# 1.2 Pipe Cross-Sectional Area

Pipe cross-sectional area is the area of a circle as seen when looking at the end of a pipe. The flow capacity of different size pipes is easily compared when the velocity is held constant. The flow capacity will vary in direct proportion to the change in area. The area varies directly with the square of the inside diameter.
When one speaks of a one inch pipe, it is a nominal one inch; the true inside diameter usually is not 1 inch. For a Class 160 PVC pipe, the inside diameter is actually 1.195 inches. The outside diameters are kept constant so pipe fittings will fit. Inside diameters vary depending on the wall thickness. Higher pressure rated pipes will have thicker walls and smaller inside diameters.

Water carrying capacity of two pipes can be compared using the diameters. A ratio of the cross-sectional areas of two pipes is used. The process uses the area equation for a circle:

The flow capacity of different size pipes is easily compared when the velocity is held constant. The flow capacity will vary in direct proportion to the change in area (diameter). The actual inside diameters of the pipes should be used for accuracy.

The comparison process is developed from the cross-sectional area of a pipe, described by the area equation for a circle:

A= 3.14 D2 / 4 = 0.785 D2 or 0.785 D x D

Where D = pipe diameter, in feet or in inches. Note that D2 reads "D squared" and means D x D

Recall the equation Q = V x A. Since the water velocity is constant, the flow Q is directly proportional to the area, A.

A ratio of the two areas will reduce to a ratio of the two diameters squared. Therefore, the flow of water in the two pipes is compared using a ratio of their diameters squared.

(D2)2 : (D1)2 which means D2 x D2 compared to D1 x D1

As an example, we can compare the flow capacity of a 3-in. diameter pipe to that of a 2-in. diameter pipe, using the diameter squared ratio, i.e:

3 x 3 : 2 x 2
9 : 4
9/4 : 4/4
2.25 : 1

So the 3-in. pipe will carry 2.25 times more water than the 2-in. pipe at the same water velocity.

Too frequently the benefits -- present and future -- of the larger pipe are overlooked. Yes, pipe costs will be greater for larger diameter pipe, but the larger pipe allows future expansion. More on that later!