Automated Readability Index calculator

Paste your text to get its Automated Readability Index (ARI) — a US grade level worked out from characters per word and words per sentence — with the working shown. Free, instant, and your text never leaves your browser.

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The Automated Readability Index formula

ARI = 4.71 × (characters ÷ words) + 0.5 × (words ÷ sentences) − 21.43

Characters ÷ words — average word length, counting letters and digits but not spaces or punctuation.
Words ÷ sentences — average sentence length. Every two extra words per sentence adds a grade.
The result — an approximate US school grade. Add five for a rough reading age.

Worked example

Here is the Automated Readability Index worked through for our sample bedtime story — press Try a sample story above to load it. It’s a short tale about an owl who can’t sleep, and it counts up like this:

Words: 118
Sentences: 18
Characters: 480

4.71 × (480 ÷ 118) + 0.5 × (118 ÷ 18) − 21.43 = 1.0

So the story scores 1.0 — Grade 1 · ages 6–7.

What your score means

Score	Level	Who reads it easily
1 or below	Kindergarten–1st grade	Ages 5–7 · picture books, first readers
2–3	2nd–3rd grade	Ages 7–9 · early chapter books
4–5	4th–5th grade	Ages 9–11 · confident young readers
6–8	Middle school	Ages 11–14 · most adults read this comfortably
9–12	High school	Ages 14–18 · news features, business writing
13–16	College	Academic and specialist writing
17+	Graduate	Dense professional or scientific prose

What is a good ARI score?

For writing aimed at most adults, an ARI of 8 or below is a sensible target. Scores in the teens usually mean long sentences that could be split.

For children’s writing, aim at or just below the reader’s school grade for independent reading. Very simple stories often score below zero on ARI, which simply means they’re easier than kindergarten level.

Where it comes from

E. A. Smith and R. J. Senter developed the ARI in 1967 for the US Air Force. It was designed to be computed automatically while text was typed, using counters for keystrokes, words, and sentences on an electric typewriter — hence “automated”.

Because it needs only character, word, and sentence counts, the ARI is fast and unambiguous to compute — which is why it still turns up in software readability checks today.

Limitations

Character counts are a rough proxy for difficulty: long, everyday words raise the score and short technical terms don’t. Formatting that hides sentence breaks — lists, headings, bullet points — also inflates the words-per-sentence figure.

Some older reference tables shift ARI scores by one grade (treating a score of 1 as kindergarten). This calculator reads the result directly as a US grade, consistent with the other formulas here.

How to calculate ARI by hand

Count the words, the sentences and the characters — letters and digits only. Divide characters by words and multiply by 4.71; divide words by sentences and multiply by 0.5. Add the two and subtract 21.43.

A handy shortcut: everyday English averages about 4.5 characters per word, and at that length the character term almost exactly cancels the 21.43. So for ordinary prose, ARI comes out close to half the average sentence length — 16-word sentences give roughly grade 7.8.

Why ARI scores simple stories so low

Children’s stories use short words, which pushes the character term below zero on its own. Our sample bedtime story averages 4.07 characters a word, worth −2.3 grades before sentence length is added.

Its 6.6-word sentences then add 3.3, for a final ARI of 1.0. Syllable-based formulas like SMOG, which can’t go below about 3, rate the same story higher — so compare a story against itself on ARI, not against another formula.

Frequently asked questions

What is the Automated Readability Index?

A readability formula from 1967 that estimates the US school grade needed to understand a text, using average characters per word and average words per sentence.

How is ARI different from other readability formulas?

It counts characters rather than syllables, so it’s quicker and more consistent to compute. Coleman-Liau takes a similar approach, while Flesch-Kincaid, Gunning Fog, and SMOG all rely on syllable estimates.

What does an ARI score of 6 mean?

The text should suit a typical 6th grader, around age 11 to 12 — comfortable reading for most adults.

Does ARI count spaces and punctuation?

No. Characters here means letters and digits only. Punctuation is used only to work out where sentences end.

Can the ARI be negative?

Yes. Short words in short sentences — typical of first readers — can push ARI below zero. Read any negative score as kindergarten level or easier.