The internal length of a pipe is defined in the image to the right. The three main factors in calculating the internal length are the frequency of the note, whether it is a stopped or open pipe and the size of the pipe. The frequency determines the wavelength of the soundwave: Λ = 13543 / f Λ is the wavelength in inches f is the frequency in Hz 13543 is the approximate speed of sound in inches per second An open pipe will contain one half of a wavelength and a stopped pipe will contain one fourth of a wavelength: For an open pipe: IL = Λ / 2 For a stopped pipe: IL = Λ / 4

The last thing to consider is a correction factor due to the effect of the air
 at the end of the pipe. There is a rule of thumb I read about on the following
 webiste that has shown to be fairly accurate at predicting the internal length.
 The rule states that you should subtract from the theoretical length two pipe
 widths for an open pipe and one pipe width for a stopped pipe. Adding in the
 correction factor and simplifying the expressions gives the following:

For an open pipe:	IL = (6772 / f) - 2 * IW
For a stopped pipe:	IL = (3386 / f) - IW	

    IL is the internal length in inches
    f is the frequency in Hz
    IW is the internal width in inches