Is it possible to find the pressure of water that leaves a pipe of certain area/diameter given only the flow rate of the water itself? OR the requires piping diameter/area given the pressure that I desire. I'm trying to design a system that requires relatively high pressure (for spraying) however, I do not want to invest money in pumps, piping, gear etc. I'd rather try find a way to figure out a combination of criteria before I do this. For this, I'm assuming that the flow regime is laminar. I have found an equation that I think may give me what I need, however I am unsure how I can apply it in this situation. Hagen-Poiseuille equation: Flow rate = πr4(P−P0)/8ηl where r is the radius of the pipe or tube, P0 is the fluid pressure at one end of the pipe, P is the fluid pressure at the other end of the pipe, η is the fluid's viscosity, and l is the length of the pipe or tube. I am unsure what pressure I should set the input (pump end) of the equation, P0? Any help would be appreciated, thanks.