Does the tester predict open rates?

No. It scores mechanical faults — length, trigger phrases, shouting, stacked bangs, concreteness. Open rates depend on sender reputation, the list and the moment, none of which the tool can see.

How long should a subject line be?

Fifty characters or fewer to survive a phone intact; sixty to seventy works on desktop but invites truncation. The tester shows the exact character count and flags the band it falls in.

Why are words like free and limited time flagged?

Because both filters and readers have learned to skip them, so the phrase costs attention without adding information. The check names each trigger so it can be replaced with the concrete offer.

What counts as a concrete hook?

A number or a personalisation token such as the recipient's name. Either gives the reader a reason to open that stands on its own; the check asks for one when the line contains neither.

Email Subject Line Tester

The subject line

Subject line

Score

Type a subject line — the score updates as you write.

Checks

Checks appear once there is a subject line to check.

How the score is built

Length (50 characters or fewer): 0/30
No spam-trigger phrases: 0/25
No ALL-CAPS words: 0/15
At most one exclamation mark: 0/15
Contains a number or personalisation: 0/15

Scope and limits

The rubric is fixed: length, trigger phrases, shouting, exclamation marks and one concrete hook. It measures the line, never the offer behind it.

Open rates depend on the sender’s reputation, the list and the moment — none of which this tool can see. A 100 here is a tidy subject line, not a promise of an open.

The email subject line tester scores a subject line against the mechanical faults inboxes punish — length past the cut, spam-trigger phrases, shouted words and stacked exclamation marks — and reports the characters and words it holds.

What is Email Subject Line Tester?

The email subject line tester scores one subject line out of one hundred against the faults inboxes punish mechanically. Length is the first: a line of fifty characters or fewer scores full points, up to seventy scores half, and past that the rubbish bin on a phone cuts it. Then trigger phrases — free, act now, limited time and their relatives — which both filters and readers have learned to skip. Then shouted words, stacked exclamation marks, and one concrete hook: a number or a personalisation token such as the recipient's name.

The rubric is fixed and the breakdown is visible, which is the point. The tester cannot predict an open rate; open rates depend on the sender's reputation, the list and the moment. What it can do is catch the mechanical mistakes that sink a perfectly good offer: a line that spends forty characters before the verb, three bangs where one would do, a spam phrase that guarantees the promotions tab. Those are facts about the line, not predictions about the reader, and the page draws that line clearly.

The checks explain each deduction in words, so the score becomes a revision list. A line at fifty-five characters gets a note to move the detail into the preview text rather than a demand to cut words; a trigger phrase is named so it can be replaced rather than merely penalised. That framing matters because subject lines are often written under time pressure, and a specific question — will this survive the cut on a phone? — is easier to answer than a general one.

Two more notes on calibrating the score. The spam trigger list is deliberately short and literal, because the phrases that reliably cost a send are the obvious ones: free, guarantee, act now, limited time and a handful of relatives. Length and shouting are matters of degree, and the checks describe the degree rather than pretending to a verdict. The second note is that a low score is often a rewrite rather than a rejection: moving the concrete detail to the front of the line commonly lifts a subject from the forties into the eighties without changing the offer at all. Keep the tested line next to the preview text as you revise, since the two are read together on a phone and a subject that repeats the preview has wasted half of the impression.

Concreteness is rewarded for a reason worth stating: a subject line with a number or the reader's name gives a reason to open that stands on its own. Your refund in 10 working days says what is inside and how long it takes, without a single marketing adjective. The rubric is calibrated for that kind of line, and the check asks for one concrete hook when the line has none.

Nothing is uploaded and nothing is stored beyond the tab, so a subject line for an unpublished send stays private. Read the score, read the checks, change one thing, and stop before the line starts sounding scored — the best subject line is the one that reads like a person wrote it, which no rubric can measure.

How to use Email Subject Line Tester

Paste the subject line into the field.
Read the score, the character count and the trigger-phrase count.
Work through the checks: length, triggers, shouting, bangs, concreteness.
Rewrite with a number or the reader's name and watch the score move.
Check the final line against the preview text, not in isolation.

When to use Email Subject Line Tester vs related tools

Use the email subject line tester on any subject line that matters — a launch, a renewal, a note from a founder. When the body behind it is one of the routine templates, the Email Template Library already supplies a subject line to test.

The same habits apply a page up in display type: the Headline Analyzer scores headlines on length, detail and power words, and testing both keeps the email and its landing page from overselling each other.

Privacy & Security

This tool runs entirely in your browser — no data ever leaves your device. There is no server round-trip, no upload, no logging, and no account required. Your input is processed locally using client-side JavaScript and is never stored, transmitted, or accessible to anyone else. When you close the tab, everything disappears.