1. The six readability formulas we compute
English readability formulas are public and well-validated. ReadLvl runs six of them in your browser — no server upload, no login. Here is what each one emphasizes:
| Index | Driver | Interpretation |
|---|---|---|
| Flesch Reading Ease | Sentence length + syllables/word | 0-100, higher = easier. 60+ is conversational English. |
| Flesch-Kincaid Grade | Same inputs, grade-mapped | US grade level. 10 ≈ HS sophomore, 12 ≈ HS senior. |
| SMOG Index | Polysyllabic-word density | Favored in healthcare/academic writing. Stricter than FRE. |
| Gunning Fog | Complex-word ratio | Typical in business/journalism. 8-12 = easy read. |
| Coleman-Liau | Letters and sentences | Resilient to syllable estimation error; good for OCR output. |
| Dale-Chall | Share of words outside the 3,000 easy list | Vocabulary-sensitive; great for CSAT / TOEIC gauging. |
2. How we map to Korean CSAT bands (1-9)
Korean CSAT English passages historically cluster around Flesch-Kincaid grade 9.5 – 12.5. We average the grade-level scores and cross-check with FRE and the CEFR vocabulary estimate.
- Band 1: avg grade 13+ · CEFR C1+ · FRE below 40
- Band 2: avg grade 11.5–13 · B2-C1 · FRE 40-50
- Band 3: avg grade 10–11.5 · B2 · FRE 50-60 (typical CSAT passage)
- Bands 4-5: avg grade 8-10 · B1-B2 · FRE 60-70
- Bands 6-9: avg grade below 8 · A2-B1 · FRE 70+
Caveat: CSAT bands depend on the test's actual cutoff and on question traps, not only on passage text. Our estimate applies to passage difficulty, not full-test scoring.
3. TOEIC RC, CEFR, and Lexile — what backs the numbers
- CEFR (A1-C2): Derived from the published FK-grade to CEFR table, refined by the advanced-vocabulary share in our bundled CEFR dictionary. Push up one tier when C-level words exceed 25% of the passage.
- TOEIC RC: Anchored to ETS-published CEFR centers (A2≈200, B1≈310, B2≈395, C1≈460, C2≈490) and nudged by the averaged grade.
- Lexile: MetaMetrics public grade→Lexile mapping (grade 1 ≈ 200L, grade 8 ≈ 1020L, grade 12 ≈ 1200L), adjusted ±60L at FRE extremes.
4. Known errors and limits
- Syllable counting uses an orthographic heuristic plus a short exception list — expect ±0.1 syllables/word noise.
- Proper nouns (Tom, Seoul) are excluded from complex-word and difficult-word tallies.
- Very short passages (fewer than 10 words) make SMOG unstable, so we weight it down.
- Overall mapping accuracy: ±1 CSAT band · ±50 TOEIC points · ±80L Lexile.
5. Who benefits
- Students: Gauge how a practice passage compares to your target band.
- Teachers: Pre-select readings for a class. CEFR distribution flags vocabulary spikes.
- Writers: Tune blog posts, newsletters and docs to the intended audience.
- Translators: Compare source and target readability.
More answers on the FAQ page.