There need not be any water at all in the hose! The capacity of the hose is 3.41 cubic feet.
A standard fire hose is 50 feet long. A hose this length with a 2-inch radius grants about 4.36 cubic feet. This volume holds 32 gallons of water.
50 feet of 2.5-inch diameter hose has a volume of: 1.7 cubic feet (12.72 liquid gallons)
A 100-foot hose with an inside diameter of five inches can hold 102 US gallons of water.
A 2.5 inch fire hose has a capacity of approximately 60 gallons per 100 ft. Therefore, a 50 ft hose would hold around 30 gallons of water.
A 20' length of 4-inch hose can hold approximately 0.38 gallons of gasoline per foot. Therefore, 20 feet of this hose can hold around 7.6 gallons of gasoline.
Hose comes from Victorian times when men wore 'hose' under there three quarter trousers. Hose is like a very fine sock.
Crankcase breather hose.
One gallon is 231 cubic inches. Assuming cyllindrical shape of hose, it contains 100 * 12 * 5 * PI * 0.25 * 52 cubin inches, which is 11780 cubic inches = 51 gallons.
I assume you want to find out how much water fits in the hose. Treat the hose as a cylinder - that should be a good approximation, even when you bend it. Measure the length and the interior diameter. Divide the diameter by 2 to get the radius. Convert everything to feet, and finally use the formula for a cylinder to get the volume.
Pi x Radius Squared x length in inches / 231 ci in a gallon 3.14 x 2 squared x 50 feet x 12 inches = 7,536/231 = 32.6 Gallons
Do you mean a fixed length of 5 inch pipe or are you asking to the amount of laminar flow through a 5 inch pipe? There is not enough info here to answer. Need length of pipe and what you are asking.