Flesch–Kincaid grade level

What this check does

Calculates the Flesch–Kincaid Grade Level using the standard formula 0.39 × (words / sentences) + 11.8 × (syllables / words) − 15.59. The check fails above grade 12 — the level at which most general readers start skimming or bouncing.

Why it matters

Reading-level data has been studied for a century, and the result is consistent: most adults read comfortably at grade 7–9, even when they hold advanced degrees. Above that level, comprehension drops, time-on-page drops, and conversion drops.

• Search ranking. Google’s Helpful Content System favors prose that matches the audience’s reading level. Technical docs can be denser; consumer pages should be lighter.
• AI extraction. LLMs that summarize web content lose detail more aggressively from high-grade text — long sentences with embedded clauses produce summaries that miss the point.
• Conversion. Nielsen-Norman, ContentSquare, and HubSpot all replicate the same finding: simpler copy on landing pages produces higher CTA clicks. The effect is biggest in finance, legal, and B2B SaaS — the categories most tempted to write at grade 14.

How to fix it

The four levers in the formula are sentence length, syllable count, abstract-noun rate, and passive voice.

Shorter sentences. Aim for an average of 14–18 words. Watch for sentences over 25 words — split them.

Before: Our enterprise-grade observability platform, leveraging next-generation
machine-learning algorithms, delivers actionable insights across heterogeneous
infrastructure environments. (24 words, grade 17)

After: Watch every server, container, and function in one dashboard. We surface
the metrics that matter so your on-call engineer can act in minutes. (24 words
across 2 sentences, grade 8)

Simpler words. Swap Latinate words for Germanic ones where the meaning is identical: utilize → use, facilitate → help, commence → start, terminate → end, prior to → before.

Concrete nouns. “The configuration of the alerting system” → “the alert config.” Every abstract noun is a syllable tax.

Active voice. “Errors are logged by the agent” → “The agent logs errors.” See Passive voice.

Tools.

• Hemingway Editor (web, free) — highlights long sentences, passive voice, adverbs, and complex phrases.
• Grammarly / ProWritingAid — same checks plus rephrasing suggestions.
• textstat (Python) or readability (Node) — run Flesch–Kincaid in CI: fail the build if a new page exceeds grade 12.
• Microsoft Word — File → Options → Proofing → “Show readability statistics.”

The exceptions. Legal terms, scientific papers, and API reference docs are supposed to be dense and precise. Don’t dumb those down. The rule applies to marketing pages, blog posts, support articles, product pages — anywhere you want a non-specialist to read to the end.

Frequently asked questions

How is the grade actually calculated?

0.39 × (words / sentences) + 11.8 × (syllables / words) − 15.59. Two levers: how long your sentences are and how many syllables your words have. Lower either and the grade drops.

Does this apply to languages other than English?

The formula was calibrated against US English. Equivalents exist for other languages — Fernández–Huerta for Spanish, Wiener Sachtextformel for German, LIX for Swedish/Norwegian. MetricSpot reports the closest equivalent for non-English pages.

Grade 7 feels patronizing for my technical audience.

Then write at grade 10 — the rule fails at 12+ for a reason. Even senior engineers prefer prose that doesn’t make them work to decode it; “simple” and “shallow” aren’t the same thing. See also Average sentence length and Long sentences.