Your email arrives in a crowded inbox. The recipient spends less than two seconds deciding whether to open it or archive it. That decision is based on three things they can see without opening: the sender name, the subject line, and the preview text. If those three elements do not earn attention, your carefully written email will never be read.

Most Shopify merchants underinvest in subject line writing. They spend an hour crafting email copy and then spend 30 seconds on the subject line. That is the wrong ratio. The subject line is the most valuable real estate in your email marketing programme.

What Actually Drives Email Open Rates?

Before looking at subject line formulas, understand what the inbox experience looks like from the recipient's side. In the inbox, they see:

Sender name: Who sent this? Is it a brand they recognise or care about?
Subject line: What is this email about? Is there a reason to open it right now?
Preview text: The grey text that appears next to the subject line, either the first line of your email or custom preview text you set.

The sender name has a significant effect on open rates that many merchants underestimate. Emails sent from a personal name ("Niko from Sutton Commerce") consistently outperform emails sent from a brand name alone ("Sutton Commerce") for most audiences. This is because personal names signal a human behind the message. Test both for your list.

Klaviyo benchmarks show that average open rates for ecommerce emails sit around 30 to 40 per cent. Welcome emails and abandoned cart emails perform significantly higher (40 to 60 per cent) due to their immediate relevance. Promotional broadcast emails typically land in the 25 to 35 per cent range for well-maintained lists.

What Subject Line Formulas Work Consistently?

No subject line formula works forever. Overused formulas become wallpaper. But these frameworks consistently perform because they tap into fundamental motivations:

Curiosity gap: Imply there is something valuable inside that requires opening to access. "We do not tell many people this." "The mistake most Shopify stores make." "What your analytics are not showing you." The key is that the payoff inside the email must match the implied promise. Curiosity that is not satisfied destroys trust.

Direct benefit: State the outcome the reader gets. "Get free next-day delivery on your first order." "Your summer wardrobe, ready in 48 hours." "Save 20 per cent on everything this weekend." This works best for transactional and promotional emails where a concrete offer exists.

Personalisation: Including the subscriber's first name in the subject line ("Tom, we picked these for you") increases open rates by 10 to 20 per cent on average. Klaviyo handles this with {{ first_name|default('there') }} which falls back to "there" if no first name is recorded.

Urgency and scarcity: "Last 24 hours." "Only 4 left in stock." "Offer ends midnight Sunday." Urgency works when it is genuine. False scarcity damages brand trust when customers discover it is not real.

Social proof: "Our best-reviewed product of the year." "3,400 people bought this last month." "Why 94 per cent of customers come back." These work because they borrow credibility from the crowd.

Question format: "Still looking for the right fit?" "Have you tried this yet?" "What is your biggest ecommerce challenge?" Questions invite engagement and feel less like broadcasting.

What Should You Avoid in Subject Lines?

Certain words and patterns reliably reduce open rates and can trigger spam filters:

Avoid	Why
ALL CAPS	Aggressive tone, spam flag
Excessive punctuation (!!!, ???)	Spam signal
"FREE" in all caps	High spam score in most email clients
"Buy now", "Act now", "Limited time offer" (alone)	Overused, low trust
Misleading subject lines	Destroys trust, increases unsubscribes
Subject lines over 60 characters	Truncated on mobile; aim for 40 to 50 characters
Emojis in every email	Dilutes effect; use sparingly

Spam triggers are assessed by email providers using a combination of content analysis and sender reputation. Even if a single email passes spam filters, a pattern of spammy subject lines erodes your sender reputation over time and reduces deliverability for all your emails.

Keep your subject lines honest. If your email is a product update, say it is a product update. If it is a sale, say it is a sale. The short-term open rate lift from clickbait is outweighed by the long-term damage to trust and unsubscribe rates.

How Do You Write Preview Text That Works?

Preview text (also called preheader text) appears next to your subject line in the inbox. It is either the first line of your email body or a custom preview text field in Klaviyo.

Always set custom preview text. If you do not, Klaviyo uses the first line of your email, which is often "View this email in your browser" or a logo image alt tag. That is wasted space.

Good preview text does one of two things:

Extends the subject line: The subject line sets up the hook; the preview completes it. Subject: "Tom, we picked these for you." Preview: "Based on your last order, you might like these three products."
Adds a second reason to open: Subject: "Our summer sale starts today." Preview: "20 per cent off everything, plus free next-day delivery until Sunday."

Keep preview text between 40 and 100 characters. Longer text is cut off in most email clients. In Klaviyo, the preview text field is in the email settings panel (the gear icon in the email editor).

How Do You A/B Test Subject Lines in Klaviyo?

A/B testing subject lines is one of the highest-return activities in email marketing. A 5 percentage point improvement in open rate across your entire list compounds significantly over time.

In Klaviyo, A/B tests can be run on:

Flows (automated sequences): Go to your flow email, select "Add A/B Test", and set up a second variation with a different subject line. Klaviyo splits incoming recipients 50/50 between the two and tracks which performs better.
Campaigns (broadcast emails): When creating a campaign, select "A/B Test" and define your variants. Klaviyo can automatically send the winning version to the remainder of your list after a defined test period.

Best practices for subject line A/B tests:

Test one variable at a time. Do not change the subject line and the preview text simultaneously.
Run tests on audiences of at least 1,000 subscribers to achieve statistical significance.
Let tests run for 48 hours minimum before drawing conclusions.
Document results. Build a log of what subject line approaches have won and lost over time. This becomes a reference guide for your whole email programme.

What Benchmarks Should You Target by Email Type?

Email Type	Average Open Rate	Strong Open Rate
Welcome Email 1	45 to 55 per cent	60 per cent+
Abandoned Cart Email 1	40 to 50 per cent	55 per cent+
Post-purchase Email	40 to 48 per cent	55 per cent+
Broadcast promotional	25 to 35 per cent	40 per cent+
Win-back campaign	15 to 25 per cent	30 per cent+
Product restock	35 to 50 per cent	55 per cent+

These benchmarks are based on Klaviyo ecommerce data and Mailchimp industry reports for UK merchants. If your open rates are consistently below the average range, investigate: list health (how many subscribers have not opened in 90 days), sender reputation (check Postmaster Tools for Gmail deliverability), and subject line quality.

Subject Line Examples for Common Shopify Email Flows

Here are working examples across different email types:

Welcome series: "Welcome. Here is what we do differently." | "Your first order, with a little something from us." | "Meet the team behind [Brand]."

Abandoned cart: "You left something behind." | "Your basket is getting cold." | "[Product name]: still available, just for now."

Post-purchase: "Your order is on its way, [first name]." | "You just bought something great. Here is how to get even more from it." | "A quick favour: how did we do?"

Re-engagement (win-back): "We miss you. Here is a reason to come back." | "Has anything changed? We updated our range." | "One last thing before we go."

Promotional (sale): "The sale is live. Here is where to start." | "48 hours only. No code needed." | "Our biggest discount of the year starts now."

Key Actions to Take Now

Audit your last ten email campaigns in Klaviyo and identify the three with the highest and three with the lowest open rates. Look for patterns in what the top performers have in common.
Change your sender name to a first name plus brand ("[Name] at [Brand]") and test it against your current sender name over the next four campaigns.
Add custom preview text to every email you send from today. Never leave it empty.
Set up an A/B test on your highest-volume flow email (typically Welcome Email 1 or Abandoned Cart Email 1) testing a direct benefit subject line against a curiosity-gap subject line.
Build a subject line blacklist: words and phrases that consistently underperform for your audience. Review every two months.
Keep a subject line swipe file. When you see a marketing email with a subject line that made you want to open it, save it. Over time this becomes your best creative resource.
Check your email open rates against the benchmarks in this post. If any flow is below average, prioritise it for subject line testing this month.

Frequently Asked Questions

Do emojis in subject lines help open rates? Sometimes. Emojis can increase open rates when used occasionally and when relevant to the content. Overuse reduces the effect and can look unprofessional. Test a version with an emoji against a version without and let your audience tell you. Many UK B2C ecommerce audiences respond positively to occasional, relevant emojis.

How important is personalisation (first name) in subject lines? It typically adds 10 to 20 per cent to open rate when used appropriately. However, it requires accurate first name data. If your list has missing or incorrect first names, personalisation will show "there" or whatever your fallback is, which can look odd. Audit your first name data quality before relying heavily on this tactic.

Should subject lines be the same length for desktop and mobile? Mobile clients typically display 30 to 40 characters before truncating. Desktop clients show 50 to 70 characters. Write your most important information in the first 35 to 40 characters so it renders fully on mobile. The key hook should not be at the end of a long subject line.

What is a good unsubscribe rate and should I worry about it in relation to subject lines? A healthy unsubscribe rate is below 0.2 per cent per email sent. Misleading or clickbait subject lines that do not match the email content are a primary driver of elevated unsubscribe rates. If yours consistently exceeds 0.3 per cent, review the alignment between your subject line promise and your email content.

Email Subject Lines for Shopify: How to Write Lines That Get Opened