Measure the length of your system. Measure in units of length that will correspond to the speed for your system. For example, if you are calculating the harmonics of a string on a musical instrument or other sonic system, the speed of sound is typically measured in meters per second. Therefore, record your length in meters.
Record the speed of your wave. For a sound wave, the speed of sound in air at 20 degrees celsius is approximated to be 344 meters per second.
Divide the speed of your wave by its wavelength. For a tube or string, the wavelength is twice the length of the system (for a tube closed at one end, the wavelength is four times the length of the system). For example, if you are calculating the harmonics of an organ pipe of length 1m, the fundamental frequency is:
344 m/s / (2 * 1m) = 172 Hz
Multiply the fundamental frequency by positive integers, beginning with 1. These correspond to the harmonics. For example:
First harmonic: 172 Hz * 1 = 172 Hz
Second harmonic: 172 Hz * 2 = 344 Hz
Third harmonic: 172 Hz * 3 = 516 Hz
Fourth harmonic: 172 * 4 = 688 Hz
...and so on.