Copywriting
Subject line engineering: length, structure and the six patterns worth testing
3 December 2025 · 7 min read
A subject line is read in a list, on a phone, in about a second, beside twenty competitors. Most of the craft is arithmetic: how many characters survive truncation, and whether the useful words are inside that window.
This article covers the display constraints, then six structures worth putting into a test rotation.
How much of it is actually displayed
Display limits vary by client, orientation and font size, but the practical planning window is narrow. Aim for roughly 30 to 50 characters and place the decisive words first.
| Context | Characters typically shown |
|---|---|
| iPhone Mail, portrait | ~35–40 |
| Android Gmail app | ~40–45 |
| Gmail web, standard density | ~60–70 |
| Outlook desktop, reading pane right | ~55–60 |
The preheader is the second half of the sentence
The preview text shown after the subject is a separate opportunity, not a repeat. Left unset, clients fall back to whatever appears first in the HTML — commonly 'View in browser'. Write it deliberately, keep it to about 40 to 90 characters, and let it complete the thought the subject started.
Six structures worth testing
- Specific benefit plus timeframe — 'Cut your reporting time to 10 minutes'.
- Concrete number — 'Three changes that lifted our reply rate'.
- Direct question — 'Still hiring for the ops role?'.
- Plain announcement — 'Your December statement is ready'.
- Named curiosity, honestly earned — 'The pricing page mistake we made'.
- Personal and short — 'Quick question, Maria'.
What to avoid, and why it is not superstition
Full capitals, stacked exclamation marks and words such as free, guaranteed and act now do not trigger an automatic block in modern filters — reputation and engagement dominate. But they correlate with the kind of mail recipients report, and complaints are what filtering actually responds to. The risk is indirect and real.
Test one variable at a time, on segments large enough to separate signal from noise, and judge on clicks rather than opens.
Key takeaways
- Plan for 30 to 50 characters and front-load the decisive words.
- Always set the preheader; otherwise the client invents one.
- Rotate a small set of structures rather than rewriting from scratch each time.
- Judge subject tests on clicks, because open data is noisy.
Sources
Put this into practice with HTsend
Everything in this guide is built into the platform: authenticate a sending domain, clean your list before you send, and watch inbox placement in real time.
