The classic formulas — Flesch Reading Ease, Flesch–Kincaid Grade Level, Gunning Fog, SMOG — count syllables per word and words per sentence. Short words in short sentences = easier text.
For general-audience marketing copy, aim for grade 7–9. Technical docs sit around 10–12. Anything above 14 loses most readers who aren't in the field.
Scores are a proxy, not a judgement — a well-written grade-12 essay can be more accessible than a poorly-written grade-6 one. Use them as a signal to break up sentences and swap jargon, then re-read the whole piece.