How do you calculate the fundamental frequency of a pipe?

This depends on whether the pipe is closed or open ended.

The fundamental frequency of a pipe is the simplest, smallest portion of a wave that can fit into a pipe. At the open end of a pipe, there is always an antinode - an area with maximum air movement.

If it is an open ended pipe, there is an antinode at each end, meaning that the length of the pipe is equal to 1/2 λ. Manipulating the formula v=fλ to solve for the fundamental frequency leaves us with f = v / 2L in an open ended pipe.