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 D^{2} /
4 = 0.785 D^{2} or 0.785 D x D

Where D = pipe diameter,
in feet or in inches. Note that D^{2} 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.

(D_{2})^{2}
: (D_{1})^{2} which means D_{2} x D_{2} compared
to D_{1} x D_{1}

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!