Use Klaviyo's AI to Optimize Subject Lines and Send Times

What This Does

Klaviyo's built-in AI generates subject line alternatives and uses machine learning to identify the best send time for each individual subscriber — so you stop guessing and start using data instead.

Before You Start

• You have a Klaviyo account (any plan, including free)
• You're building or editing a campaign (not a flow — Smart Send Time applies to campaigns only)
• Your list has at least 500+ subscribers for meaningful send time data

Steps

1. Open a campaign and reach the subject line field

In your Klaviyo dashboard, click Campaigns in the left sidebar. Either start a new campaign or open an existing draft. Work through the setup until you reach the Email Content step where you see the Subject Line field.

2. Use the AI Subject Line Assistant

Next to the subject line text field, click the Use AI button (it appears as a small sparkle or wand icon). A panel will open asking what your email is about — describe your campaign in 1-2 sentences.

What you should see: A list of 5-6 subject line suggestions with different tones and angles.

Troubleshooting: If you don't see the AI button, your account may be on an older interface version — try refreshing or check the Klaviyo changelog for your plan.

3. Select or edit your favorite variant

Click any suggested subject line to paste it into the field. You can edit it directly if you want to blend the AI suggestion with your own phrasing. Add preview text separately (Klaviyo's AI doesn't auto-generate this — use ChatGPT for preview text).

4. Enable Smart Send Time

Scroll down to the Sending options section of your campaign setup. Find the Smart Send Time toggle and switch it on. Klaviyo will analyze each subscriber's historical open-time patterns and deliver the email at their individual optimal window — within a 24-hour delivery period.

What you should see: A note saying "Emails will be sent over a 24-hour window based on each subscriber's optimal time."

Troubleshooting: Smart Send Time is not available on the free plan. If you don't see it, check your plan tier under Account settings.

5. Schedule and send

Complete the rest of your campaign setup and schedule. Your campaign will deploy over the 24-hour window, with Klaviyo staggering sends per subscriber.

Real Example

Scenario: You're sending a spring sale announcement to 12,000 subscribers for a home goods brand.

What you type/do: In the subject line AI panel, type: "Spring sale — 20% off all rugs and bedding, limited time." Klaviyo suggests: "Your spring refresh starts now 🌿", "Up to 20% off — this weekend only", "Finally: the sale you've been waiting for."

What you get: Three usable variants to choose from; you pick "Finally: the sale you've been waiting for" and edit it to: "Finally: 20% off rugs and bedding this weekend."

Smart Send Time result: Instead of all 12,000 emails arriving at 10 AM, they arrive throughout the day when each subscriber is most likely to be in their inbox.

Tips

• Use the AI subject line suggestions as a starting point, not a final answer — edit the best one to add brand-specific language or the actual offer amount.
• Smart Send Time works best on lists of 1,000+ where there's enough historical behavior to learn from. On smaller lists, the model has less signal.
• You can still A/B test subject lines with Smart Send Time enabled — Klaviyo will split your list and apply Smart Send Time to both variants.

Tool interfaces change — if a button has moved, look for similar AI/magic/smart options in the same menu area.