The Gunning Fog Index: Measuring Readability in Communication

Most People Aren’t Reading What You Write

That policy document you spent three weeks drafting? The all-staff email announcing the restructure? The strategy deck needed took two senior leaders and a consultant to produce? There is a reasonable chance that the majority of your intended audience skimmed the first paragraph, understood perhaps half of it, and moved on. This is not because your colleagues are lazy or unintelligent. It is because most organisational writing is significantly harder to read than it needs to be, and people are busy, distracted, and reading on their phones between meetings.

Robert Gunning understood this problem in the 1950s. His solution was characteristically blunt: a formula that could tell you, with uncomfortable precision, just how impenetrable your writing had become. He called it the Fog Index.

Robert Gunning and the War on Fog

Gunning was an American businessman who made his living telling newspapers and corporations that their writing was, unnecessarily difficult. Working initially with American newspapers in the 1940s, he observed that circulation figures correlated with readability: papers that wrote more clearly sold more copies. This was not a revolutionary insight, but Gunning had the good sense to build a measurement tool around it, which gave the observation commercial power.

His 1952 book, The Technique of Clear Writing, laid out both the philosophy and the formula. Gunning argued that unclear writing was not a sign of sophistication but of muddled thinking, and that the “fog” of unnecessarily complex prose was costing businesses real money in misunderstandings, wasted time, and poor decisions.

He went on to consult for over a hundred major American organisations, including newspapers, insurance companies, and government agencies. In our experience, the problems he identified in the 1950s have not appreciably improved; if anything, the proliferation of corporate jargon and the pressure to sound “strategic” have made things worse.

How the Formula Works

The Gunning Fog Index is calculated in three steps:

1. Calculate the average sentence length (total words divided by total sentences) in a sample of text.
2. Calculate the percentage of complex words, defined as words with three or more syllables, excluding proper nouns, familiar compound words, and common suffixes like “-ed,” “-es,” or “-ing.”
3. Add the average sentence length to the percentage of complex words, then multiply the result by 0.4.

The resulting number is an estimate of the years of formal education a person would need to comfortably understand the text on a first reading. A score of 6 suggests the writing is accessible to someone with a primary school education. A score of 12 corresponds roughly to a secondary school leaver. Scores above 17 indicate prose dense enough to require postgraduate training, or at least a strong cup of coffee and a willingness to re-read sentences.

For context:

• most popular newspapers aim for a Fog Index between 9 and 12.
• Academic journals routinely score above 17.
• The average internal corporate communication is around 14 to 16.

The Readability Landscape: Fog Among Friends

The Gunning Fog Index is not the only formula in this territory. Several other readability measures emerged during the mid-twentieth century’s enthusiasm for making communication measurable, and it is worth knowing how they differ.

The Flesch Reading Ease score, developed by Rudolf Flesch in 1948, produces a number between 0 and 100, where higher scores indicate easier reading. It uses average sentence length and average syllables per word. The closely related Flesch-Kincaid Grade Level converts this into a US school grade, making it functionally similar to the Fog Index in output if not in method.

The SMOG Index (Simple Measure of Gobbledygook, which is a name that deserves more appreciation than it gets) focuses specifically on polysyllabic words and is often favoured in health communication research, where ensuring comprehension can be a matter of patient safety rather than mere convenience. The Coleman-Liau Index uses character counts rather than syllable counts, which makes it easier to compute programmatically and slightly less subjective in application.

All of these formulas share the same basic intuition: shorter sentences and simpler words are, on average, easier to understand. They differ in the mathematical weighting, the definition of “complex,” and the scale of the output.

Why This Matters in Organisations

If readability were merely an aesthetic concern, it would be a nice-to-have for communicators and an irrelevance for everyone else. But unreadable organisational writing has real consequences that compound quietly over time.

Consider the humble policy document. Most organisations have dozens of them, covering everything from expenses to data protection to grievance procedures. These documents exist, in theory, so that people know what to do and what is expected of them. In practice, many are written in a style so dense and legalistic that employees either never read them or, having read them, come away with a confident misunderstanding of what they actually say. When something goes wrong and a manager says, “But it’s in the policy,” the uncomfortable follow-up question is: was the policy written in a way that anyone would actually absorb?

Strategy documents present a different flavour of the same problem. Leadership teams often produce beautifully designed strategy decks that score above 18 on the Fog Index. The language is abstract, the sentences are long, and the key messages are buried under layers of qualifying clauses. The irony is that these documents are supposed to create alignment and clarity, yet the writing itself works against both.

Even everyday emails contribute to organisational fog. A manager who writes in long, complex sentences is not demonstrating rigour; they are creating work for everyone who has to decode their messages. Multiply that across an organisation of several hundred people sending dozens of emails a day, and the cumulative cost in time, confusion, and quiet frustration is substantial.

The Paradox of Simplicity

However, not all complexity in writing is bad. Some ideas are inherently complex, and attempting to express them in short sentences with small words can distort or trivialise them. A legal contract needs precision. A clinical protocol needs specificity. A philosophical argument sometimes needs a sentence that unfolds across forty words because the thought itself has that structure.

There is also the question of condescension. Writing that is aggressively simplified can feel patronising to an educated audience, and there are contexts where your readers are specialists who find technical vocabulary more efficient, not less. A cardiologist does not need you to say “the tube that carries blood from the heart” when “aorta” will do. Among experts, jargon is not fog; it is shorthand, and the Fog Index (and other similar tools) cannot distinguish between the two.

The real skill, then, is not in achieving the lowest possible score but in achieving the appropriate score for your audience and purpose. A company-wide email about a benefits change should probably sit around 10 to 12. A technical specification shared between engineers might legitimately score 16. The question is not “Is this too complex?” but rather “Is this more complex than it needs to be for the people who will actually read it?”

Practical Applications for Leaders and Communicators

Despite its limitations, the Fog Index remains a genuinely useful tool when treated as a diagnostic rather than a directive. Here are some practical ways to use it:

Audit your most important documents. Run your key policies, strategy summaries, and all-staff communications through a Fog Index calculator (there are many free ones available online). If your employee handbook scores above 16, it is worth asking whether that level of complexity is serving anyone, or merely protecting the writer from the effort of writing more clearly.

Know your audience. Different audiences within your organisation have different reading levels, different levels of familiarity with your terminology, and different amounts of time to spend decoding your prose. A communication aimed at frontline workers should not read like a board paper, and a board paper should not read like a press release. Adjust accordingly.

Use the score as a conversation starter, not a verdict. If a document scores 18 on the Fog Index, the right response is not necessarily to simplify it but to ask why it scores that high. Is it because the subject matter demands technical language? Or is it because the writer has defaulted to long, convoluted sentences out of habit or a misguided sense that complexity signals authority? The former is defensible. The latter is fixable.

Pair readability with structure and design. A Fog Index of 14 in a well-structured document with clear headings, short paragraphs, and good visual design is vastly more readable than the same score in a wall of unbroken text. Readability formulas measure linguistic complexity, but actual readability is also a function of layout, typography, and information architecture.

Write, then simplify. Most of us write our first drafts at a higher reading level than necessary because we are thinking through the ideas as we write. The revision process is where clarity happens. Read your draft aloud. If you stumble over a sentence, your reader will too. Cut unnecessary qualifications. Break long sentences where the thought naturally divides. Replace jargon with plain language wherever doing so does not sacrifice meaning.

Learning More

To go deeper, explore these resources on people-shift.com:

• The 7 C’s of Communication
• The ABCs of Communication
• The Five Canons of Rhetoric
• The Rhetorical Triangle
• Think, Feel, Do

The People Shift View

We like and dislike tools like this. Just like we like and dislike SEO tools that provide feedback on our articles etc. They’re helpful sign-posts, but they also really annoy us. We very much recognise that lots of writing is long and convoluted unnecessarily, but that’s not always the case. Sometimes we need complex words, and sometimes style and personality need latitude to present themselves. 

Our basic advice to to really think about who you’re writing for. Who is the actual audience? And to ask yourself, are you writing what they want to read? Or are you writing what you want to write? 

Also – remember that long words and sentences don’t always make you look smart and powerful. Some people think they do, that they add gravitas etc. But our view is that they often do the opposite really. It’s often considerably more effective (and commensurately more difficult) to express things in the simplest way possible that is still sufficient to convey what needs to be conveyed. 

We love the old and well-worn quote covering all of this that is often attributed to Mark Twain too: “I didn’t have time to write a short letter, so I wrote a long one instead”.