Free tool · No email, no signup

Subject Line Tester

A solid influencer outreach subject line names the creator, says plainly what you are offering, and stays around 25 to 50 characters. Paste yours below to see which of seven written rules it meets and get scored variants. It checks writing rules; it does not predict replies.

Tester

Type or paste a subject line. The score is a count of rules met, listed in full below, and is not a prediction of replies. The detail fields are examples; replace them with yours.

Your subject line

Scored as you type. Nothing is sent anywhere.

37 characters, 7 words

Rules score

Out of 100. Each row is one rule from the methodology table.

Passes our checklist

100 / 100

It clears most of our writing rules. Still read it aloud and ask whether you would open it.

Length
37 characters, inside the 25 to 50 target.
Pass 20/20
Personalisation
Uses the creator's name.
Pass 15/15
Clear about the offer
Makes clear what the email is about.
Pass 15/15
Spam phrases
No phrases from our spam-phrase list.
Pass 20/20
Formatting
No shouting, no stacked punctuation, at most one emoji.
Pass 15/15
Not misleading
Does not pretend to be a reply or forward.
Pass 10/10
Specific, not generic
Avoids stock pitch phrases.
Pass 5/5

Variants to try

Built from your details with fixed patterns, then scored with the same rules and sorted best first. Click "Test" to load one into the tester.

  • Gifting Glow Serum to Maya100/100
  • A gift from Acme Skin for Maya100/100
  • Would you try Glow Serum, Maya?100/100
  • Maya, a free Glow Serum (no strings)100/100
  • Maya, a skincare gift from Acme Skin100/100
  • Maya, free Glow Serum from Acme Skin?100/100
  • Acme Skin x Maya: want to try Glow Serum?100/100
  • Can we send you Glow Serum?85/100
  • Loved your skincare content, Maya85/100
  • Can we send a skincare gift your way?85/100
  • Quick question about sending you Glow Serum85/100
  • Your skincare posts and Acme Skin78/100

A high score only means the line meets the rules. Read it as the creator would before you send.

Subject line sorted. Now ship the gift.

Once a creator replies yes, Seed sends them one branded link to pick a product and enter an address. A $0 draft order lands in your Shopify admin.

Install Seed on Shopify

Example subject lines and how they score

These six were scored by the same function that runs the tester, using the example details Maya, Acme Skin and Glow Serum. They show how the rules behave, not which line wins in your inbox. When you are ready to write the email body, use the outreach message generator.

Rules score for six example subject lines
Subject lineScoreWhy
Maya, free Glow Serum from Acme Skin?100Names the creator, says what is offered, plain language.
Can we send you Glow Serum?85Short and clear, but not personalised with a name.
Loved your skincare content, Maya85Personal, but does not say there is a gift.
Collaboration opportunity80Generic phrase, no name, no offer.
URGENT!!! 100% FREE GIFT - ACT NOW55Hits the caps, punctuation and spam-phrase rules.
Re: our chat44Fakes a reply thread, so it fails the honesty rule.

Frequently asked questions

What subject line gets influencers to reply?

No tool can promise replies. As a sound default, name the creator, say plainly what you are offering, keep it short enough for a phone screen, and avoid hype and fake reply prefixes. Test it by sending small batches and tracking your own open and reply rates rather than trusting any universal formula.

How long should an outreach subject line be?

Our checklist treats 25 to 50 characters as the target. That is an assumption we chose, not a measured benchmark: mobile mail apps cut subjects off at different points, so shorter is safer. If your key words appear in the first few words, a slightly longer line can still work.

Should I put the creator's name in the subject line?

Usually yes. A name shows the email was written for them and not blasted to a list. If you are sending in bulk, use a merge tag and check that it fills in correctly before sending. A subject reading "Hi {name}" in a real inbox is worse than no personalisation.

Do spam words really matter?

Spam filters use many signals, including your sending domain, authentication and recipient behaviour, so a word list is only one small part. We flag a short list of hype phrases because they also make a pitch look generic to a human reader. Treat a flag as a prompt to reword, not a verdict.

Is a fake "Re:" subject line a good idea?

No. Starting a subject with Re: or Fwd: to imply an earlier conversation is misleading. In the US, the FTC's CAN-SPAM guidance says commercial email must not use deceptive subject lines. It also damages trust with the very creator you are trying to win over.

Methodology and sources

Last updated: October 29, 2026

The tester applies seven fixed rules and adds up the points. The thresholds are our own writing judgement and editable assumptions, not benchmarks from a study. We do not have reply-rate data behind them.

The seven rules and their points
RulePointsHow it is tested
Length2025 to 50 characters scores full points; 15 to 24 or 51 to 65 scores 12; anything else scores 4.
Personalisation15Contains the creator's name (from the field above) or a merge tag like {name}, [Name] or {{first_name}}.
Clear about the offer15Contains an offer word (free, gift, send, try, collab, partnership, and similar) or your product name; naming only your brand scores 8.
Spam phrases20No phrases from our list scores 20; one scores 10; two or more scores 0.
Formatting155 points each for: no ALL-CAPS words of 3+ letters, at most one exclamation mark, at most one emoji.
Not misleading10Does not start with Re: or Fwd:.
Specific, not generic5Avoids stock phrases such as "collaboration opportunity" and "business proposal".

Spam-phrase list (hand-written, illustrative, not exhaustive): act now, urgent, 100% free, guarantee, guaranteed, winner, you won, cash, $$$, click here, limited time, buy now, order now, congratulations, risk-free, risk free, earn money, make money, free money, once in a lifetime, exclusive deal, dear friend, no cost, double your, special promotion, final notice, don't miss. Real spam filters weigh many other signals such as sender reputation and authentication.

  • Honesty rule: the US FTC's CAN-SPAM Act compliance guide says commercial email must not use deceptive subject lines. Other countries have their own rules. This page is not legal advice.
  • Length target (25 to 50 characters) is an assumption; mail apps truncate at different points.
  • Variants come from eight to twelve hand-written patterns filled with your details.
Who made this. Built by the team behind Seed, a Shopify app that sends free product to creators. This page uses fixed rules and templates that run in your browser: no AI model, no account, and nothing you type is sent to our servers. Spot a mistake? Email sam@seedinfluencers.com.

More free tools

Better subject lines get replies. Seed gets the gift shipped.

Share one branded link. The creator picks a product and types their address, and a $0 draft order lands in your Shopify admin.

Install on ShopifySee how it works