Watering (Test) Watering Calculator - Small Trees Calculates the number of gallons needed to irrigate with 10 gallons per inch of trunk diameter. "*" indicates required fields Trunk Circumference*Enter the number of inches around the tree trunk at about chest height.Gallons NeededThis is the number of gallons required to irrigate with 10 gallons per inch of trunk diameter. Watering Calculator - Large Trees Calculates the number of gallons needed to irrigate with ½ inch of water throughout the entire root zone of a mature tree. "*" indicates required fields Radius*Enter the number of feet (decimals for part feet, eg 8.5) from the tree trunk to the edge of the canopy.Gallons NeededThis is the number of gallons required to irrigate with ½ inch of water throughout the tree's root zone.HiddenNumber