Preheader text: the second half of every subject line
TL;DR
The preheader is the second sales line in every inbox row. Most operators waste it by leaving it empty (the client fills the space with the unsubscribe link) or by repeating the subject. Optimised preheaders lift open rate 15-25 percent. Target 40-90 characters; write the subject and preheader as one continuous sentence; never overlap more than 20 percent of words.
You spent 30 minutes on the subject line. You spent zero on the preheader. The reader sees both at the same time, the same size, on the same row. That math is wrong.
What the inbox actually shows
Most newsletter operators underestimate how much preheader real estate exists in the inbox. The character budget per major client:
Apple Mail (iOS + macOS): up to 140 characters
Gmail web (desktop): up to 90 characters
Gmail mobile (iOS app): up to 40 characters
Outlook 365 web: up to 55 characters
Outlook desktop preview: up to 50 characters
The 40-90 character band is the safe target. It survives Gmail web, Outlook, and Apple Mail without truncation, and the first 40 of those characters are the ones Gmail mobile readers actually see.
The overlap trap
The most common preheader failure: repeating most of the subject in different words. "Read this week's report" → "This week's report is now available." The reader's eye scans both, recognises them as the same content, and discards the second. You spent two character budgets to deliver one sales line.
The fix is reframing. Subject and preheader should read as one sentence with a comma in the middle. "Read this week's report, plus three trends nobody is writing about yet" splits cleanly: subject "Read this week's report" + preheader "Plus three trends nobody is writing about yet." Same total length; double the information density.
Our preheader optimizer calculates word-level overlap between your subject and preheader and flags anything over 20 percent.
The empty-preheader trap
The other common failure: leaving the preheader empty. The inbox client does not leave the space blank - it fills it with the first text it finds in the email body. For most newsletter templates that is "View in browser" or "If you can't see this email." Both are visible to the reader before they even open. Both are dead real estate.
The repair is mechanical: every send must have a deliberate preheader. If your ESP has a preheader field, use it. If not, drop a hidden div at the top of the HTML body:
<div style="display:none;font-size:1px;color:#fff;line-height:1px;
max-height:0px;max-width:0px;opacity:0;overflow:hidden;">
Your deliberate preheader text here.
</div>Pairing patterns that work
The continuation
Subject sets up; preheader pays off. "We made a big mistake this quarter / Here's the post-mortem and what we changed."
The proof point
Subject states; preheader proves. "The 4% click-through trick / Tested across 12 ecommerce sends in March."
The contrast
Subject claims; preheader complicates. "Why we cut email frequency in half / And why open rate climbed."
The teaser
Subject names; preheader hooks. "This week's brief / Plus a weird thing happening at Mailchimp."
What the optimizer scores
Subject and preheader together score on three things beyond what the subject scores alone:
- Preheader length budget - is it in the 40-90 char band, or wasted at 15 chars / truncated at 130?
- Word overlap - n-gram overlap between subject and preheader. Under 20 percent is healthy.
- Complementary framing - does the preheader add new information or restate?
Test your subject + preheader as a unit
The preheader optimizer shows you exactly what the email looks like in Gmail, Outlook, and Apple Mail, scores the joint pairing, and flags overlap and truncation in real time.
Open the preheader optimizer →Frequently asked questions
What is a preheader?
The snippet of preview text shown after the subject in the inbox row. Gmail web 90 chars, Outlook 55, Apple Mail 140.
Optimal length?
40-90 characters. Under 30 wastes the space; over 100 truncates on most clients.
Repeat the subject?
Never. Write subject + preheader as one continuous sentence. Overlap above 20% wastes characters.
Does it move open rate?
Yes - 15-25% lift over no preheader, 8-12% over a redundant one.
How do I add one without an ESP field?
Hidden div at the top of the HTML body with display:none and font-size:1px.