This "calculator" is provided for comparison purposes only, and the resulting PSI can be dramatically different in actual field conditions.
This calculator is used for system which will employ a non-pressurized water source such as a cistern or rain barrel. This will calculate your PSI levels based on the height of your water supply. The height of your water supply is often refered to as "head". So a water supply that is 10 feet high has 10 feet of head.
Measure the vertical height from your watering target to the top of your water source. Remember, as your water source empties your PSI will drop.