cone angles

i have a 20degree transducer
what is the formula to figure the bottom circle size if the depth is known

thanks
al

1/3rd the depth. For example if you are in 30 ft of water you are seeing 10ft of the bottom.

That is what I was told also. The reason you don't see too many fish in shallow water is because your cone angle is usually less than a foot wide at its widest.