There are 86,400 seconds in one 24-hour day.86,400 because there are 24 hours in one day and each hour has 60 minutes and each minute has 60 seconds so 24 hours times 60 minutes equals 1440 times 60 seconds equals 86,400.However, a sidereal day (Earth's rotational period) is slightly shorter. The sidereal day is 86164.0916 seconds long.