ARI uses characters per word and words per sentence to produce a U.S. grade level for your text.
Built and maintained by Paul Clark, Redmoon Software · Last verified · Sources
Use ARI when you need a fast, no-syllable readability check on technical text or live data streams. Its original purpose — grading Air Force training materials in real time — still makes it ideal for documentation, transcripts, and chat-log analysis.
ARI is essentially a faster cousin of Coleman–Liau. Both produce similar scores; ARI weights sentence length more, Coleman–Liau weights it less. Compared to Flesch–Kincaid, ARI is much more robust to vocabulary surprises.
The Automated Readability Index was developed in 1967 for the US Air Force as a fast readability check.
Like Coleman-Liau it relies on character counts, but it also factors in average sentence length directly.
ARI is particularly suitable for evaluating technical and instructional material.
Both rely on characters per word, but ARI also uses average sentence length while Coleman-Liau uses sentences per 100 words. They typically produce similar grade levels.
ARI was developed for the US Air Force in 1967, originally to grade training material.
Characters per word can be measured exactly by a computer, whereas syllable counting requires error-prone estimation. This makes ARI fully reproducible and well suited to automated tools.
ARI outputs a U.S. grade level, so a score of 8 corresponds to roughly an eighth-grade reading level. Lower scores indicate easier text and higher scores indicate more demanding writing.
Input
ARI uses characters per word and average sentence length to estimate a grade level.
Output
Automated Readability Index: 9.6 — 9th–10th grade.
Average 4.4 characters per word and 14 words per sentence. ARI adds the two with empirical weights to land on a US grade level near 10.
The Automated Readability Index, like Coleman–Liau, works from characters per word and words per sentence rather than syllables, and for the same reason: it was built in 1967 for machine scoring of US Air Force technical material, where syllable counting was impractical.
ARI and Coleman–Liau use the same two inputs with different coefficients, so they rarely disagree by much. Where they do, ARI tends to weight sentence length slightly more heavily. Running both is mostly a consistency check rather than two independent opinions.
ARI's output maps to a US grade level, and unlike most of the family it was designed with a direct age interpretation in mind — grade 8 corresponds to roughly a 13–14 year old reader. That makes it convenient when writing for a specific age band rather than a school system.
The usual caveat applies with more force than usual on short text: both of ARI's inputs are averages, and averages over three sentences are not stable. Give it a few paragraphs before treating a decimal place as meaningful.
The method behind this tool, so you can check it rather than take it on trust.
Smith, E.A. & Senter, R.J. (1967). “Automated Readability Index.” Aerospace Medical Research Laboratories, AMRL-TR-66-22.
Character-based like Coleman–Liau, and built for machine scoring of technical material.
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