How different B2B cold email subject lines perform: Belkins’ 2025 study

Through our extensive research, analyzing 5.5 million emails and leveraging our partnership with Reply.io, we've gathered explicit data that paints a clear picture of what makes cold email subject lines truly effective and what doesn't. We'll present the data on:

• The impact of subject line personalization on open and reply rates
• Which words and phrases lift and lower open rates
• The impact of subject line length on email opens.
• How casing styles sway user engagement.
• The real effect of numbers in subject lines

Key takeaways

• Emails sporting personalized subject lines have a 46% open rate versus just 35% without, which is a 31% boost in visibility.
• Subject lines framed as questions hit a 46% open rate, outperforming all other types by sparking curiosity and hinting at genuine value.
• Terms steeped in marketing hype, urgency (“ASAP”), and generic greetings (“Hello, friend”) drag open rates below 36%, signaling a clear shift towards authenticity and clarity.
• Subject lines with 2–4 words yield the highest open rates (46%). Extremely short (1-word) or long (9–10 word) lines underperform due to a lack of clarity or visual clutter.
• ALL CAPS subject lines edge out others at 30%, though the difference is marginal.
• Subject lines featuring numbers (27%) actually perform slightly worse than those without (28%).

How personalization in subject lines affects open and reply rates

Personalized subject lines, whether they include a relevant event or the recipient's name, company, or location, significantly boost both open and reply rates. Our research lays bare just how much a small tweak can improve visibility and overall email performance:

• Emails with personalized subject lines boast a 46% open rate, compared to a mere 35% without. That’s a 31% leap.
• Reply rates jump from 3% (no personalization) to 7% (with personalization), a whopping 133% increase.

 📚 Relevant reading: Personalized cold email examples and best practices

Which words and phrases in subject lines increase open rates

This chart breaks down average open rates by subject line type, laying bare how different elements like questions, urgency, and greetings steer user behavior. Here's the rundown:

• Subject lines framed as questions (e.g., “Are you the right person to talk with?”) are top performers, averaging a 46% open rate.
• Subject lines that include a call-to-action (44.6%), numbers (44%), and questions (44%) also significantly boost open rates.
• Descriptions/adjectives (39%) and company names (38%) land in the middle ground. While not detrimental, these terms might not grab attention unless paired with a more compelling hook.
• Subject lines loaded with marketing jargon, generic greetings (e.g., “Hello, friend”), and urgency-driven phrasing (e.g., “now”, “ASAP”) actively push engagement below 36%.

How subject line length impacts email open rates

• Email subject lines hitting the 2–4 word sweet spot consistently reel in the highest open rates, reaching 46%.
• Single-word subject lines, while concise, surprisingly underperform at a 38% open rate. They often lack context, appear automated, or get flagged as spam.
• Open rates start to drop noticeably beyond 7 words (39%), dropping further after 9 words (35%), with 10-word subject lines showing the lowest engagement (34%). This dip likely happens because longer lines look cluttered, are tougher to process quickly, dilute the core message, and get truncated after 35–50 characters on mobile.

How subject line casing affects user engagement 

While casing style alone won't drastically alter performance, it's the very first visual cue your readers pick up. Here's a fascinating glimpse into how casing plays out:

• Surprisingly, ALL CAPS subject lines pull ahead with the highest open rate at 30%. Despite their reputation for being aggressive or spammy, their advantage over other formats is present, if minimal (just 1–2%).
• Title case and lowercase both hold steady with solid performance (29%), hitting a sweet spot between readability and drawing the eye.
• Randomly mixing uppercase and lowercase letters (e.g., "GreAt OffEr") results in slightly lower engagement (28%), often appearing unprofessional or like a typo.
• Unexpectedly, the most “natural” style (with only the first letter capitalized) yielded the lowest open rate at 25%. While easy on the eyes, it likely lacks the visual punch needed to stand out in a jam-packed inbox.

How numbers in subject lines affect open rates

Though the difference is slight, this finding directly challenges the widespread assumption that throwing numbers or data points into subject lines always boosts performance.

The data reveals a marginally lower open rate (27%) for subject lines with numbers compared to those without (28%). This means using numbers isn't a guaranteed win.

“Audiences have grown more discerning, only responding when numbers are genuinely tied to curiosity or a clear, immediate value. Without meaningful context, numerical subject lines risk fading into the background noise rather than cutting through.”

Kiril Potapkin, head of content at Belkins

Relevant reading:

• What are B2B cold email response rates? Belkins’ 2025 study
• LinkedIn B2B outreach statistics: Belkins’ 2025 study
• What are B2B cold email deliverability rates? Belkins’ 2025 study
• Sales follow-up statistics in B2B: Belkins’ 2025 study

Sources

Unless otherwise specified, all deliverability figures and statistics in this report stem from Belkins’ proprietary dataset of over 5.5 million emails sent to our clients between January 1, 2024, and December 31, 2024. To enrich our internal data, we also teamed up with Reply.io, gathering their proprietary insights on how subject line formatting directly impacts open and reply rates.