Best Email Subject Line Testers (2026): 10 Tools Compared

Your subject line decides whether your email gets opened or ignored, and you usually get one go at it. A subject line tester gives you a second opinion before you press send. It checks things like length, spammy words and how the line looks in an inbox, and some suggest rewrites.

Here's a list of ten subject line testers, in no particular order, sorted alphabetically. Two are better known and the rest are smaller makers. I've taken every description from each tool's own website and directory listings, so check the current details on the site itself before you rely on one. If you'd like help writing the lines in the first place, my pack of email subject line prompts is a good place to begin.

Ten subject line testers in a table

Nearly every subject line tester is a free page run by an email, outreach or marketing company. In the free plan column, Yes means the maker's own site describes the tester as free to use. Trial only would mean a free trial but no ongoing free access, and No would mean neither is stated. Some makers limit how often you can use a free tester, so check the terms.

Tool	Type	Good for	Free plan
Brevo	Email platform tool	Inbox previews and spam checks	Yes
Designmodo	Email builder tool	A score and an expected open range	Yes
Hunter	Email finder tool	Scores and rewrites for outreach	Yes
Landerlab	Landing page tool	Length, power words and preview	Yes
Mailmeteor	Mail merge tool	Scores plus AI alternatives	Yes
Ortto	Marketing platform tool	AI-written subject line options	Yes
SendCheckIt	Email testing site	Scanning for common subject line problems	Yes
SendKit	Email platform tool	Scoring many factors at once	Yes
SubjectLine.com	Dedicated tester	Scores, rewrites and spam trigger checks	Yes
SuperSend	Cold email tool	Previewing cold email subject lines	Yes

Subject line testers, listed one by one

Brevo

Best for: Newsletter senders who want to see where a subject line gets cut off in common inboxes.

Designmodo

Best for: Owners who build emails in a visual editor and want a quick AI-based score first.

Hunter

Best for: People who send outreach emails and want a score plus a few rewritten options.

Landerlab

Best for: Owners who want a no-signup score with a live inbox preview while they type.

Mailmeteor

Best for: Gmail and Sheets users who send mail merges and want alternative subject lines.

Ortto

Best for: Marketers who want an AI model to suggest wording that might lift opens.

SendCheckIt

Best for: Senders who also want deliverability checkers in the same place as a subject line test.

SendKit

Best for: Owners who want a quick score covering length, spam risk and personalization.

SubjectLine.com

Best for: Anyone who wants a single-purpose page for testing one line at a time.

SuperSend

Best for: Cold emailers who want to see sender name, subject and preview text together.

What a subject line tester can and can't tell you

Most testers run your line through a set of rules. They count characters and words, spot spam-trigger wording, look at punctuation and capital letters, and sometimes preview how the line looks in different inboxes. A few use AI to guess how it might perform.

That's useful for catching mistakes, but it isn't a prediction you can bank on. A tester doesn't know your readers, your brand or what you sent last week. The only way to learn what your own audience opens is to try lines on them.

Features worth comparing in a subject line tester

• Inbox previews. Does it show where the line is cut off on a phone and in a desktop inbox?
• Preview text. The text after the subject line matters too, so see if you can add it.
• Spam checks. Look for explanations rather than a bare score, so you understand what to change.
• Rewrites. AI suggestions are handy for ideas, but read them before you use them.
• Audience fit. A tester built for cold outreach may judge newsletters differently, and the reverse is true as well.
• Signup. Some run with no account, and some ask you to register.

Using a subject line tester well

• Write three or four versions first, then test them side by side.
• Read each line aloud. If it sounds like an advert, rewrite it.
• Keep the line honest about what's inside, since a misleading line brings unsubscribes.
• Check the preview on a phone, because that's where many people read.
• Run a real split test on your list when you can. My ways to increase email conversion cover what to try beyond the subject line.

Which subject line tester suits which sender

• Newsletter writers usually want previews and a readability check.
• Shop owners often care about spam wording and how promotional lines look.
• Cold emailers tend to prefer short, plain lines and a tool built for outreach.
• Anyone using AI help can ask a tester for alternatives, then pick the one that sounds like them. My ChatGPT prompts for email marketing show how to keep that human.

Why small subject line testers are worth trying

Smaller makers often build a tester to show off their main product, so the page is quick and simple. You can usually try several in a few minutes, which suits a job where you only need a second opinion.

Before you rely on one, check what it does with the text you type, since the maker's privacy page should say. Don't paste anything confidential into a free page. No tester can promise opens, and scores from different tools often disagree, so use them as a nudge.

Do you make a subject line tester or email optimization tool? Tool makers can ask to be considered for a featured place on this list.

Before you settle on a subject line tester

Pick two that suit the kind of email you send, then run the same set of lines through each. Compare how clear the advice is and whether it matches your own instinct. Keep the one you'll enjoy using and note anything you'd want to ask the maker.

Frequently asked questions

What is a subject line tester?

It's a tool that checks an email subject line for length, spam-trigger words, punctuation and sometimes inbox appearance. Some also suggest rewrites or estimate how well a line might perform.

Are subject line testers accurate?

They're good at catching obvious problems. They can't know your audience, so treat any score as a guide rather than a forecast.

How long should a subject line be?

Many inboxes cut off long lines, especially on phones, so short and clear tends to be safer. A tester with previews shows where your line is trimmed.

Is there a free subject line tester?

Yes, many. Most are free pages from email or marketing companies. The table on this page shows what each maker's own site says.

Should I still run a split test?

If your list is big enough, yes. A tester is a quick check, while a split test shows what your own readers do.