What is the smallest unit of time?
The shortest meaningful unit of time is the Planck time, about 5.39 × 10−44 seconds. The shortest time ever measured is 247 zeptoseconds (247 × 10−21 s), and the smallest official SI unit is the quectosecond (10−30 s).
Key facts
| Planck time | 5.39 × 10⁻⁴⁴ s |
|---|---|
| Shortest measured | 247 zeptoseconds (2020) |
| Smallest SI prefix | Quecto (10⁻³⁰), added 2022 |
| Attosecond Nobel Prize | 2023, Agostini, Krausz, L'Huillier |
| 1 zeptosecond | 10⁻²¹ s |
| 1 yoctosecond | 10⁻²⁴ s |
| Unit | Length | What happens in that time |
|---|---|---|
| Second (s) | 1 | A heartbeat, roughly |
| Millisecond (ms) | 10−3 s | A camera flash lasts about 1 ms |
| Microsecond (µs) | 10−6 s | GPS clocks must agree to within tens of nanoseconds to microseconds |
| Nanosecond (ns) | 10−9 s | Light travels about 30 cm |
| Picosecond (ps) | 10−12 s | Fastest transistors switch |
| Femtosecond (fs) | 10−15 s | Atoms move in chemical reactions (Nobel Prize 1999) |
| Attosecond (as) | 10−18 s | Electrons move (Nobel Prize in Physics 2023) |
| Zeptosecond (zs) | 10−21 s | Light crosses a hydrogen molecule in 247 zs |
| Yoctosecond (ys) | 10−24 s | Lifetimes of some particles |
| Rontosecond (rs) | 10−27 s | Prefix added in 2022 |
| Quectosecond (qs) | 10−30 s | Smallest SI prefix (2022) |
| Planck time | 5.39 × 10−44 s | Shortest time with physical meaning, as far as we know |
Planck time: the shortest meaningful time
Planck time, about 5.39 × 10−44 seconds, is the time light takes to cross one Planck length (1.6 × 10−35 m). It is built from three fundamental constants: the speed of light, the gravitational constant and Planck's constant. Below that scale, our theories of quantum mechanics and gravity both break down, so physicists cannot say what a shorter time would even mean. It is not proven to be a "pixel" of time, but nothing shorter has any known meaning.
The shortest time ever measured
In 2020 physicists at Goethe University Frankfurt measured how long it takes a photon to cross a hydrogen molecule: 247 zeptoseconds. In 2023 the Nobel Prize in Physics went to Pierre Agostini, Ferenc Krausz and Anne L'Huillier for making attosecond pulses of light, which act like a camera flash fast enough to film electrons.
How small is a zeptosecond?
A zeptosecond is a trillionth of a billionth of a second. There are about as many zeptoseconds in one second as there are seconds in 30 trillion years, more than 2,000 times the age of the universe.
For the other end of the scale, see all units of time, how a second is defined, or the clock with milliseconds.
Questions people ask
What is the smallest unit of time?
The smallest unit with physical meaning is the Planck time, about 5.39 × 10−44 seconds. The smallest named SI unit is the quectosecond, 10−30 seconds.
What is the shortest time ever measured?
About 247 zeptoseconds (247 × 10−21 s), the time a photon took to cross a hydrogen molecule, measured in 2020.
What is a zeptosecond?
One trillionth of a billionth of a second, 10−21 seconds.
What is a yoctosecond?
10−24 seconds, a thousand times shorter than a zeptosecond. Some unstable particles exist for only a few yoctoseconds.
Is time made of tiny pieces?
Nobody knows. Planck time is the scale where current physics stops working, and some theories of quantum gravity suggest time may be granular there, but there is no experimental evidence yet.
Last updated October 23, 2026.