Flesch Reading Ease Explained

The Flesch Reading Ease score rates any text from 0 (nearly unreadable) to 100 (a children's book), using only sentence length and syllable counts. Plain English scores 60–70. Paste your draft into our free readability score calculator to get your score instantly; the formula, chart, and targets are below.

The formula, piece by piece

The Flesch Reading Ease formula, published by Rudolf Flesch in 1948, is: 206.835 − (1.015 × average sentence length) − (84.6 × average syllables per word). Three inputs, all countable: total words, total sentences, total syllables. Average sentence length (ASL) is words divided by sentences; average syllables per word (ASW) is syllables divided by words. The constants are calibrated, not arbitrary: 206.835 anchors the scale so that ordinary prose lands mid-range, 1.015 prices each extra word per sentence, and 84.6 prices each extra syllable per word — note how much heavier the syllable weight is. A single additional syllable per word costs 84.6 points, while a single additional word per sentence costs only 1.015: the formula punishes Latinate vocabulary roughly 83 times harder than long sentences. That is why swapping 'utilize' (3 syllables) for 'use' (1 syllable) moves the needle more than trimming a sentence by a few words. Scores can theoretically dip below 0 for extremely dense academic text — the formula has no floor — and cap at 100 for the simplest writing. Two measurements feed everything, so a readability calculator only needs accurate counts of words, sentences, and syllables to reproduce the score exactly. If you also track length, pair it with our word counter to keep drafts within their word budgets while you tune readability.

The 0–100 score chart and what each band means

Flesch's original scale maps score bands to school levels: 90–100 is very easy (5th grade — comics and children's books); 80–90 is easy (6th grade — conversational writing); 70–80 is fairly easy (7th grade — most marketing copy); 60–70 is plain English (8th–9th grade — the target for general-audience writing); 50–60 is fairly difficult (10th–12th grade — quality journalism); 30–50 is difficult (college — academic papers); 0–30 is very difficult (college graduate — legal and scientific prose). For calibration: Hemingway's novels score around 75, the Harvard Law Review around 30, and insurance policies famously cluster near 10–15. The US military adopted the companion Flesch-Kincaid grade-level version precisely because these bands are actionable — a manual that must be understood by every recruit gets a target band, not a vibe. Note that the bands describe reading difficulty, not quality: a score of 95 on a legal contract would be alarming, and a score of 35 on a children's story would be a failure. Context sets the target; the formula only measures the distance. Writers sometimes ask whether the chart is 'still valid' after 75 years — it is, because the inputs (sentence length, syllable density) track cognitive load the same way they did in 1948, even though the vocabulary of any given era shifts.

Two worked examples with real numbers

Example one — a dense paragraph: 200 words, 14 sentences, 320 syllables. ASL = 200 ÷ 14 = 14.29; ASW = 320 ÷ 200 = 1.60. Score = 206.835 − (1.015 × 14.29) − (84.6 × 1.60) = 206.835 − 14.50 − 135.36 = 56.98 — 'fairly difficult,' roughly a 10th-grade level. Example two — the same ideas rewritten plainly: 120 words, 12 sentences, 168 syllables. ASL = 120 ÷ 12 = 10.00; ASW = 168 ÷ 120 = 1.40. Score = 206.835 − (1.015 × 10) − (84.6 × 1.40) = 206.835 − 10.15 − 118.44 = 78.24 — 'fairly easy,' 7th-grade level. The rewrite gained 21 points by cutting 4 words per sentence and 0.2 syllables per word — small edits, large swing, which is typical: most professional writing sits within a 15-point band of its target, and two editing passes close the gap. Notice the asymmetry again: dropping ASW from 1.60 to 1.40 contributed 16.9 points (0.2 × 84.6), while dropping ASL from 14.29 to 10 contributed only 4.4 points (4.29 × 1.015). Vocabulary simplification is the highest-leverage edit the formula rewards. Run both versions of your own draft through a readability score calculator and you will see the same pattern: word choice moves the score, sentence surgery fine-tunes it.

What score should you actually target

Targets depend on audience, not ambition. General web content: 60–70 — plain English, readable by the ~50% of US adults who read at or below a 9th-grade level. This is the band most style guides (including the US federal plain-language guidelines) converge on. Marketing and email: 70–80 — skimmable under distraction; every 10-point gain measurably lifts comprehension in eye-tracking studies. Technical documentation: 50–60 — precise terminology costs syllables, and that is fine as long as the audience is specialists. Academic and legal: 30–50 — the terminology is the content; chasing 70 here means dumbing down. A useful rule: target the band one level easier than your audience's reading level, because people read faster and retain more with headroom — a college-educated audience still prefers 60–70 prose when scanning. Avoid two traps: treating 100 as the goal (it reads as condescending for adult audiences) and comparing scores across genres (a 55 in a medical journal and a 55 in a blog post mean different things). Measure consistently — same tool, same draft stage — and track the delta between drafts rather than the absolute number. For SEO-adjacent writing, pair readability checks with our keyword density guide and reading-time calculator guide to tune length, density, and difficulty together. Writers who bill by the hour can convert editing sessions with our minutes to hours converter, and those publishing in markdown can preview the final layout with the markdown to HTML converter before measuring readability on the rendered page.