The short version: the back-in-stock flow notifies people who explicitly asked to know when a product returns, which makes it the purest intent signal in the account and the numbers show it: Klaviyo's data puts back-in-stock email conversion at 7.9%, with SMS at 8.5%, and in fashion the engagement runs 55 to 72% opens with 10 to 18% clicks, the strongest bands of any flow in the category. Most brands treat it as a checkbox in an app setting. Built properly, it is a demand-capture system: the waitlist is presold inventory, and the flow is just the delivery mechanism.
Why this flow overperforms everything else
Every other flow interrupts someone; this one keeps a promise. The subscriber saw the product, wanted it, found it gone, and raised their hand, which means the flow starts where other flows end: past awareness, past consideration, at blocked purchase intent. The only jobs left are speed (getting there before they bought elsewhere), fairness (not notifying 4,000 people about 40 units), and coverage (making sure the opt-in exists everywhere stock runs out). Everything below serves one of those three.
What the stockout is quietly costing: the switching data
Speed matters because a stockout is a switching event, and the research quantifies how fast the intent leaks: 74% of consumers hit an out-of-stock product they regularly buy in the past year, and when it happens, 45% buy from a different retailer and 32% switch to a competing brand at least temporarily. 62% have switched brands at least once because of a stockout, 82% would try a competitor if their go-to were frequently out of stock, and a quarter say stockouts damage their trust in the brand outright. Every day between sell-out and restock notification is a day those numbers work against you; the waitlist plus a fast, fair notification is the mechanism that turns a switching event back into a sale.
The opt-in: coverage is the whole ballgame
The flow can only be as big as the waitlist, and the waitlist is built at the moment of disappointment:
- Every sold-out variant shows the notify-me control, replacing the add-to-cart button, not hiding below the fold. A sold-out size on an otherwise in-stock product counts: variant-level opt-in is where apparel's volume lives, per the size-out handoff in the cart flow.
- The flows feed it: the cart flow converts sold-out carts to alerts, browse abandonment offers it when the viewed item is gone, and the welcome flow plants it as a feature of being on the list.
- Anonymous visitors get captured too: the notify-me form is an email capture that outperforms the generic popup because it arrives attached to a specific want. For drop-culture brands, this is quietly the best list-growth engine on the site.
- Offer the SMS lane at opt-in: restocks are time-sensitive and phones win time-sensitive: Klaviyo's client data shows SMS click rates of 5.3% against 1.9% for email on the same restock moment, so the dual-channel build captures the same intent on the faster channel.
The build: the notification and its two follow-ups
Email (and SMS) 1: the moment inventory updates. It's back
Subject says the product name and that it is back; body is the product, the size or variant they wanted, one button to a cart-ready link. Nothing else: no cross-sell, no brand story, no discount (they already wanted it at full price; a restock discount is pure margin donation). Urgency here is honest by definition: it sold out once, and saying so is fact, not theater.
Email 2: 24 hours later, only if still in stock and unpurchased
The still-available nudge, with social proof (the reviews that accumulated, how fast it went last time) and, for apparel, the fit notes that de-risk per the fit-first recovery pattern. If stock is nearly gone again, say so truthfully.
Email 3: only on repeat sell-out. The waitlist consolation
If it sold out again before they acted: acknowledge it plainly, keep them on the list automatically, and offer the nearest alternative honestly. This email converts frustration into either an adjacent sale or deepened intent for the next drop; silence converts it into a competitor's customer, and the 82%-would-try-a-competitor statistic says how quickly.
Batching: the fairness mechanics
The classic self-injury is notifying the full waitlist about a sliver of stock: the email arrives, the product is already gone, and the flow that should build trust burns it. Klaviyo's own settings exist for exactly this: set a minimum inventory threshold before notifications fire at all, and batch the sends (notify in waves sized to stock, first-come by opt-in date) so early hand-raisers get first shot and no wave outnumbers the units behind it. For chronically undersupplied products, tell the list how the queue works; fairness explained is loyalty built.
Merchandising the waitlist beyond the flow
- The waitlist is a demand forecast: opt-in counts by variant are purchase-intent data your buying team should see. Restock quantities informed by waitlist depth turn the flow from recovery into planning, and they are the structural fix for the frequently-out-of-stock pattern that the switching data punishes hardest.
- Pre-restock warm-up for big returns: for a hero product coming back, a day-before tease to the waitlist (it returns tomorrow at 10am) converts the notification moment into a drop event.
- Suppression discipline: purchasers of the item exit instantly; the alert never sends for trivially restocked perpetual SKUs (reserve the flow for genuine scarcity, or it becomes noise).
What to measure
- Conversion against the 7.9% email and 8.5% SMS marks, and engagement against the 55 to 72% open band; a healthy back-in-stock flow should be the account's best performer per recipient, full stop
- Opt-in coverage: the share of sold-out variant views that see a notify-me control, the metric that caps everything else
- Sell-through speed of notified restocks, and the notified-but-missed rate (wanting stock that sold before their wave), which tunes batch sizes
- Waitlist-to-restock lead time: how long hand-raisers wait, because intent decays even here, and chronic 60-day waits are an inventory problem wearing an email costume
Frequently asked questions
What does a back-in-stock flow convert at?
Klaviyo's published data shows 7.9% conversion for back-in-stock email and 8.5% for SMS, with fashion open rates running 55 to 72%: the strongest per-recipient performance of any standard flow.
Why do stockouts matter beyond the lost sale?
Because they trigger switching: 45% of shoppers buy from a different retailer when they hit one, 62% have switched brands over one at least once, and 82% would try a competitor if a go-to product were frequently unavailable. The waitlist-plus-notification system exists to intercept that leak.
Should back-in-stock notifications include a discount?
No. The subscriber asked to buy at full price; the restock itself is the offer. Discounting it donates margin and teaches the list that waiting produces coupons.
How do you avoid notifying more people than there is stock?
Set a minimum inventory threshold before the flow fires and batch notifications in waves sized to stock, first-come by opt-in date. Fairness mechanics are what keep the flow trust-building instead of trust-burning.
Email or SMS for restock alerts?
Both, with SMS leading for speed: the same restock moment shows 5.3% SMS clicks against 1.9% email in Klaviyo's client data. Offer the SMS lane at opt-in.
Sources
- Klaviyo. Back-in-stock notification data and batching guidance.
- Doss. Stockout Stigma Index consumer research.
- Landing Partners. Klaviyo email benchmarks for fashion brands.
- Klaviyo. Email marketing benchmarks.