https://www.washingtonpost.com/news/wonk/wp/2014/11/24/the-single-chart-that-shows-that-grand-juries-indict-99-99-percent-of-the-time/

Edit: this shows indictment rate, not conviction rate. I can't find a good source for the conviction rate.

While true it's also fleetingly rare for a grand jury indictment not to lead to a conviction. Federal Grand juries have a conviction rate of over 99%.

Edit: I was mistaken. Grand juries almost always indict, but conviction rates seem pretty hard to track down if anyone knows what the conviction rate is, please elucidate the rest of us.