To calculate the hole size needed for a specific flow rate in a tank or reservoir, one can apply energy conservation principles. The kinetic energy of the liquid exiting the hole must equal the work done by the pressure pushing it out. By using the formula Psv = ρsv(v²/2), where P is pressure, s is the area of the hole, and v is the velocity, the relationship between pressure and flow can be established. Ignoring friction simplifies the calculations, allowing for a direct correlation between pressure and the required hole size. This approach effectively determines the necessary dimensions for achieving the desired flow rate.