How many small pipes does it take to carry as much mass as one large pipe? If you do not have room or the money for one large pipe, how many smaller and narrower pipes are required to carry the same volume? The answer to that question depends on the substance being transported and the sizes of the pipes.
Calculation for Pipes Carrying Gases or Steam

Determine the diameters of the pipes. If you're comparing two pipes, call the diameters D1 and D2.

Add 3.6 to D2. Then take the square root. Multiply this value by D1 cubed.

Add 3.6 to D1. Then take the square root. Multiply this by the cubed value of D2.

Divide the answer at step 2 by the answer of step 3. This result is the relative carrying capacity of two pipes carrying steam or atmospheric gases.
Calculation for Pipes Carrying Water

Determine the two pipe diameters to be compared. Call them D1 and D2.

Divide D2 by D1. This provides the ratio of the diameters of the two pipes.

Raise the result to the power of 2.5. This result is the relative carrying capacity of two pipes carrying water.
Tips & Warnings
 Relative carrying capacity of standard sized pipes can be determined from reference tables in hydraulic engineering and fluid mechanics books. However, if the tables are not available or the pipe size is not referenced in the table, the calculations for relative carrying capacity for pipes are necessary.
 These equations do not take into account turbulent water or airflow, which reduces pipe capacity.
 When performing these calculations for air or gas flow, differences in pressure are not taken into account. Higher pressure air flows will carry more gas than lower pressure air flows, even given the same diameter of pipe.
References
 Photo Credit abstract red piping image by marilyn barbone from Fotolia.com