The second (s) is the base unit of time in the International System of Units. NIST states that it is defined by fixing the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom at exactly 9,192,631,770 Hz.
In everyday conversions, NIST lists the minute as exactly 60 s, the hour as exactly 3,600 s and the day as exactly 86,400 s.
Conversion
s = min × 60
s = h × 3,600
s = days × 86,400
Example
A week is 7 × 86,400 = 604,800 s, and 3 hours is 3 × 3,600 = 10,800 s. See how many hours in a week for minute, hour and year totals, and hours to seconds for any value.
Common mistakes
Minutes and seconds are base 60, not base 100: 1.5 minutes is 90 seconds, not 150.