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What is a second?

The SI base unit of time — defined by atomic caesium transitions

Definition

The second is the SI base unit of time. It is defined by the caesium-133 atomic transition: 9,192,631,770 cycles of this oscillation equal one second. Modern atomic clocks maintain the second to about 10⁻¹⁶ accuracy — you would lose one second in 100 million years.

Common uses

  • Everyday timekeeping
  • Scientific and engineering measurements
  • Sports timing (100 m sprint in ~10 s)

🌍 Real-world scale

A blink takes 100–400 ms. Heartbeat at rest ~1 s. The 100 m sprint world record is 9.58 s (Usain Bolt).

Convert from seconds

9 conversions
Second to Millisecond1 s = 1,000 msSecond to Minute1 s = 0.01667 minSecond to Hour1 s = 0.0002778 hSecond to Day1 s = 1.1574e-5 dSecond to Week1 s = 1.6534e-6 wkSecond to Month1 s = 3.8026e-7 moSecond to Year1 s = 3.1688e-8 yrSecond to Decade1 s = 3.1688e-9 decSecond to Century1 s = 3.1688e-10 c

Convert to seconds

9 conversions
Millisecond to Second1 ms = 0.001 sMinute to Second1 min = 60 sHour to Second1 h = 3,600 sDay to Second1 d = 86,400 sWeek to Second1 wk = 604,800 sMonth to Second1 mo = 2,629,746 sYear to Second1 yr = 31,557,600 sDecade to Second1 dec = 315,576,000 sCentury to Second1 c = 3,155,760,000 s

Tips

  • 60 s = 1 minute; 3600 s = 1 hour; 86,400 s = 1 day.
  • For sub-second intervals use ms (milliseconds), µs (microseconds) and ns (nanoseconds).
  • The symbol is s (lowercase). "sec" is informal.

Common mistakes

  • Writing S instead of s for the second.
  • Confusing second of time with second of arc in astronomy.
  • Assuming microsecond and millisecond are similar — 1 ms = 1000 µs.

FAQ about the second

What is a second?
The second is the SI base unit of time. It is defined by the caesium-133 atomic transition: 9,192,631,770 cycles of this oscillation equal one second. Modern atomic clocks maintain the second to about 10⁻¹⁶ accuracy — you would lose one second in 100 million years.
Where is the second used?
The second is commonly used for: Everyday timekeeping; Scientific and engineering measurements; Sports timing (100 m sprint in ~10 s).
What is an example of a second in real life?
A blink takes 100–400 ms. Heartbeat at rest ~1 s. The 100 m sprint world record is 9.58 s (Usain Bolt).
What category does the second belong to?
The second is a unit of time, part of the 10-unit time category on UnitConv.
How accurate are second conversions?
All conversion factors follow NIST and BIPM international standards to 8 significant figures — the highest practical accuracy.
Explore all 10 time units and their conversions.⏱️ Time hub →