Old Faithful “plays” prior to an eruption with splashes of boiling water that may reach as high as 20 feet (6 m). A full eruption usually begins from this play. Once the eruption starts, the splashes grow larger and larger until the eruption reaches its full height.

Predicting an Old Faithful eruption in Yellowstone is a mix of timing and specific physical "tells." The geyser erupts roughly every 90 minutes, with about 90% of predictions being accurate within a 10-minute window. The most reliable physical sign that an eruption is imminent is when the geyser begins to "play"—this involves small splashes of boiling water that can reach up to 20 feet (6 meters) in height. These "pre-play" splashes occur a few minutes before the main event. Once the eruption truly begins, the splashes grow rapidly in size until the water column reaches its full height of 90 to 184 feet. If you see these consistent, rhythmic splashes after a long period of dormancy, find a seat on the boardwalk immediately. It is a grounded peer recommendation to check the official "National Park Service" app or the visitor center boards for the latest predicted time, as the interval can fluctuate based on local seismic activity or changes in the groundwater. Once it starts, the display usually lasts between 1.5 and 5 minutes.