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Calculations for pit depth.

Cave Pit Depth
by Philip Rykwalder

At pits, we all throw in rocks then try to guess how deep it is. There is a ROUGH equation that can be used to tell how deep pits really are. It is basically the free fall equation with a little modifier/adjustment to compensate for wind resistance.

This empirical equation is good up to about 5 seconds.

depth = (16t^2)-11t

t = time in seconds
depth = output in feet.

I've used it for years. There are only four values to remember:

  • 2 sec (42')
  • 3 sec (111')
  • 4 sec (212')
  • 5 sec (345')
Based upon these values, I split time into about half or quarter seconds, then estimate the rough depth (plus add 10% for rigging).

To estimate pit depth reach over the lip and DROP a rock. Do not throw the rock or add any horizontal component to its path. Remember to start counting at ZERO. Many people drop rocks and start at one. If you hit 'go' on a stopwatch to time that rockfall the timer first has to count up to 1 from zero. Count slowly. If the rock hits things all this goes out the window, so it works until you hear the first impact and not the final one.

I was on a trip in Mexico and a friend came back to camp one night and told me he found a pit that was around 200m deep. He estimated 200m because he told me when he dropped a rock it took 6 seconds to hit. I was skeptical. We got to the pit the next day and it looked deep-ish---but when he dropped the rock he started counting at 1 and then he very quickly counted up to 6. It was like a race to see how many counts he could get in before the rock hit. He also threw the rock in a somewhat horizontal arc and did not drop it vertically, which affected the time some. I leaned over the edge and dropped a rock vertically and its fall at 3.5 sec. It ended up being a ~150' deep pit, so the calculator was pretty close: 3.5 sec = 157'.