Decoding the Fog Index: A Readability Yardstick for Technical Writing

Every technical document carries an invisible barrier between the writer and the reader. Sometimes that barrier is thin, and the message lands clearly on the first read. Other times it is thick with long sentences and heavy vocabulary, leaving the reader confused. The Fog Index is a tool that measures the thickness of this barrier. It gives writers a single number that tells them how hard their text is to understand. For anyone studying technical writing, where clarity is the entire point, this number is one of the most useful diagnostic tools available.

What is the Fog Index? 🔗

The Fog Index, more formally called the Gunning Fog Index, is a readability test for English writing. It estimates the number of years of formal education a person needs to understand a piece of text on a first reading. A score of 12, for example, means the text demands roughly the reading level of someone finishing high school. The lower the score, the easier the text is to read.

The index was developed in 1952 by Robert Gunning, an American businessman with experience in newspaper and textbook publishing. Gunning had set up a consultancy in 1944 to help businesses write more clearly. He believed that most public and business writing was full of “fog” – unnecessary complexity that obscured the message rather than conveying it. His goal was to give writers a simple yardstick they could apply by hand, without any special equipment, to check whether their writing matched their audience.

The name captures the idea well. When writing is foggy, readers cannot see the meaning clearly. They have to slow down, reread, and work hard just to follow the argument. The Fog Index puts a number on that struggle.

Why it matters for technical writing 🔗

Technical writing exists to transfer information accurately and efficiently. Whether the document is a user manual, an installation guide, a government circular, or an academic abstract, the reader needs to understand it without effort. The main goal of any technical communicator is to make information readable so the audience can engage with and understand it as effectively as possible.

This is exactly where the Fog Index earns its place. It flags two of the most common problems in technical prose: sentences that run too long and vocabulary that is heavier than it needs to be. By measuring both at once, it gives the writer an early warning that the text may be drifting away from its intended reader.

Calculating Fog Index 🔗

The strength of the Fog Index is its simplicity. The whole calculation rests on two ideas: how long your sentences are, and how many heavy words you use. The complete formula is:

Fog Index = 0.4 × [ (Total words ÷ Total sentences) + 100 × (Complex words ÷ Total words) ]

Each part of this formula has a clear meaning. Working through it step by step makes the logic easy to follow.

Step one: average sentence length 🔗

First, you choose a sample of writing at least 100 words long and count the total number of words. Then you divide that by the number of sentences. This gives the average sentence length. The reasoning is straightforward. Longer sentences hold more ideas, more clauses, and more pauses, all of which make the reader work harder. Shorter sentences are easier to process, so they lower the fog.

Step two: percentage of complex words 🔗

Next, you count the complex words. In Gunning’s system, a complex word is one with three or more syllables. However, the rules exclude certain categories: proper nouns, familiar jargon, and compound words do not count, and common suffixes such as -es, -ed, or -ing should not be treated as an extra syllable. You divide the number of complex words by the total number of words, then multiply by 100 to get a percentage. This is the share of “heavy” vocabulary in your text.

Step three: combine and scale 🔗

Finally, you add the average sentence length to the percentage of complex words, then multiply the result by 0.4. This scaling factor converts the combined figure into a number that lines up with school grade levels. A result of 10.4, for instance, points to roughly a tenth-grade reading level.

To see this in practice, imagine a 100-word passage written in 5 sentences, containing 12 complex words. The average sentence length is 100 ÷ 5 = 20. The percentage of complex words is (12 ÷ 100) × 100 = 12. Adding these gives 32, and multiplying by 0.4 gives a Fog Index of 12.8. That score tells you the passage is pitched at around the level of a high school senior or a first-year college student.

What the formula reveals to a writer 🔗

One useful feature of the Fog Index is that it points directly to the fix. If your score is too high, you have only two levers to pull. You can break long sentences into shorter ones, or you can replace some complex words with simpler alternatives and cut unnecessary padding. For technical documents that genuinely require long terms, the more practical route is usually to shorten sentences and structure paragraphs better, which improves flow without losing precision.

Ideal Fog Index ranges 🔗

There is no single “good” score that applies to every document. The right range depends entirely on who will read the text. The purpose of the Fog Index is not to push every piece of writing toward the lowest possible number, but to match the writing to the audience.

As a general guide, text for a wide audience usually needs a Fog Index below 12, and text meant for near-universal understanding should sit below 8. Many writing experts treat a score of around 8 or 9 as a comfortable target for general readers, since this matches the reading level of a large portion of the public.

Ranges by type of document 🔗

Different kinds of writing naturally fall into different bands. A general, non-technical audience typically reads somewhere between the 7th and 11th grade level, while a reputable newspaper may run closer to grades 12 to 14. Expert technical audiences can handle higher reading levels because they already know the vocabulary of their field.

For everyday technical documentation aimed at end users, many practitioners recommend keeping the Fog Index below 13. Public-facing materials such as government notices, instruction sheets, and safety information benefit from even lower scores, so that readers with varying literacy levels can follow them. This matters in a country like India, where documents often need to reach a population with a very wide range of educational backgrounds.

Why higher is not always wrong 🔗

A high score is not automatically a failure. If you are writing a manual for trained engineers or a research abstract for specialists, a higher Fog Index may be perfectly appropriate. The key principle is to write a couple of grades below where your readers actually are. This gives readers headroom, so they spend their energy on understanding the content rather than decoding the language.

Knowing the limits of the index 🔗

The Fog Index is a guide, not a verdict. It has a well-known blind spot: it treats every three-syllable word as equally difficult. As one analysis notes, the index sees the everyday word “message” and the obscure word “heterozygote” as carrying the same degree of complexity, even though one is far harder than the other. So a long but familiar word can inflate your score unfairly. Because of this, the index works best when paired with your own judgement. You remain the final arbiter of whether your writing is clear, and the number is simply there to keep you honest.

Putting the Fog Index to work 🔗

For a technical writer, the most valuable habit is to check readability while drafting and revising, not only at the end. A quick Fog Index calculation on a sample paragraph can reveal whether your sentences have crept too long or your vocabulary has grown too dense. Over time, the practice trains your instinct. Interestingly, Gunning himself reportedly began favouring simpler prose only after his own writing was tested and found difficult, which shows how the act of measuring can quietly reshape how you write.

Used this way, the Fog Index becomes more than a score. It becomes a steady reminder that the reader, not the writer, decides whether a document has succeeded.

What do you think? Would the technical documents you read most often pass a Fog Index test for their intended audience, or are they written above the level of the people meant to use them? And where would you draw the line between simplifying language for clarity and oversimplifying it to the point of losing precision?

References

1. https://en.wikipedia.org/wiki/Gunning_fog_index
2. https://readable.com/readability/gunning-fog-index/
3. https://courses.lumenlearning.com/suny-esc-technicalwriting/chapter/usability/
4. https://arxiv.org/pdf/2001.01863
5. https://library.guilford.edu/c.php?g=111810&p=723930
6. https://readabilityformulas.com/the-gunnings-fog-index-or-fog-readability-formula/
7. https://clickhelp.com/clickhelp-technical-writing-blog/gunning-fog-index-how-to-calculate-and-example/
8. https://medium.com/technical-writing-is-easy/readability-metrics-and-technical-writing-b776422eaba
9. https://readable.com/blog/readability-for-technical-writers/
10. https://arxiv.org/pdf/2210.00553