Text Readability Score API

About the Text Readability Score API

A readability score API that grades how hard a piece of text is to read. Readability Score works by analyzing the text provided and calculating the readability score with seven established readability formulas, including the Flesch Reading Ease score, Flesch Kincaid grade level, Gunning Fog index and Dale Chall readability, in a single call.

Alongside the raw scores you get plain-language labels such as "fairly difficult", an averaged reading grade level and a suggested target audience from elementary to post-graduate, plus the syllable, word and sentence counts behind the numbers. Content teams use it to check articles against a house reading level, publishers and educators use it to match texts to readers, and product and legal teams use it to keep help pages, notices and consent text readable.

Every field is returned on every plan, including the Free plan, so you can build a full readability report or a simple pass and fail check from the same response.

How it works

Readability Score first replaces every tab, carriage return and line feed in text with a space and trims it; empty text returns a 400 error. Because line breaks become spaces, a heading or bullet without terminal punctuation runs into the next sentence, which raises the sentence-length based scores. Add periods to list items if you want them counted as separate sentences.

textCounts.syllableCount, lexiconCount (words) and sentenceCount come from the text-readability library, as do Flesch Reading Ease, Flesch-Kincaid Grade, Gunning Fog, Coleman-Liau, the Automated Readability Index and the New Dale-Chall score. smogIndex is taken from the readability-scores library instead. These are standard formulas over syllables per word, words per sentence, characters per word, polysyllabic word counts and, for Dale-Chall, the share of words outside a list of familiar words.

All grade-style metrics and the Dale-Chall score are floored at 1. fleschReadingEase is not floored: it runs roughly 0 to 100 with higher meaning easier, and very dense text can go below 0. Its label in readabilityText uses lower bounds of 90 very easy, 80 easy, 70 fairly easy, 60 standard, 50 fairly difficult, 30 difficult, and very confusing below that. The Dale-Chall label maps whole-number bands from "average 4th-grade student or lower" up to "13th to 15th-grade (college) student" for 9 and above.

averageGradeLevel is the mean of Flesch-Kincaid, Gunning Fog, Coleman-Liau, SMOG and ARI, rounded to one decimal. targetAudience reads from it: 16 and above post-graduate, 13 college, 9 high school, 6 middle school, otherwise elementary. Short samples of a sentence or two produce unstable scores. Results are identical on every plan.

What people use it for

Editorial CMS Publishing

Before publishing articles, newsroom CMS editors check the Flesch-Kincaid grade level and word counts to match target reader expectations.

Patient Instructions Review

When clinicians draft care summaries, health portals flag passages scoring above an elementary target audience so staff can rephrase dense medical terms.

Curriculum Reading Assessment

To assign books to suitable grade bands, edtech apps evaluate uploaded chapters against Dale-Chall scores and the average grade level.

Consumer Contract Auditing

Flag convoluted policy clauses during drafting by checking consumer agreements against the Gunning Fog index and reading ease ratings.

Ways to call it

One endpoint, many ways in — REST with JSON, XML, YAML and CSV, plus GraphQL and an MCP interface for AI agents.

JSON

Default REST response

XML

Markup format

YAML

Human-readable

CSV

Tabular export

Beta

GraphQL

Query language

New

MCP

For AI agents

Request parameters — POST /v1/readabilityscore

Sent as JSON in the request body. Premium parameters are accepted on every plan but only take effect on plans that include them. Anything not listed here is dropped rather than passed through.

Required

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curl -X POST "https://api.apiverve.com/v1/readabilityscore" \
  -H "x-api-key: YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"text":"Western astrology is founded on the movements and relative positions of celestial bodies such as the Sun, Moon and planets, which are analysed by their movement through signs of the zodiac (twelve spatial divisions of the ecliptic) and by their aspects (based on geometric angles) relative to one another."}'

Authentication

Send your key in the x-api-key header. That is the only auth step — no token exchange, and no per-endpoint scope to configure.

Response — POST /v1/readabilityscore

Every APIVerve endpoint returns the same three top-level keys, so one response handler covers your whole integration: status, error and data. Only data changes shape.

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{
  "status": "ok",
  "error": null,
  "data": {
    "textCounts": {
      "syllableCount": 82,
      "lexiconCount": 49,
      "sentenceCount": 1
    },
    "readability": {
      "fleschReadingEase": 13.28,
      "fleschKincaidGrade": 23.6,
      "gunningFog": 26.95,
      "colemanLiauIndex": 13.19,
      "smogIndex": 22.08,
      "automatedReadabilityIndex": 27.8,
      "daleChallReadabilityScore": 17
    },
    "readabilityText": {
      "fleschReadingEase": "very confusing",
      "daleChallReadabilityScore": "average 13th to 15th-grade (college) student"
    },
    "averageGradeLevel": 22.7,
    "targetAudience": "post-graduate"
  }
}

Response fields

Paths are relative to data. Premium fields are absent rather than zeroed on plans that do not include them, so check for presence instead of comparing to 0.

Response headers

Every response carries your balance, so your own code always knows where it stands without polling anything. All four are exposed to browsers through CORS.

Errors

Read the HTTP status first, then error for the specific reason. The body names the parameter that has to change.

What a call costs — 2 credits per call

Credits are shared across every APIVerve API — one balance, one key, one bill. Failed requests never count against it, so a rejected input costs nothing.

Calls are what the credits buy at this API's rate — spend them here, on any of the other APIs, or across both. Nothing is reserved per endpoint.

Zero-code embed — one snippet, no backend

Drop an interactive Text Readability Score form onto any page. The widget calls the API for you, so no key ever appears in your markup and there is nothing to deploy.

Changelog — Text Readability Score API

Every change to this endpoint, newest first. Breaking changes ship as a new version and the previous one keeps serving — response fields are added, never removed or retyped in place, so an integration written against v1 keeps working.

Currently v1