Subject Line & Preheader Tester
Gmail's app shows 33 characters of a subject, Apple Mail on iPhone 48 and Gmail on the web 88. Type below to see where each inbox cuts yours.
@emailens/engine 0.18.22 · data last changed 10 Nov 2026
Length: fits all 8 inboxes.
Inbox preview
Apple Mail on iPhone · lock screen
Acme Supply
Your order #82914 has shipped
Estimated delivery is Thursday via Express. Track your parcel here.
Cut at 48 characters, as in All clients.
Limits from the Emailens engine 0.18.22. Runs in your browser; nothing is sent.
This checks the inbox row. To check the email behind it, open the checker and paste its HTML: five clients, no account.
No signup · Five clients free, Outlook Classic included · All 21 with an account
Which clients need an account
Gmail Android, Gmail iOS, Outlook 365, Outlook iOS, Outlook Android, Outlook for Mac, Apple Mail, Yahoo Mail Android, Yahoo Mail iOS, Samsung Mail, Thunderbird, HEY Mail, Proton Mail, AOL Mail, Fastmail, Superhuman open with a free account.
The checker needs a computer screen.
Your next question
- What will a spam filter penalise in the email? Check your content, free.
- Does the email have a plain-text version? Generate one, free.
Other free tools: AI Fix-Prompt Generator · Gmail Clipping & Size Checker · HTML Email Minifier · Email Link Checker · UTM Link Builder · Accessibility Checker
Run this check on every email
The open-source CLI checks the subject and preheader of every template,
free: npx @emailens/cli lint email.html
(CLI docs).
Questions
How long should an email subject line be?
Front-load the words that matter into the first 33 characters, the tightest view in the engine's table. Gmail's app shows 33, Apple Mail on iPhone 48, Outlook 51 on the web and 55 on the desktop, and Gmail on the web 88.
What is preheader (preview) text?
The preheader is the snippet shown after the subject in the inbox list. Gmail on the web shows up to 90 characters of it, Outlook on the desktop only 35. If you leave it empty, clients pull the first text in the email, often 'View in browser' or an address block.
Do emojis in subject lines help or hurt?
One emoji can lift attention, but some clients and corporate Outlook installs strip it or show an empty box. Use one at most, and make sure the subject still reads without it.