Readability Score API
Readability metrics for any text: Flesch Reading Ease, Flesch-Kincaid Grade, Gunning Fog, SMOG, Coleman-Liau, ARI and LIX for English, plus Fernandez-Huerta and Szigriszt-Pazos (INFLESZ scale) for Spanish. Also word, sentence and syllable counts and reading/speaking time.
Authentication & base URL
Subscribe through one of our marketplace listings; your marketplace API key and base URL are shown there. All endpoints return JSON over HTTPS, served from a global edge network (typically 20–150 ms).
Endpoints
GET /v1/readability
Readability metrics
| Parameter | In | Description | Example |
|---|---|---|---|
text * | query | Text to analyse (or POST it as the body) | The quick brown fox jumps over the lazy dog. It was a sunny day. |
lang | query | en or es | en |
POST /v1/readability
Readability metrics (POST text body)
Request body: text/plain
| Parameter | In | Description | Example |
|---|---|---|---|
lang | query | en or es | en |
Example
GET /v1/readability?text=The+quick+brown+fox+jumps+over+the+lazy+dog.+It+was+a+sunny+day.&lang=en
{
"language": "en",
"words": 14,
"sentences": 2,
"syllables": 17,
"characters": 64,
"letters": 49,
"complexWords": 0,
"averageWordsPerSentence": 7,
"averageSyllablesPerWord": 1.21,
"readingTimeSeconds": 4,
"speakingTimeSeconds": 6,
"scores": {
"fleschReadingEase": 97,
"fleschKincaidGrade": 1.47,
"gunningFog": 2.8,
"smogIndex": 3.13,
"colemanLiauIndex": 0.55,
"automatedReadabilityIndex": -1.45,
"lix": 7
},
"level": "Very easy",
"averageGradeLevel": 1.3
}Readability metrics for any text: Flesch Reading Ease, Flesch-Kincaid Grade, Gunning Fog, SMOG, Coleman-Liau, ARI and LIX for English, plus Fernandez-Huerta and Szigriszt-Pazos (INFLESZ scale) for Spanish. Also word, sentence and syllable counts and reading/speaking time.
Use cases
Content and SEO tools, writing assistants, education platforms (grade level), plain-language compliance (health, legal, government), checking AI-generated copy before publishing.
Why this API
- Fast: global edge network, answers in milliseconds.
- 7 English formulas + an average grade level, and Spanish formulas that most APIs don't offer.
- Plain-language level label (Very easy ... Very difficult).
- Up to 200,000 characters per request.
- Fair pricing: Free plan 500 requests/month, Pro 25,000 for $4.99, Ultra 150,000 for $14.99.
Support
Questions or a feature you need? Open a thread in the Discussions tab.
Pricing
Free plan with 500 calls per month, then Pro (25,000 calls for $4.99/month) and Ultra (150,000 calls for $14.99/month). See all plans.