Which Readability Formula to Use: A Practical Guide
Six readability formulas compared: who made each one, what it counts, and how to choose. Includes two passages scored on all six.
There are dozens of readability formulas. Six of them cover almost every tool you will meet. This post explains what each one counts and how to choose.
The six formulas
| Formula | Published | What it counts |
|---|---|---|
| Flesch Reading Ease | Rudolf Flesch, 1948 | Words per sentence, syllables per word. Score from 0 to 100, higher is easier |
| Flesch-Kincaid Grade Level | Kincaid and colleagues, US Navy, 1975 | The same two inputs, turned into a US grade |
| Gunning Fog | Robert Gunning, 1952 | Words per sentence, plus the share of words with 3+ syllables |
| SMOG | G. Harry McLaughlin, 1969 | Words of 3+ syllables in a 30-sentence sample |
| Coleman-Liau | Coleman and Liau, 1975 | Letters per 100 words, sentences per 100 words |
| Automated Readability Index (ARI) | Smith and Senter, 1967 | Characters per word, words per sentence |
They fall into two groups. The first four count syllables. The last two count letters, which a computer can do without guessing.
All six measure the same two things in different ways: how long your sentences are, and how long your words are. None of them reads for meaning.
Two passages, six scores
Scored on this site's calculator on 9 October 2026. The text was written for this post.
A medicine instruction
Take one tablet by mouth twice a day with food. Do not take more than two tablets in one day. If you miss a dose, take it as soon as you remember. Call your doctor if you feel dizzy, have trouble breathing, or notice swelling in your face or hands.
A developer note
The application authenticates each request using a bearer token issued by the authorization server. Tokens expire after sixty minutes, and the client must request a replacement before continuing. Configure the redirect address in the developer console before deploying to production.
| Formula | Medicine instruction | Developer note |
|---|---|---|
| Flesch Reading Ease | 92.6 | 17.8 |
| Flesch-Kincaid | 3.4 | 14.1 |
| Gunning Fog | 5.8 | 14.3 |
| SMOG | 5.7 | 12.5 |
| Coleman-Liau | 4.4 | 17.3 |
| ARI | 3 | 14 |
Two things stand out. Every formula agrees on which passage is harder. And they disagree on the number, by more than two grades for the first passage and more than four for the second.
That spread is normal. A 2013 review found formulas varied by up to five grade levels on the same text. Short samples like these make it worse.
How to choose
Use what your reviewer uses. If your client or compliance team checks a Flesch-Kincaid grade, use that. Microsoft Word reports Flesch Reading Ease and Flesch-Kincaid, so those two are the ones most people can check for themselves.
For health materials, consider SMOG. The same 2013 review concluded that SMOG "appears best suited for health care applications." SMOG aims at full comprehension, so it gives a more cautious number. See our SMOG guide.
For text full of unusual terms, add a letter-based formula. Product names and technical terms can fool a syllable counter. Coleman-Liau and ARI avoid that. See how Coleman-Liau works.
When in doubt, look at several. If five formulas say grade 8 to 10 and one says 14, look at what that one counts. This site's calculator shows all six and an average.
Rules that apply to every formula
- Name the formula. "Grade 8" means little unless you say which formula produced it.
- Use the same formula before and after. Do not compare a Flesch-Kincaid score with a SMOG score.
- Score enough text. A few sentences can swing by several grades on one word. SMOG was designed for 30 sentences.
- A score is a prediction. FDA's guidance on medical device labeling says formulas "should be used to predict (not measure) the reading level of the text."
- No law names a formula. The Plain Writing Act of 2010 requires clear writing and sets no formula or grade. Our posts on government writing and healthcare rules give the details.
Suggested starting points
These are our suggestions, not standards.
| Writing | A reasonable first choice |
|---|---|
| General web and business writing | Flesch-Kincaid |
| Patient and health materials | SMOG |
| Technical docs with many product terms | Coleman-Liau or ARI as a cross-check |
| Anything you must defend to a reviewer | Whichever formula the reviewer names |
For more on each formula, see the guide to all six.
How we checked
Formula authors and dates come from the original papers and from the 1975 Navy report that compared several of them. Both passages were scored on this site's calculator. An earlier version of this post included a table of scores that did not reproduce, and it has been replaced.
Frequently asked questions
Which readability formula is the most accurate?
None is proven best for all text. A 2013 review found the formulas varied by up to five grade levels on the same health text. Pick one that suits your field, name it when you report a score, and use it consistently.
Which formula should I use for health materials?
SMOG is a sound choice. A 2013 review concluded it is best suited for health care applications, and a 2010 study also recommended it. No regulator requires it.
Does any law require a specific readability formula?
No US federal rule we have read names one. The Plain Writing Act of 2010 requires clear writing and sets no formula or grade level. Your own organisation or client may set one, so ask.