Old Faithful Geyser is the world’s most famous geyser and the symbol of America’s first national park. Compared to other big geysers, which can sometimes remain dormant for days or even years between eruptions, the size and consistency of Old Faithful’s eruptions have not changed dramatically during the entire recorded history of the park.
To predict Old Faithful’s next eruption, rangers examine factors like the time interval between eruptions and the length and character of the last eruption. The rangers claim to be accurate (within 10 minutes) over 90% of the time. Historically, the interval has ranged between 30 minutes to two hours. If you thought Old Faithful performed every hour on the hour, you’re in for a disappointment. These days, it’s in the range of every 90 minutes. In general, the longer the eruption, the longer the interval will be. Park Rangers post the predicted eruption times for Old Faithful and several other major geysers at the visitor center.