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Automated Readability Index

A readability grade based on characters rather than syllables.

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Developed for the US Air Force in 1967 so readability could be calculated by a machine that could count keystrokes but not syllables. That makes it exactly reproducible.

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US school grade level

Your longest sentences

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Quick Tool Overview

The Automated Readability Index counts characters rather than estimating syllables, which makes it fully reproducible — two implementations always agree on the same text, which is not true of any syllable-based formula. It returns a US grade level.

What it does
Calculates the Automated Readability Index
You give it
A hundred words or more
You get back
A US grade level from exact character counts
Best for
Reproducible scores and automated pipelines
Text Tools Free, unlimited use No sign-up Runs in your browser

About the Automated Readability Index

Every syllable-based readability formula has the same weakness: syllable counts are estimated by rule, and the rules get English wrong often enough to move a score. The Automated Readability Index avoids that entirely by counting characters, which is exact.

It was developed in 1967 for the US Air Force, specifically so that readability could be calculated automatically by a typewriter that could count keystrokes but had no idea what a syllable was. That constraint turned out to be a strength: ARI is fully reproducible, and two implementations will always agree on the same text, which is not true of any syllable-based formula. It returns a US grade level like Flesch-Kincaid, and the two usually land within a grade of each other.

Key features of the Automated Readability Index

No syllable estimation

Characters are counted exactly, so the score is identical wherever it is calculated.

Exact inputs shown

Characters per word and words per sentence, both measured rather than approximated.

Comparison with Flesch-Kincaid

Shown side by side, with the disagreement explained when the two diverge.

Correct character definition

Letters and digits only, with punctuation and whitespace excluded as the specification requires.

Its origin explained

Built for a typewriter that could count keystrokes but not syllables — which is why it works this way.

How to use the Automated Readability Index

  1. 1

    Paste your text

    At least a hundred words for a meaningful result.

  2. 2

    Read the grade level

    A US school grade, comparable to Flesch-Kincaid.

  3. 3

    Check the two inputs

    Characters per word and words per sentence, both counted exactly rather than estimated.

  4. 4

    Compare with a syllable formula

    Where ARI and Flesch-Kincaid disagree, the text usually has short words with many syllables, or the reverse.

Frequently asked questions

Because character counts are exact and syllable counts are estimates. Every syllable-based formula uses a rule that misjudges some English words, so two tools can disagree on the same text. ARI cannot.

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