The SI base unit of time is the second (s).
The second (s) is defined by taking the fixed numerical value of the caesium-133 atom’s unperturbed ground-state hyperfine transition frequency ΔνCs to be exactly 9,192,631,770 when expressed in hertz (s−1). One hertz is one cycle per second.
Symbol and quantity
- Unit name: second
- Symbol: s
- Base quantity: time
Common SI prefixes for time
| Prefix | Symbol | Relation to second | Example |
| millisecond | ms | 1 ms = 10−3 s | Blink ≈ 100 ms |
| microsecond | µs | 1 µs = 10−6 s | CPU cycle scale |
| nanosecond | ns | 1 ns = 10−9 s | Network latency scale |
| picosecond | ps | 1 ps = 10−12 s | Ultrafast optics |
| kilosecond | ks | 1 ks = 103 s | 16.67 minutes |
Non-SI units accepted for use with SI
- 1 minute (min) = 60 s
- 1 hour (h) = 3600 s
- 1 day (d) = 86,400 s
Quick examples
- 2.5 minutes = 2.5 × 60 = 150 s
- 3 hours = 3 × 3600 = 10,800 s
- 750 ms = 0.750 s
