The formula to calculate the pressure in feet of water of an elevated water tank is:

Pressure (feet of water) = Height of the tank (feet) x Specific gravity of water (1)

For example:

If the height of the elevated water tank is 100 feet:

Pressure (feet of water) = 100 feet x 1

= 100 feet of water

Therefore, the pressure at the base of the elevated water tank is 100 feet of water.