Two questions most email marketers can't answer with confidence: how long should this email actually be, and is it easy enough to read? The honest answer to both is: it depends — but not on gut feeling. There are clear, measurable signals that tell you whether your email copy is working for your reader or against them.
This guide covers the right word count for different email types, what the Flesch readability score actually means in plain English, how sentence length affects whether people reach your CTA, and why certain words in your email body copy — not just your subject line — can damage your deliverability without you ever knowing.
How long should a marketing email be?
There's no single correct answer, but there is a range that works for most email types — and clear reasons why straying too far in either direction costs you opens, clicks, and conversions.
What is the Flesch readability score — and why does it matter for email?
The Flesch Reading Ease score is a formula developed in 1948 by Rudolf Flesch, a readability researcher who wanted a way to objectively measure how easy a piece of writing is to understand. It's been used by publishers, government agencies, and educators ever since — and it turns out to be remarkably useful for email copy too.
In plain English: it measures how hard your writing is to read. It does this by looking at two things — how long your sentences are on average, and how many syllables your words have on average. Short sentences with short words score high (easy). Long sentences with long words score low (difficult).
The score runs from 0 to 100. Here's what each range means in practice for email:
The target for most marketing emails is 70 or above. That's not dumbing things down — it's respecting the fact that your reader is probably on their phone, scanning quickly, and hasn't dedicated focused attention to your email. It's the register of a good conversation, not a university essay.
How to improve a low Flesch score
Two levers move the score significantly. You don't need to change what you're saying — just how you're saying it.
Shorter sentences. If your average sentence length is over 20 words, cut it. Split long sentences in two. Use a full stop where you'd naturally pause in speech. The most effective technique: read your email out loud. Every time you run out of breath, that's a sentence break.
Simpler words. Replace long words with shorter equivalents where you can. "Use" instead of "utilise". "Help" instead of "facilitate". "Show" instead of "demonstrate". You're not losing precision — you're removing the friction between your words and your reader's understanding.
Same information. Same message. The second version takes less than half the words, scores 40 points higher on Flesch, and actually reads like a human wrote it.
Sentence length — the metric most copywriters ignore
Average sentence length is one of the most underrated signals in email copy. It's closely related to the Flesch score, but it tells its own story too — because a long average sentence length means your reader is having to hold more information in working memory before reaching the relief of a full stop.
The target for email copy is under 15 words per sentence on average. That's not a strict rule — variety in sentence length is actually what makes copy feel natural and engaging. Short punchy sentences create pace. Longer ones slow down for detail. But if your average creeps above 20, you'll start losing readers before they reach your CTA.
Skimmability — because most people won't read every word
Here's the uncomfortable truth about email marketing: most of your subscribers won't read your email. They'll skim it. They'll look at the headline, scan a few lines, catch a bullet point or two, and decide whether to click based on that 5-second impression.
That's not laziness — it's how humans process information when they're busy. The question isn't "how do I make them read every word?" It's "how do I make the important stuff impossible to miss for a skimmer?"
Three things make emails skimmable:
- Short paragraphs. One idea per paragraph. Two or three sentences maximum. White space is not wasted space — it's visual breathing room that makes the copy feel approachable rather than dense.
- Bullet points for lists. Any time you find yourself writing "firstly... secondly... thirdly...", stop and use a bullet list instead. Three or more items in a sentence belong in a list. The Read Time Optimiser will flag when your email is long enough to benefit from bullet points but doesn't have any.
- A CTA that stands out. Your call to action should be impossible to miss for a skimmer. If it's buried in the third paragraph of dense prose, a skimmer will never see it. Put it on its own line. Make it obvious.
The bit most people don't think about — inappropriate language in email body copy
You're probably already aware that spam trigger words in your subject line can damage deliverability. What many marketers don't realise is that spam filters scan your entire email — not just the subject line. Inappropriate language anywhere in your email body can trigger the same filters, with the same consequences.
This matters more than it sounds. It's not just obvious profanity — it's any language that spam filter algorithms associate with unsolicited or harmful content. And the damage isn't always immediate. Every time one of your emails gets caught by a spam filter, your sender score takes a small hit. Enough hits over time and your emails start defaulting to junk for everyone on your list — including the subscribers who genuinely want to hear from you.
The Read Time Optimiser scans your full email body for inappropriate language and flags it with a clear warning before you send. It's not prudish — it's practical. A single inappropriate word in an otherwise excellent email can send it to junk for your entire list.
The Email Read Time Optimiser — what it checks
The Email Read Time Optimiser on Mailfois analyses your full email body and gives you an instant, plain-English breakdown of everything that matters for readability and deliverability. It's free, runs in your browser, and requires no account.
Quick reference — email readability checklist
- ✅ Aim for 50–150 words for most marketing emails
- ✅ Target a Flesch score of 70 or above — easy and conversational
- ✅ Keep average sentence length under 15 words
- ✅ Use bullet points for any list of three or more items
- ✅ One idea per paragraph — short paragraphs, plenty of white space
- ✅ CTA on its own line, visible to a skimmer
- ✅ Check for inappropriate language before every send
- ❌ Don't send emails under 20 words — spam filter risk
- ❌ Don't write sentences over 25 words without a very good reason
- ❌ Don't bury the CTA in dense prose
- ❌ Don't assume spam filters only look at the subject line
