How many seconds are in a day?
Exactly 86,400 seconds (24 × 60 × 60), in standard non-leap-second days.
Time calculator
Time converter
Last updated:
Time is the dimension of duration and the progression of events. The SI base unit is the second (s), defined by the cesium-133 atomic transition: exactly 9,192,631,770 cycles of the radiation between two specific hyperfine levels. Common time units include milliseconds (10⁻³ s), seconds, minutes (60 s), hours (3,600 s), days (86,400 s), and years.
Example time conversions
1 second1,000 ms, 10⁶ μs, 10⁹ ns
1 minute60 s (exact)
1 hour3,600 s, 60 minutes (exact)
1 day86,400 s, 24 hours (exact)
Open the live time converter for any input value.
The second and its decimal subdivisions
Since 1967 the second has been defined atomically rather than astronomically. The current definition was chosen because atomic transitions are vastly more reproducible than Earth's rotation, which slows and wobbles. The Boltzmann, Planck, and elementary-charge constants pinned to specific values in the 2019 SI redefinition all chain back to the cesium second.
Standard decimal prefixes apply: millisecond (10⁻³ s), microsecond (10⁻⁶ s), nanosecond (10⁻⁹ s), picosecond (10⁻¹² s), and femtosecond (10⁻¹⁵ s). These are the natural units of digital electronics (clock cycles), networking (latency), audio processing, and ultrafast optics.
Calendar and clock units
Above the second, time units lose their decimal structure. 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day. A week is 7 days; a typical year is 365 days, with leap years of 366 days every four years (except century years not divisible by 400, in the Gregorian calendar).
The non-decimal multipliers are historical (Babylonian sexagesimal counting at the second/minute/hour level; lunar and solar cycles for the day/month/year level). They survived because they were practical, not because they were elegant. Modern timekeeping standards reconcile atomic time (TAI) with astronomical time (UT1) by occasionally inserting leap seconds to keep UTC aligned with Earth's rotation.
When to use which time unit
Physics and engineering calculations use seconds because the second is the SI coherent unit and integrates cleanly with the rest of SI (joules, watts, hertz, newtons). Software performance work uses milliseconds, microseconds, and nanoseconds; CPU instruction timings are usually quoted in nanoseconds and picoseconds.
Project planning, schedules, and everyday work use minutes, hours, and days. Astronomy adds the Julian year (365.25 days, used as a standard interval) and longer units like the megayear and gigayear for geological and cosmological timescales.
Common time conversions
| From | Equivalent |
|---|---|
| 1 second | 1,000 ms, 10⁶ μs, 10⁹ ns |
| 1 minute | 60 s (exact) |
| 1 hour | 3,600 s, 60 minutes (exact) |
| 1 day | 86,400 s, 24 hours (exact) |
| 1 week | 7 days, 604,800 s (exact) |
| 1 common year | 365 days, 31,536,000 s |
| 1 leap year | 366 days, 31,622,400 s |
| 1 Julian year | 365.25 days, 31,557,600 s |
| 1 millisecond | 0.001 s, 10⁶ ns |
| 1 nanosecond | 10⁻⁹ s, 1,000 ps |
Frequently asked questions
What is the SI definition of a second?
9,192,631,770 cycles of the radiation emitted by the hyperfine transition of cesium-133, in the unperturbed ground state. Adopted in 1967 and refined in the 2019 SI redefinition.
How many seconds are in a year?
31,536,000 in a 365-day year; 31,622,400 in a 366-day leap year. The Julian year used in astronomy is 31,557,600 s (365.25 days).
What is a leap second?
An occasional one-second adjustment to Coordinated Universal Time (UTC) inserted to keep atomic time aligned with Earth's slightly slowing rotation. The most recent leap second was added in 2016. The international body that governs UTC has voted to abolish leap seconds by 2035.
Why isn't time decimal above the second?
Historical inertia. The 60-second minute and 60-minute hour come from Babylonian sexagesimal arithmetic, which divides cleanly into 2, 3, 4, 5, 6, 10, 12, 15, 20, and 30. The 24-hour day comes from Egyptian timekeeping. Several attempts at decimal time (notably during the French Revolution) failed to displace the entrenched practice.