There are 86,400 seconds in a day. This calculation is based on the conventional definition of the length of a day; for convenience, a day is defined as 24 hours.
A day is considered to be the amount of time it takes the earth to rotate once around its axis. This rotation takes 23 hours, 56 minutes and 4.1 seconds, which is exactly 86,164.1 seconds long.
If you measure the length of a day in solar time, it varies depending on the time of the year. Using solar time, the length of a day in seconds varies from 86,325 seconds to 86,471 seconds.