Back-in-Stock by Size: Turn Waitlists Into Restock Data
Every sold-out variant in your store is a sale you didn't make. For apparel and accessories, sold-out sizes are worse — because the shopper liked the product, was ready to buy, and bounced for one specific reason you can fix. This post is the operator playbook for capturing that demand and turning it into a smarter restock decision. Part of our Store Operations pillar.
Skip ahead and ship it → Install ShopMySize on the Shopify App Store — captures back-in-stock demand by size + builds your restock-priority dashboard automatically.
The customer pain point
Every Shopify apparel merchant has the same blind spot. You can pull inventory reports. You can see what's selling. You can see what's sold out. What you can't see — natively, without instrumentation — is demand for the sold-out sizes.
A shopper hits a product page, sees their size is out, and leaves. No event fires. No record exists. They might come back; usually they don't. Multiply by hundreds of shoppers a week and you're flying blind on:
- Which sizes to restock first
- Which products are bleeding the most lost demand
- Who's waiting on what and would buy immediately if you emailed them
Restocking on guess-feel is fine when you're small. By the time you're doing $50K/month it's the single biggest margin leak in apparel. There's a real fix.
Table of Contents
- What "Restock Priority" Actually Means
- Why the Standard "Notify Me" Setup Falls Short
- Capturing Demand by Size: The Operator Workflow
- Turning the Email List into a Restock Email
- Connecting It to Klaviyo
- The Restock-Priority Dashboard
Key Takeaways
| Decision | Operator default | What you actually want |
|---|---|---|
| Who decides what to restock first? | The buying team's gut + last cycle's velocity | Real waitlist demand by size, ranked |
| Who knows the size was wanted? | Sometimes a manual support ticket | Every sold-out-by-size click recorded |
| What happens after restock? | A general newsletter blast | A targeted email to exactly the people waiting on that product in that size |
| What's the conversion rate on that targeted email? | n/a | Materially higher than any general send — these are high-intent waitlist subscribers |
The non-negotiable: the demand signal has to be captured at the moment of intent, on the product page, in the size, by the shopper. Anything else is reconstruction. Reconstruction is always partial.
What "Restock Priority" Actually Means
Most merchants treat restock as a buying-team decision. It's not — it's a demand-driven decision wrapped in a buying-team decision. The buying team picks what to bring back; demand data answers what first and in what depth.
A real restock-priority report says:
Product A, Size M: 47 shoppers requested
Product A, Size L: 31 requested
Product B, Size 8: 38 requested
Product C, Size Plus: 22 requested
That ranks your reorder POs. It tells you which depth ratio to ask the supplier for. It tells you which products to spotlight when the stock lands.
Most Shopify stores have none of this data. The shoppers who would have given it to you bounced silently.
Why the Standard "Notify Me" Setup Falls Short
The default Shopify experience for out-of-stock is poor. A few merchants install a "back in stock" app and assume they're covered. They aren't, usually, because:
- The button is hidden behind the variant dropdown. Shopper has to know to select their size to see "Sold Out — Notify Me." Most don't.
- The button is generic — captures email + product, but not the specific variant/size. So when stock lands you can't email "we restocked your size."
- The data sits in the app's own tool, not in your operator workflow. You'd have to log in, export, filter, and reassemble to make a buying decision.
- There's no aggregate view. You can see one product's waitlist. You can't see "what are the top 10 most-requested sold-out sizes across my whole store" — the actual buying-team question.
ShopMySize was built around the actual operator question instead of the surface-level shopper-facing one.
Capturing Demand by Size: The Operator Workflow
The shape of the workflow that works:
- Shopper arrives at a product page in their pre-selected size (ShopMySize asks them up front, once, so every subsequent page knows which size to surface).
- If the variant for that size is sold out, a clean "Notify me when this is back" badge appears in the size's spot — no dropdown spelunking required.
- One email field, one tap. The email gets captured against both the product and the specific size variant. Not just "Mark wants something from Brand X" — specifically "Mark wants Product A in Size 10."
- Both signals get stored. The email list (with size + product tagging) goes into the operator dashboard and the analytics layer that aggregates "most-wanted sold-out sizes" across the entire store.
You end up with two artifacts every merchant wants but few have:
- A targeted email list per product per size you can fire at the second restock lands.
- A store-wide restock-priority dashboard that ranks which products in which sizes are bleeding the most demand right now.
Turning the Email List into a Restock Email
The email itself matters. Don't blast the entire newsletter list with "we restocked." That gets a 1-2% click rate at best and burns engagement.
Send the specific people waiting on the specific product in the specific size. The subject line writes itself:
Your size is back: Product A in Size 10
CTR on that is 25-40% in our experience. Conversion rate is 8-15%, because every recipient signaled intent on this exact item. Compare to your general newsletter and the gap pays for the app many times over within a single restock cycle.
ShopMySize stores the captured emails tagged with product + size, exportable to CSV, and filterable by product so you can pull "everyone waiting on Product A" in three clicks.
Connecting It to Klaviyo
Most apparel brands run Klaviyo. The back-in-stock waitlist should live there too so the rest of your flows (welcome, browse abandonment, post-purchase) can speak to those people consistently.
ShopMySize syncs captured emails into Klaviyo as a tagged subscriber so:
- Your welcome flow can mention "by the way, we noticed you wanted X — we'll let you know"
- A custom back-in-stock flow fires automatically when inventory returns for the product/size combo
- Your segmentation can treat "people waiting on a restock" as its own audience for offers
If you run Branva Email as your AI-run email layer, this integration is configured for you out of the box.
The Restock-Priority Dashboard
The analytics layer is the part the buying team cares about. ShopMySize surfaces:
- Most-wanted sold-out sizes, ranked by waitlist demand, across all products
- Engagement rate — % of shoppers who picked a size on the popup (so you can tell whether the personalization is being used)
- Auto add-to-cart events — when a shopper's known size meant a one-tap add, this is the friction reduction working
- Collections filtered — how often shoppers are filtering by size, validating the personalization premise
- Emails captured — back-in-stock list growth over time
- Size distribution — what your actual buyer-base looks like by size, which informs every buy cycle for the next year
That's the data you wish you had every time the buying meeting comes around.
Ship size-aware back-in-stock on your store
Configure the popup, define your size mapping, and start capturing back-in-stock-by-size demand in about 15 minutes. The restock-priority dashboard fills in as shoppers hit sold-out variants. Free to install.
Install ShopMySize on the Shopify App Store →
Frequently Asked Questions
How long until I have useful restock-priority data?
Two to four weeks for a brand doing 5K+ monthly sessions. Faster if you're spending on paid acquisition. The data fills in as shoppers hit sold-out sizes; the more traffic, the faster the picture.
Does this work for footwear?
Yes. Footwear sizing (US 7, EU 38, UK 5) is the same shape of problem as apparel sizing (M, 10, L). The size-equivalence layer (covered in the Shopify size-filter post) handles both.
Will this conflict with my existing back-in-stock app?
Possible. Run one or the other, not both. ShopMySize's back-in-stock module is more operator-aware than most standalone back-in-stock apps because it ties the request to the same shopper's known size.
What if I sell across multiple regions with different size systems?
The size-equivalence config handles cross-region sizing (US 8 = UK 12 = EU 38 = M). Set it once in the admin; the popup and product pages respect the shopper's locale.
Does it slow my store down?
It uses Shopify's native filtering for collection pages (no client-side rerendering at scale), so collection pages stay fast and paginate correctly. The popup is a single async asset.
Related reading
- Why Your Shopify Size Filter Doesn't Work (Letter vs Numeric Sizing) — the technical pain post.
- Cut Apparel Bounce Rate with Shopify Size Personalization — the conversion-rate angle.
- Branva Email — the AI-run email layer to pair with the restock flow.
- Shopify Store Operations — The Branva Playbook — the broader on-site systems framework.
- The AI Workflows pillar — every workflow we run.