The Automated Readability Index, or ARI, gives a rough school grade level for a piece of English writing. It only needs three counts: characters, words, and sentences. That simplicity is its strength, and also its limit, so use it as a guide rather than a verdict.
How it works
The formula is ARI = 4.71 x (characters / words) + 0.5 x (words / sentences) – 21.43. Characters means letters and numbers, not spaces or punctuation. Words per sentence captures sentence length, and characters per word captures word length. The result is rounded to the nearest whole number, which maps to a US school grade: 8 means about eighth grade, 12 about twelfth grade, and 14 and above means college level.
Step by step
- Pick a sample of at least 100 words from the text, ending at a sentence boundary.
- Count the words and the sentences in the sample.
- Count characters: letters and digits only, no spaces or punctuation.
- Divide characters by words, and words by sentences.
- Multiply the first result by 4.71 and the second by 0.5, add them, and subtract 21.43.
- Round to the nearest whole number and read it as a grade level.
Worked example
A sample has 120 words, 8 sentences, and 560 characters. Characters per word = 560 / 120 = 4.67. Words per sentence = 120 / 8 = 15. ARI = 4.71 x 4.67 + 0.5 x 15 – 21.43 = 22.0 + 7.5 – 21.43 = 8.07, which rounds to 8, about eighth-grade level. If the same sample had longer words, say 640 characters, the score would rise to about 11, showing how sensitive the index is to word length.
Common mistakes
- Counting spaces as characters, which inflates the score.
- Using a tiny sample, where one long sentence swings the whole result.
- Treating the grade number as exact. It is an estimate, and two tools can differ by a grade.
- Applying ARI to non-English text, which it was not designed for.
Try it with the free tool
You can follow every step above by hand, but the free tool below does the same calculation instantly. Try your own numbers in it, then check one result by hand so you trust the method as well as the answer. Open the full tool here: open the tool.
Checklist before you finish
- Write down every input number first, with its unit, before you calculate anything.
- Work through the steps in order, and do not skip the unit conversions.
- Check the answer against the worked example above: is it the right size, and does the direction make sense?
- Round only at the end, and keep the unrounded figure for any follow-up calculation.
Practice with your own numbers
Pick three cases of your own and work each one by hand. First, try a simple round-number case you can check mentally. Second, try a realistic case with awkward numbers, the kind you will actually meet. Third, try an edge case, such as a very small or very large input, and confirm the answer still behaves sensibly. If any case gives a surprising result, re-check the units and the formula step by step before you trust it.
If your answer looks wrong
Most errors come from a small set of causes. Check that you used the same units throughout, that you divided where the formula divides and multiplied where it multiplies, and that you applied any percentage as a decimal. Check the order of operations: powers and brackets come before multiplication and division, which come before addition and subtraction. Finally, test the method on the worked example in this guide. If your method reproduces the example answer, the method is right and the inputs deserve a second look.
Quick tips
Check two or three samples from different parts of a long document and average them. If the score is far above your audience, shorten sentences first; that usually moves the score more than swapping single words.