What is the formula for calculating the inner area of pipe?

The inner area of a pipe can be calculated using the formula for the cross-sectional area of a circle, which is ( A = \pi r^2 ), where ( r ) is the inner radius of the pipe. If you have the diameter ( d ) of the pipe instead, you can use the formula ( A = \frac{\pi d^2}{4} ). This area represents the space available for fluid flow within the pipe.