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Email Subject Line Tester

Preview an email subject line and preheader across Gmail, Outlook, Apple Mail, Yahoo, and Apple Watch. Live truncation, length, emoji, and spam-trigger checks.

Preview

Inbox previews across Gmail, Outlook, Apple Mail, Yahoo, and Apple Watch

12 chars

The sender name as it will appear above the subject. Most inboxes show 20 to 30 characters.

30 chars
OptimalWithin the 28 to 50 character band that most mainstream studies recommend for open rate.
48 chars

The hidden snippet most inbox lists render after the subject. Most clients show 80 to 100 characters.

Words

5

Emoji

0

Clients cut off

1/8

Gmail

Web, default inbox

Subject fits

Acme Updates

Your June order ships tomorrow - Tracking, returns, and a small thank-you inside.

9:24 AM

Subject px
0 / 360
Char limit
30 / ~70
Preheader
Fits

Gmail uses one row that shares space between subject and preheader; long subjects push the preheader off the line.

Gmail

iPhone / Android app

Subject fits

Acme Updates

9:24 AM

Your June order ships tomorrow

Tracking, returns, and a small thank-you inside.

Subject px
0 / 320
Char limit
30 / ~35
Preheader
Fits

Mobile Gmail stacks From, Subject, and Preheader. Subjects are usually cut around 35 to 40 characters in portrait.

Outlook

Windows desktop

Subject fits

Acme Updates

9:24 AM

Your June order ships tomorrow

Tracking, returns, and a small thank-you inside.

Subject px
0 / 260
Char limit
30 / ~55
Preheader
Fits

Outlook desktop uses a compact two-line list. Subjects past about 55 characters are cut off on standard zoom.

Outlook.com

Web

Subject fits

Acme Updates

9:24 AM

Your June order ships tomorrow

Tracking, returns, and a small thank-you inside.

Subject px
0 / 320
Char limit
30 / ~73
Preheader
Fits

Outlook.com web shows about 73 subject characters before truncation in the default reading pane.

Apple Mail

iPhone, portrait

Subject fits

Acme Updates

9:24 AM

Your June order ships tomorrow

Tracking, returns, and a small thank-you inside.

Subject px
0 / 320
Char limit
30 / ~41
Preheader
Fits

iPhone Mail in portrait shows roughly 41 subject characters, then truncates with an ellipsis.

Apple Mail

iPad, split view

Subject fits

Acme Updates

9:24 AM

Your June order ships tomorrow

Tracking, returns, and a small thank-you inside.

Subject px
0 / 380
Char limit
30 / ~60
Preheader
Fits

iPad split view fits about 60 subject characters in the mail list column.

Yahoo Mail

Web

Subject fits

Acme Updates

9:24 AM

Your June order ships tomorrow

Tracking, returns, and a small thank-you inside.

Subject px
0 / 280
Char limit
30 / ~50
Preheader
Fits

Yahoo Mail truncates subjects around 50 characters and shows a short snippet of the preheader.

Apple Watch

41 to 45 mm

Subject cut off

Acme Updates

Your June order ships tomorrow

Tracking, returns, and a small thank-you inside.

Subject px
0 / 160
Char limit
30 / ~22
Preheader
Cut off

Apple Watch only shows the first 20 to 25 subject characters. The first words must do all the work.

How the inbox limits are measured

  • The tool measures the actual rendered pixel width of your subject and preheader using a font that approximates each client (Segoe UI for Outlook, SF Pro for Apple Mail, Roboto for Gmail mobile, and so on). System fallbacks are used when the exact font is missing.
  • Each client has a documented inbox-list width budget. When your text exceeds that width the preview is truncated with an ellipsis, the same way real clients render it.
  • Character counts use Unicode grapheme segmentation, so emoji, ZWJ sequences, and combining marks are counted as one character each, matching how mail clients render them.
  • No network requests are made. Your subject, sender name, and preheader stay on your device.

Subject line best practices

  • Aim for 28 to 50 characters so the line fits the iPhone Mail and Gmail mobile cut-off and still has room for the value proposition.
  • Put the most important words first. The Apple Watch and Gmail mobile previews only show the first 20 to 35 characters.
  • Pair the subject with a deliberate preheader. Most clients render the preheader inline after the subject; if you do not set one, the inbox shows the first sentence of the email body instead.
  • Use emoji and punctuation sparingly. ALL CAPS, exclamation stacks, and well-known spam-trigger phrases lower trust in the inbox and risk flagging by mainstream filters.
  • A/B test on a real send before betting on a subject. This tool previews truncation and writing quality; it does not predict open rate.

How to use

  1. Type or paste your sender name into the From field. Most inboxes show 20 to 30 characters before truncation.
  2. Type your subject line. The Optimal band turns green when the length lands between 28 and 50 characters; the chips below the field flag emoji, ALL CAPS, excessive punctuation, and spam-trigger phrases.
  3. Add your preheader (preview text). Aim for around 90 characters; longer text is cut off by Apple Mail and Outlook.
  4. Scan the inbox preview cards. Each card shows From, Subject, and Preheader rendered with that client's truncation budget, plus subject pixel width, character count, and a Cut off or Fits badge.
  5. Use Load sample to compare against a working subject, or Clear to start from a blank slate. Click Copy summary to export a plain-text report with every signal and per-client verdict.

About this tool

Email Subject Line Tester previews how an email subject line and preheader will render across eight common inbox clients: Gmail web, Gmail mobile, Outlook desktop, Outlook.com, Apple Mail on iPhone, Apple Mail on iPad split view, Yahoo Mail web, and Apple Watch. Each preview measures the actual pixel width of the subject using a font that approximates the client (Segoe UI for Outlook, SF Pro for Apple Mail, Roboto for Gmail mobile) with the Canvas 2D API, then truncates with an ellipsis the same way the real client does. Above the inbox previews the tool surfaces the writing-quality signals that affect open rate: subject length banding (28 to 50 characters is the optimal window cited by most mainstream studies), word and emoji counts using Unicode grapheme segmentation, ALL CAPS shouting detection, excessive punctuation detection, and a conservative scan for well-known spam-trigger phrases. The preheader is treated as a first-class field with its own character ceiling, so you can see when the preview text gets cut off by the inbox list. Everything runs locally in your browser; the sender name, subject, and preheader are never sent to a server.

Free to use. Works in your browser. No signup, no login.

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