ɪf juː mˌʌltɪplˈaɪ ðə swˈɛl pˈiəɹɪəd baɪ wˈʌn.fˈaɪv juː wɪl kˈælkjʊlˌeɪt ðə spˈiːd ɪn nˈɑːts ʌvðə wˈeɪv ɡɹˈuːp fɔːɹ dˈiːp wˈɔːɾɚ.
If you multiply the swell period by 1.5 you will calculate the speed in knots of the wave group for deep water.