Old Faithful is a cone geyser located in Yellowstone National Park in Wyoming, United States. It was given the name "Old Faithful" in 1870 during the Washburn–Langford–Doane Expedition and was the first geyser in the park to receive a name. It is a predictable geothermal feature that erupts about every 92 minutes on average. The time between eruptions can sometimes be as short as 35 minutes or as long as 120 minutes. The geyser and the nearby Old Faithful Inn are part of the Old Faithful Historic District.
History
On the afternoon of September 18, 1870, members of the Washburn-Langford-Doane Expedition traveled down the Firehole River from the Kepler Cascades and entered the Upper Geyser Basin. The first geyser they saw was Old Faithful. Nathaniel P. Langford wrote in his 1871 account of the expedition:
"It spouted at regular times nine times during our stay. The water shot up between 90 and 125 feet high with each eruption, which lasted 15 to 20 minutes. We named it 'Old Faithful.'"
— Nathaniel P. Langford, 1871
In the early years of the park, Old Faithful was sometimes used as a laundry. An 1883 Yellowstone guidebook explained:
"Old Faithful was sometimes used to wash clothes. Garments placed in the crater during quiet periods were ejected and thoroughly cleaned during eruptions. In 1882, General Sheridan’s men discovered that linen and cotton fabrics were not damaged by the water, but woolen clothes were shredded."
Eruptions
More than 1,000,000 eruptions have been recorded. In 1938, Harry Woodward explained how the length of an eruption and the time between eruptions are connected mathematically. Old Faithful is not the tallest or largest geyser in the park. That title belongs to Steamboat Geyser, which is less predictable. Old Faithful is reliable because it is not connected to other thermal features in the Upper Geyser Basin.
Each eruption can send 3,700 to 8,400 US gallons (14,000 to 32,000 liters) of boiling water up to 106 to 185 feet (32 to 56 meters) high. An eruption usually lasts between 1½ to 5 minutes, with an average height of 145 feet (44 meters). The time between eruptions has varied from 34 to 125 minutes. In 1939, the average time between eruptions was 66½ minutes. Since 2000, this average has increased to about 90 minutes. This change may be caused by earthquakes that affect underground water levels. Although these changes made earlier mathematical predictions less accurate, they have made it easier to predict when Old Faithful will erupt next. For example, after the Borah Peak earthquake in Idaho in October 1983, the time between eruptions became longer.
The time between eruptions follows two common patterns. The average time is either 65 or 91 minutes, depending on how long the previous eruption lasted. If an eruption lasts less than 2½ minutes, the next eruption usually happens about 65 minutes later. If an eruption lasts more than 2½ minutes, the next eruption usually happens about 91 minutes later. This pattern is accurate within a 10-minute range.
Measurement
Between 1983 and 1994, four probes equipped with tools to measure temperature, pressure, and video cameras were lowered into Old Faithful. These probes reached a depth of 72 feet (22 meters). At this depth, the water temperature was measured at 244 °F (118 °C), which matches the temperature recorded in 1942. Video cameras were lowered to a maximum depth of 42 feet (13 meters) to study the conduit and the processes inside it. Observations included fog forming when cool air from above mixed with hot air from below, water entering the conduit and expanding from below, and superheated steam as hot as 265 °F (129 °C) entering the conduit. In 2025, scientists measured the amount of water erupted from Old Faithful. They found that the average volume of water erupted is 27.9 ± 9.4 cubic meters. These measurements also helped calculate the heat flow as 2.2 to 2.4 megawatts, and the average annual discharge of chloride (63 tons), fluoride (3.9 tons), and arsenic (241 kilograms).