Predict Old Faithful

A crowd looks on as Old Faithful erupts
Old Faithful, named by members of the 1870 Washburn Expedition, was once called “Eternity’s Timepiece” because of the regularity of its eruptions. Despite the myth, this geyser has never erupted at exact hourly intervals, nor is it the largest or most regular geyser in Yellowstone National Park.

NPS / Jim Peaco


Though most geysers are not predictable, Old Faithful Geyser is famous for its regularity. Use the rangers’ formula below to predict its next eruption. You will need a place to record your work and answers. For kids ages 8+.


Step 1

Open the Old Faithful Geyser Streaming Webcam (the other Old Faithful Webcam is a different view and refreshes every 30 seconds). Wait for Old Faithful to erupt and time its eruption from when water begins splashing continuously out of the cone until only steam is coming out.

A. Starting Time: ______

B. Ending Time: ______

C. Length of eruption to nearest half minute: ______


Step 2

Using the table below, find the length of the eruption (C) in the first column. Read directly across to the second column in the same row and note the interval or number of minutes (D) until the next eruption.

Length of Eruption
Interval Until Next Eruption
less than 3 minutes 68 minutes
more than 3 minutes 94 minutes


Step 3

Add the starting time for the first eruption (A) with the interval number of minutes until the next eruption (D) to predict the time for the next eruption:

starting time (A) + interval until next eruption (D) = time next eruption predicted

______ + ______ = ______


Step 4

Visit the Old Faithful Geyser Streaming Webcam or @GeyserNPS on Twitter to find the rangers’ prediction for the next eruption: ______

Is your prediction similar? ______


Step 5

If you are still watching Old Faithful for the next eruption, note the time it started: ______ . Compare the actual starting time to your prediction and the rangers’ prediction. Remember that Old Faithful is a natural feature and changes often, so any prediction may not prove to be exactly right.


More Information

Last updated: June 14, 2019

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