Old Faithful is a cone-type geyser. Its intervals range from 35 to 120 minutes, its duration is 1 1/2 to 5 minutes and its height is 90 to 184 feet. The rangers say that 90% of their predictions are within +/- 10 minutes. During the last decade or so, long interval eruptions have become more common (during the summer of 2001 short interval eruptions were quite rare) as a result, the average interval for Old Faithful has increased to nearly 90 minutes.

Old Faithful was named by the Washburn expedition of 1870. They were impressed by its size and frequency. It is not the biggest or most regular geyser in Yellowstone but it is the biggest regular geyser. Furthermore, it has been erupting in nearly the same fashion throughout the recorded history of Yellowstone. Through the years, it has become one of the most studied geysers in the park. One result of this close observation is that the Park Rangers are able to predict its eruptions fairly accurately. This makes Old Faithful geyser one of the easiest geysers in Yellowstone to see.