To calculate a water wave's wavelength, you need a number of values, including an estimate of the wavelength itself. Calculating therefore requires numerous iterations, which computer programs can manage far more easily than humans. But a slightly simpler formula exists. It determines the wavelength from the wave's period and height.

Multiply pi by two.
π * 2
= 6.2831

Divide your answer by the wave's period. If it has a period of 5;
6.2831 / 5
=1.26

Square your answer.
1.26 * 1.26
= 1.58

Multiply your answer by the wave's height, in meters. If the wave is 2 m high:
1.58 * 2
= 3.16

Divide your answer by the gravitational constant  9.81
3.16 / 9.81
= 0.322

Raise your answer to the power of 0.75.
0.322 ^ 0.75
= 0.428

Find the tanh of your answer, using a scientific calculator.
tanh (0.428)
= 0.403

Raise your answer to the power of 0.66.
0.403 ^ 0.66
= 0.549

Square the wave's period.
5 * 5
= 25

Multiply your answer by the gravitational constant.
25 * 9.81
= 245.25

Divide your answer by your answer from step 1:
245.25 / 6.2831
= 39

Multiply your answer by the answer to step 8 to determine the wave's wavelength in meters.
39 * 0.549
= 29.4 meters
References
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