Measured across text samples, passive-voice ratio tracks formality almost perfectly: social media captions and email sit at 0 percent, blog posts around 4 percent, news briefs 14 percent, software documentation 18 percent, research abstracts 46 percent, and law review writing 62 percent.
The tempting conclusion is that passive voice causes difficulty. It does not. It correlates with the other things that do: longer sentences, more Latinate vocabulary, higher lexical density. A law review article at 62 percent passive also carries a grade level near 20 and a reading-ease score under 10. The passive is riding along with those, not driving them.
That matters for editing. Mechanically converting every passive clause to active in a dense academic paragraph usually produces an awkward active sentence that is still hard to read, because the actual difficulty was sentence length and vocabulary. Cutting a 26-word sentence to two 13-word sentences does far more.
Use the passive ratio as a signal that a passage may be denser than intended, then fix the density directly.
A writing assistant that surfaces these measures is at belikenative.com.