How to Set Up a Klaviyo Back-in-Stock Flow for UK Ecommerce Brands

A shopper lands on a product page, wants to buy, and finds the item sold out. That's a lost sale — unless you capture the intent. A Klaviyo back-in-stock flow does exactly that: it lets shoppers ask to be alerted when a product returns, then emails or texts them automatically the moment it does. It captures validated demand from people who already tried to buy but couldn't, which makes it one of the highest-intent automations you can run.

The setup is technically straightforward — most brands can build it in under an hour. This guide walks through the full build for UK ecommerce brands, including SMS, the account settings that quietly control performance, and the PECR and UK GDPR rules you need to respect.

A back-in-stock flow is an automated Klaviyo workflow triggered when a shopper subscribes to a restock alert on a sold-out product page. When someone clicks the "Notify me when available" button, Klaviyo logs a Subscribed to Back in Stock event on their profile. That event is the metric used to trigger the flow.

From there, the subscriber waits at a unique Back in Stock Delay component. Unlike a standard time delay, this component holds each person until the specific item they wanted is restocked in your inventory. When stock returns, the message fires automatically — no manual sending, no batch scheduling.

The performance comes down to intent. Everyone in the flow has already demonstrated they want the product — they got as far as the sold-out page and asked to be told when it came back. UK agency Pea Soup Digital reports these flows typically convert at 10–15%, putting them among the highest-performing automations in any ecommerce email programme.

Open rates back this up. Multiple agencies note back-in-stock alerts rank among the highest open rates they track — often above 55% when the message is sent within roughly two hours of a restock. The combination of proven demand and immediate relevance is hard to beat.

Back-in-stock is the closest thing email has to selling to a customer who already had their card out.

Before you build the flow, Klaviyo needs to be integrated with your store so it can read inventory. Back-in-stock is supported natively for Shopify, BigCommerce, Magento 2, PrestaShop, SFCC and Shopware, plus inventory-aware catalogues synced via a custom feed or API.

You also need the "Notify me" button and signup form live on your sold-out product pages. Klaviyo's native form builder is available for Shopify and BigCommerce, and the button appears and disappears automatically based on inventory — so it only shows when an item is out of stock. Enable it, customise it to match your brand, and publish.

Once the integration and button are in place, the build itself is quick. Create a flow from scratch or from Klaviyo's pre-built library, then work through these steps.

• Set the trigger. Choose the Subscribed to Back in Stock metric as the flow trigger. Add no trigger filters and no flow filters at this stage.
• Add the Back in Stock Delay. Drag the Back in Stock Delay component in directly after the trigger. This is what holds each subscriber until their item is restocked.
• Add the notification email. Place the notification message after the delay. For most brands a single email is enough, though Klaviyo recommends keeping the flow to 1–3 messages and some agencies extend to a short sequence over a 3–7 day window.
• Turn Smart Sending OFF. This is critical. Smart Sending suppresses recipients who received a recent message — leave it on and some waiting subscribers won't get their alert. Turn it off so every subscriber is notified.

If you extend to more than one message, keep the cadence tight. The value of this flow is speed, so a second email a day or two later is about the limit before urgency fades.

Two settings control how the flow behaves, and they live outside the flow builder under Settings > Other > Back in stock settings. This tab only appears after at least one Subscribed to Back in Stock event has been recorded, so if you can't see it, trigger a test subscription first.

• Minimum inventory rules. Set a threshold so subscribers are only notified once a meaningful quantity is back — for example, 10 or more units. Without this, two units returning can trigger alerts to dozens of people who then find it sold out again.
• Notification strategy / customer notification rules. Control how many customers are notified and how frequently. This is useful when you restock in small batches and want to avoid over-sending against limited stock.

One limitation to plan around: after the initial message sends, Klaviyo cannot re-check inventory before a second message. It can't verify whether the item has sold out again. That's another reason a single, well-timed email often outperforms a longer sequence.

SMS is a natural fit for this flow because everything hinges on timing. Text messages are typically read within about three minutes — far faster than email — which gives SMS subscribers a genuine head start on limited stock.

Klaviyo's recommended omnichannel approach adds a conditional split after the Back in Stock Delay: SMS-consented subscribers are routed to a text message, while everyone else receives the email. One Klaviyo case study showed the email version converting at 7.9% while the SMS version converted at 8.5% on the same restock moment — additional revenue captured through a stronger channel.

SMS back-in-stock is available for Shopify, BigCommerce, PrestaShop and Magento 2, and recipients must be current SMS subscribers. You can't text someone who only opted in to email.

For UK brands, SMS marketing sits under the Privacy and Electronic Communications Regulations (PECR) alongside UK GDPR. That means the rules are non-negotiable, whatever the channel's performance.

• Explicit opt-in. Subscribers must actively consent to SMS marketing — subscribing to a restock alert by email is not consent to text them.
• Clear opt-out. Include unambiguous opt-out language such as "Reply STOP to unsubscribe" in your messages.
• Respect quiet hours. Avoid sending during unreasonable hours. A restock alert firing at 3am does more harm than good, and quiet-hours handling keeps you on the right side of best practice.

Keep the notification short, personal and urgent. Focus on a single product with one clear call to action. Adding cross-sells or full site navigation dilutes urgency and reduces conversion — the subscriber wants one thing, so give them a direct path to it.

Use dynamic event and product variables to pull in the correct product image, name and buy link automatically. This is what makes the message feel personal at scale, and it's essential for a variant-specific flow where the wrong image would undermine trust immediately.

Use honest scarcity

If only a few units are left, say so — genuine scarcity is a powerful nudge for an audience that already wants the product. Don't exaggerate. Fabricated urgency erodes trust with exactly the high-value customers this flow is built to serve.

Always test the full flow end-to-end before going live. Sign up on a sold-out product, restock it in your platform, and confirm the notification arrives with the correct product data. Check the profile event timeline to verify the event fired against the right variant.

The most common technical pitfall is hardcoding the variant ID in a custom form. Do this and customers can be notified about the wrong variant — the right product in the wrong size or colour. Build the flow to be variant-specific and confirm it via the event timeline before launch.

One more timing consideration: if you add a flow filter to exclude recent purchasers, allow enough time for order confirmations to sync before the email sends. Fire too fast and you may still alert someone who just bought the item elsewhere in your funnel.

Done properly, this flow does more than recover lost sales. It grows your email and SMS lists with proven high-intent subscribers, and it hands you clean demand data — which products and variants customers are willing to wait for. That's a direct signal for your reorder decisions, and it's the kind of insight most brands never think to capture.