How do you calculate fluid capacity in gallons of a piece of pipe?

The internal capacity of a pipe is pi x internal radius x internal radius x length of pipe where pi = 3.142 This is the volume of a cylinder; pipe capacity is usually amount of flow per unit of time...

You need to know two important dimensions of the pipe: the length and the inside radius (1/2 the inside diameter). From there, you plug the dimensions in to this formula: radius2 x Pi (3.1416) x...

Start with the maximum flow speed (metres per second) for your liquid, then mutiply by the area of the pipe in square metres, the result is maximum volume flow rate in cubic metres per second.

Either by determining the core diametre of the pipe and how long the pipe would be to hold a gallon of water , or simply by running a gallon of water through said pipe and timing it to fill a...

You need more information to perfom the calculation such as the fluid flow rate. You would then divide the flowrate by the area get velocity.