This is a representative preview, not a live inbox test. Actual rendering varies by client version, device, settings, personalization, sender reputation, and message markup.
This tool runs locally in your browser.
Results are estimates based on your input only; entered values and result contents are not stored by BoringToolsKit.
Your inputs and results stay in this browser. This page may send privacy-bounded aggregate interaction events described in Privacy; it does not send your entered values or result contents.
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What does this tool show?
Email preview tester: check how your subject line and preheader render in Gmail, mobile inboxes, and truncated views before you send. Free, no signup.
How this calculator works
Formula
The simulator removes unsupported control characters, applies a documented character budget for the selected representative viewport and client card, and adds an ellipsis when a subject or preheader exceeds that simulation budget.
Worked example
Enter a sender name, Subject A, an optional Subject B, and preheader text. Switch between compact mobile, wide mobile, and desktop to compare how the same copy may truncate in four representative inbox layouts.
Assumptions to verify
- Character budgets are deterministic BTK simulation limits, not measurements of every live inbox client.
- Actual display varies with client version, device, settings, personalization, sender reputation, and email markup.
- The tool evaluates presentation only. It does not send email, inspect HTML, score spam risk, or predict inbox placement or deliverability.
Frequently asked questions
Does this send my email content anywhere?
No. The preview runs in your browser and does not upload, send, or automatically store the sender, subject, or preheader.
Is this an exact Gmail, Apple Mail, Outlook, or Yahoo rendering test?
No. These are representative layouts with documented simulation budgets. Live clients can display different text lengths and styling.
Does a good preview mean my email will reach the inbox?
No. Presentation and deliverability are different. This tool makes no spam, reputation, placement, open-rate, or delivery prediction.
Cite this tool
BoringToolsKit. “Email Preview Tester.” boringtoolskit.com/email-preview-tester/ (reviewed July 2026). Free to reference in articles, syllabi, and answer posts with a link.
Privacy: Inputs and results stay in this browser. Any future sponsored recommendation or advertisement will be clearly labeled and kept separate from the calculation.
A worked example: one subject line across four desktop previews
Take the example subject line built into the tool: 'Know the number before money changes hands', which is 42 characters, with the preheader 'Private calculators with transparent assumptions, no signup required.' at 69 characters and the from name BoringToolsKit at 14 characters. Under the desktop viewport, the four clients have these subject limits: Gmail 72, Apple Mail 78, Outlook 66, Yahoo Mail 70. Because 42 is below all four limits, no subject truncation fires, and the spare headroom is 30 characters on Gmail, 36 on Apple Mail, 24 on Outlook, and 28 on Yahoo Mail. The preheader fits too: limits are 96, 110, 88, and 94, so the spare room is 27, 41, 19, and 25 characters respectively, and no warning appears.
Now extend the subject to 'Know the exact monthly payment and total interest cost before you sign anything today', which is 85 characters. Every client caps it. The tool slices to one character below the limit and appends a single ellipsis character, so a 72-character cap shows 71 subject characters plus the ellipsis. The cut points fall at different words: Gmail ends after 'you sign ', Apple Mail after 'you sign anythi', Outlook after 'before you', and Yahoo Mail after 'you sig'. Outlook is the binding constraint: it cuts roughly 19 characters of meaning earlier than Apple Mail. If the key phrase sits past character 65, Outlook readers never see it, which is why the last 20 characters of any subject line are the risk zone rather than the headline zone.
Boundary cases: what triggers and clears each check
Truncation is the only check tied directly to the budgets, and it switches state exactly at one character. A 65-character subject on desktop Outlook (limit 66) is clean; 67 characters adds one ellipsis and drops one character. The same string that passes desktop Yahoo Mail at 70 would truncate in the compact viewport, where Yahoo Mail allows only 48, so a line that reads as safe on a laptop can be cut on a phone. Switching viewport to compact moves the Gmail subject limit from 72 down to 34: the 42-character example subject, untouched on desktop, truncates to 33 visible characters plus an ellipsis there, with roughly a third of the line gone.
Other checks are input-driven rather than length-driven. An empty from name or empty Subject A produces a single completeness warning per field; the simulation still renders, using placeholder text in the cards. Control characters are stripped silently before display, but the tool compares the cleaned string to the raw input and warns when the two differ, so pasted text with tab or carriage-return characters surfaces instead of disappearing. The preheader check only fires when the preheader starts with the subject text case-insensitively; a preheader that merely contains the subject later, or differs by one word, passes. Subject B is off by default: an empty field renders exactly 8 cards (4 clients x 1 variant), while any non-empty Subject B, even a single character, doubles the grid to 16 and can turn one shared preheader into two separate truncation warnings. The ellipsis replaces a character rather than being appended after the cap, so a truncated line is never longer than its client's limit, and trailing spaces are trimmed before the ellipsis is added, which can shave one further character.