit, not popularity
When the size is gone and no restock is coming, show alternatives, but rank them by fit similarity to the sold-out product, not by what sells best overall. Same cut, same fabric behavior, same size range: that is what this shopper wants. A generic carousel of bestsellers feels like the store did not listen.
Use what you know about the shopper's fit. Quiz answers, past purchases, and return reasons all sharpen the ranking. A shopper who returns for being too tight in the shoulders should see alternatives known to run roomy there. This is where fit data earns its keep: not in a recommender widget, but in the rescue moment when the first choice fails.
Closing the loop after the restock
The follow-up sequence after a restock alert is where most stores leave money behind. One alert is not a campaign: if the shopper does not buy within 48 hours, follow with the alternatives you would have shown, then with new arrivals in their size. Each touch should reference the original product so the thread stays coherent.
Measure size-out as its own funnel: sold-out size clicks, alert signups, alert sends, and purchases attributed to the sequence. Most stores are shocked by the volume in the first stage, because they never counted how many shoppers hit the wall. Once you see the number, the business case for size-out personalization writes itself.