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About this tool
A practical marketing tool for managing email subject line previewer workflows efficiently.
The Email Subject Line Previewer shows exactly where your subject line and preview text get cut off in four inboxes — Gmail Mobile at 38 characters, Apple Mail at 46, Outlook at 54 and Gmail Desktop at 68 — and scores the line out of 100 as you type. It is built for anyone about to press send on a campaign who wants to know whether the important half of the sentence survives truncation. The score starts at 100 and moves on measurable traits: length, word count, missing preview text, spam-trigger words, numbers, questions, personalisation tokens and emoji.
Open Email Subject Line Previewer on AltFTool — it loads instantly in your browser.
In the Subject Builder, type into Subject line (counter reads 'ideal: 30-55') and Preview text (reads 'ideal: 40-95'), and set Sender name and Recipient inbox.
Watch the four inbox previews cut the subject at Gmail Mobile 38, Apple Mail 46, Outlook 54 and Gmail Desktop 68 characters, with preview text capped between 72 and 96.
Read the score out of 100 with the Readiness, Mobile Fit, Preheader and Risk Level tiles, then take an A/B option from Variant Lab using its 'Use variant' or 'Copy variant' button.
Each of the four inbox previews applies its own subject and preview-text limit, so you can see the same line survive on desktop and break on mobile.
Every point is traceable: -18 for going over 60 characters, -12 for empty preview text, -9 per spam word, +5 for including a number.
The preheader is previewed and scored alongside the subject, with its own per-client limits from 72 to 96 characters.
Aim for roughly 20 to 60 characters. This tool deducts 10 points below 20 characters and 18 points above 60, because Gmail Mobile shows about 38 characters, Apple Mail about 46 and Outlook about 54 — anything past that is invisible on the screens where most email is first opened.
Between 72 and 96 characters depending on the client — Gmail Mobile shows about 72, Apple Mail 82, Outlook 88 and Gmail Desktop 96. Preview text over 110 characters costs 8 points in the score, and leaving it blank costs 12.
Ten common spam triggers each cost 9 points: free, guarantee, urgent, winner, cash, limited time, act now, risk-free, click here and 100%. Five weak filler words — newsletter, update, important, announcement and reminder — cost 3 points each because they say nothing about the actual contents.
In this scorer, yes, but modestly: a number adds 5 points, a personalisation token like {first_name} adds 4, a question mark adds 3 and an emoji adds 2. These reflect common email-marketing patterns rather than a guarantee — the only reliable measure for your own list is an A/B test.