Pressure calculation.

Say I have a pipe of some kind with a given radius r. If a fluid of some kind flows through the pipe with a given velocity v. What then is the pressure the fluid exerts on the walls of the pipe?
The only i can say is that it should be directly proportional to the radius r of the pipe and the density of the fluid. Whether or not it is proportional to the velocity I don't know. And I am not shure how to come up with an equation that describes the pressure properly.