Old Faithful spouts an incredible 20 times a day, blowing at least 3,700 gallons of near boiling water and hot steam skyward with each eruption. Naturalists predict its timely eruption with 90 percent accuracy by observing the previous eruption’s duration and spout height. These stalwart observers use simple tools like a stopwatch and log-book to make their predictions. Old Faithful usually erupts every 74 minutes, but intervals can range from 60-110 minutes. You can find eruption predictions posted clearly around Old Faithful and online.