Work out text complexity score instantly with clear inputs, formula shown and shareable results.
The Gunning Fog index is 0.4 times the sum of average sentence length and the percentage of complex words, giving the years of education a reader needs. Twenty-two word sentences with 18% complex words scores 16 - undergraduate level. Adding a passive voice penalty produces a composite that captures a further real barrier to comprehension.
Gunning Fog index
0.4 x (average sentence length + percent complex words)
Composite
fog x 4 plus a passive voice penalty, capped at 100
Because it hides the actor and adds words, increasing processing load. Plain language guidance targets under 10% passive constructions.
They correlate strongly. Fog and Flesch-Kincaid are the most cited; use one consistently rather than comparing across formulas.