Email subject line A/B tester
Write every variant side by side, score each one against length and deliverability best practices, and see which is worth sending as the A/B winner candidate.
Your variants
- +12 · Ideal length (47 chars)
- +6 · Personalised with a merge tag
- +4 · Includes a specific number
Mobile inbox preview
Oitomail
{{first_name}}, 3 accounts went qui…
Two minutes to see who's slipping away.
- +12 · Ideal length (31 chars)
- +5 · Speaks to the reader (you/your)
- +4 · Uses clear, active wording (your, ready)
Mobile inbox preview
Oitomail
Your churn risk report is ready
Two minutes to see who's slipping away.
Shared context
Recommendation
Highest scoring variant
A. {{first_name}}, 3 accounts went quiet this week
92/100 · 47 characters
Is your list big enough?
2 variants · 2,500 recipients each
That's enough volume to read a real difference in open rate.
Run the test for real
Oitomail splits any campaign or sequence step across subject line variants, tracks opens and clicks per variant, and stops the sequence the moment the goal is met.
Start 7-day trialHow to run a subject line A/B test properly
- Change one thing per variant — personalisation, curiosity, specificity — not everything at once.
- Split the audience randomly and send at the same time of day; time of send skews opens more than wording.
- Give each variant at least ~1,000 recipients before you trust the result.
- Judge on clicks or conversions, not opens alone — privacy protection inflates open rates.
- Roll the winner into your template library so the next campaign starts ahead.
Frequently asked questions
How many subject lines should I A/B test?
Two variants is the cleanest test. Three or four are fine on large lists, but every extra variant splits your audience further and needs more recipients to reach a confident result.
How big does my list need to be for an A/B test?
As a rule of thumb you need roughly 1,000 recipients per variant to detect a meaningful difference in open rate. Below that, treat the winner as a hint rather than a result.
What makes a high-scoring subject line?
30–50 characters, the value in the first few words, one specific idea, a personalisation token where it is genuinely relevant, no shouting, no stacked punctuation and no cash-flavoured spam wording.
Does the score guarantee a higher open rate?
No. The score checks length and well-known best practices; your audience decides the rest. Use it to eliminate obviously weak variants, then let the real test pick the winner.
Is anything sent to a server?
No. Every variant is scored in your browser and nothing is stored or uploaded.
Turn activity into retained revenue.
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