Should I use the subscriber's first name in subject lines?

It used to work. Now it's so common that it doesn't move the needle for most brands. Our testing shows personalization in subject lines provides a 1-2% lift at best. It's not bad — it's just not the magic bullet people think it is. Focus on relevance and brevity first.

How many subject lines should I test?

Two per campaign. Just two. Testing more than that splits your sample too thin to get statistically significant results unless your list is 100K+. One variable per test: change the length, the hook, or the format — never all at once.

What about questions in subject lines?

Questions work well for curiosity-driven opens. "Still thinking about it?" outperforms "Don't Forget Your Cart" in our abandoned cart tests. But avoid questions that can be answered "no" without opening — "Want 10% off?" is easy to ignore. "What's in your personalized box?" creates genuine curiosity.

Do numbers in subject lines help?

Specific numbers do. "Save $47 today" outperforms "Save big today." "3 new arrivals you'll love" outperforms "Check out our new arrivals." The number adds specificity and credibility. Vague claims get ignored because they feel like every other marketing email.

What's the best day and time to send eCommerce emails?

Tuesday through Thursday, 10am-12pm in your subscriber's timezone is the safe answer. But the real answer is: test it. Use Klaviyo's Smart Send Time feature which optimizes delivery timing per subscriber based on their historical open patterns. Our clients who use it see 5-10% higher opens than fixed-time sends.

Email Copywriting Guide

Subject lines under 40 characters get 12% higher open rates. Here's how to write them.

You spend an hour writing the perfect email. Beautiful product images. Compelling offer. Great copy. Then you spend 30 seconds on the subject line. And nobody opens it.

Subject lines are the gatekeepers. It doesn't matter how good your email content is if the subject line doesn't get the open. And most eCommerce brands are still writing subject lines the way they did in 2019 — all caps, emoji spam, and vague promises like "Don't Miss Out!!!" Gmail literally uses patterns like that to filter emails into Promotions or spam.

We've A/B tested thousands of subject lines across 150+ Klaviyo accounts. The patterns are clear, and they're probably the opposite of what you'd expect. Shorter beats longer. Specific beats vague. Boring beats clever. Here's the playbook.

<40 chars

Optimal Subject Line Length

12%

Higher Opens with Short Subjects

1,000+

Subject Lines A/B Tested

35-45%

Target Open Rate (Klaviyo)

How to fix this — step by step

Keep subject lines under 40 characters

Klaviyo's own benchmark data across billions of sends shows subject lines under 40 characters outperform longer ones by 12%. On mobile — where 60%+ of emails are opened — long subject lines get truncated. "Your order ships free" beats "Amazing Limited-Time Offer: Free Shipping On All Orders This Weekend Only!" Every character should earn its place. If you can say it in fewer words, do it.

Use sentence case, never Title Case or ALL CAPS

Title Case looks like marketing. Sentence case looks like a message from a person. "New arrivals just dropped" feels like a text from a friend. "New Arrivals Just Dropped For A Limited Time" feels like a billboard. And ALL CAPS triggers spam filters — "HUGE SALE TODAY" is basically asking Gmail to flag you. Sentence case consistently outperforms other formats in our testing. It's subtle, but at scale those percentage points compound.

Front-load the benefit or curiosity element

Most people see the first 4-6 words of a subject line before deciding to open or scroll past. Put the most important word first. "Free shipping on your next order" is better than "Psst — we have a special surprise: free shipping." "Your results are ready" beats "We wanted to share something exciting with you today." The benefit or curiosity hook needs to hit in the first few words or it doesn't hit at all.

Drop the emojis (yes, really)

Emojis in subject lines were a competitive advantage in 2018. Now they're so overused that they're actually a spam signal for many inbox filters. Our A/B test data across 150+ brands shows emoji subject lines perform the same or worse than plain text in 2025-2026. The exception: if your brand voice is genuinely playful and your audience skews Gen Z, a single relevant emoji can work. But "🔥🔥🔥 HUGE SALE 🔥🔥🔥" is always a loss.

Use preview text as a complement, not a repeat

Preview text (the gray text after the subject line in the inbox) is your second chance to hook someone. Don't waste it by repeating the subject line. If the subject is "Your cart is waiting," the preview text should add new info: "Plus: free shipping if you complete today." In Klaviyo, you can set preview text for every email. Use it on every single send. Brands that optimize preview text see 5-8% higher open rates versus those that leave it blank.

A/B test every campaign subject line

Stop guessing. Klaviyo lets you A/B test subject lines on every campaign — send two versions to 20% of your segment, then automatically send the winner to the remaining 80%. Test one variable at a time: length, tone, personalization, or format. Over 3-6 months of consistent testing, you'll have a data-backed playbook of what your specific audience responds to. Our best-performing clients test every single campaign. No exceptions.

Want us to handle this?

These six principles will immediately improve your open rates. You can implement them starting with your next campaign. And we genuinely recommend you do.

But subject lines are one piece of the email puzzle. Open rates depend on deliverability, segmentation, send timing, sender reputation, and list health. If your open rates are still below 25% after applying these principles, the problem is deeper than subject lines. We do complete email audits — deliverability, flows, segmentation, and campaigns — for Klaviyo accounts. The first one is free.

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