The smallest unit of time with an official SI name is the quectosecond (10^-30 s), added in 2022. The shortest interval ever measured is about 247 zeptoseconds (247 x 10^-21 s), and the smallest meaningful time in physics is the Planck time, about 5.39 x 10^-44 seconds.
So which answer is right? In short, it depends on what you mean by “smallest unit of time.” In fact, there are three different answers, and the old version of this page mixed them up. First, there is a naming question: what is the smallest prefix the International System of Units (SI) allows in front of “second”? Second, there is a lab question: what is the shortest stretch of time anyone has actually measured? Finally, there is a physics question: is there a smallest possible time at all?
This guide therefore answers all three with numbers you can check. It also explains the one unit everything else is built on, the second, and why it is defined by caesium atoms. Then it shows how tiny units like the femtosecond, attosecond and zeptosecond compare with things you can picture.
Convert Seconds to Milliseconds and Back
For everyday timing, the smallest unit most people use is the millisecond, one thousandth of a second. For example, stopwatches, sports timing and computer logs all work at this level. The converter below therefore covers milliseconds up to millennia. For anything smaller, use the prefix table in the next section, because each step down is simply another factor of 1,000.
The Smallest Unit of Time by SI Prefix
In general, the SI builds small units by putting a prefix in front of “second.” Also, each prefix below the milli level is 1,000 times smaller than the one before it. In November 2022, the General Conference on Weights and Measures (CGPM) added four new prefixes, including ronto (10^-27) and quecto (10^-30), in its Resolution 3, published by the BIPM. As a result, the quectosecond is now the smallest named unit of time in the SI.
| Unit | Symbol | Fraction of a second | Where you meet it |
|---|---|---|---|
| Millisecond | ms | 10^-3 s (one thousandth) | Stopwatches, reaction times, a camera flash |
| Microsecond | µs | 10^-6 s (one millionth) | Electronics, radar, lightning strokes |
| Nanosecond | ns | 10^-9 s (one billionth) | Computer clock cycles, GPS timing |
| Picosecond | ps | 10^-12 s | Fast lasers, one caesium cycle (about 108.8 ps) |
| Femtosecond | fs | 10^-15 s | Chemical bonds vibrating, laser pulses |
| Attosecond | as | 10^-18 s | Electrons moving inside atoms |
| Zeptosecond | zs | 10^-21 s | Shortest interval measured so far |
| Yoctosecond | ys | 10^-24 s | Lifetimes of some particles (calculated) |
| Rontosecond | rs | 10^-27 s | Named in 2022; no practical use yet |
| Quectosecond | qs | 10^-30 s | Smallest SI name; no practical use yet |
In other words, a name for a unit is not the same as a measurement. For instance, the SI can name a quectosecond, but no instrument comes anywhere close to timing one.
The Second: The Base Unit Behind Every Smaller Unit
Because every unit in the table above is a fraction of the second, the second has to be defined very precisely. As a result, since 1967 it has been tied to the caesium-133 atom, not to the turning of the Earth. According to the BIPM definition of the second, one second is the duration of exactly 9,192,631,770 cycles of the radiation from a specific energy jump (the ground-state hyperfine transition) in caesium-133. However, the previous version of this article said calcium atoms, which was wrong.
Because the number is fixed by definition, atomic clocks around the world can count those cycles and agree to within billionths of a second per day. In addition, one caesium cycle lasts about 1/9,192,631,770 of a second, or roughly 108.8 picoseconds. For everyday scale, that same second builds up to 86,400 seconds in a day, as our guide to how many seconds are in a day explains.
The Smallest Unit of Time Ever Measured
The record for the shortest interval directly measured belongs to a team at Goethe University Frankfurt. In a study published in the journal Science in October 2020, physicists led by Reinhard Doerner, with Sven Grundmann as lead author, timed how long a photon takes to cross a hydrogen molecule. The answer was about 247 zeptoseconds, reported by Phys.org as the shortest time span measured to date.
How do you time something that short? Certainly not with a clock. Instead, the team fired X-rays from the PETRA III source at DESY in Hamburg at hydrogen molecules. Next, each photon knocked an electron out of the molecule, first from one atom and then from the other. Because the two electron waves interfered, the shape of the interference pattern revealed the tiny delay between the two events. In short, the time was read from a pattern, not counted tick by tick.
So, 247 zeptoseconds is about 0.247 attoseconds, or 247 x 10^-21 seconds. For comparison, attosecond physics (the study of electron motion with light pulses of about 10^-18 s) earned the 2023 Nobel Prize in Physics. Zeptosecond work, by contrast, goes roughly a thousand times shorter.
Planck Time: The Smallest Unit of Time in Theory
Below the measured record lies a theoretical floor called the Planck time. Specifically, it combines three constants of nature: the reduced Planck constant, the gravitational constant and the speed of light. Using the values in the NIST CODATA list of constants, the Planck time works out to about 5.391 x 10^-44 seconds.
As you can see, that is far below even a quectosecond. In fact, one quectosecond is about 18.5 trillion Planck times, and the 247-zeptosecond record is about 4.6 x 10^24 Planck times. Moreover, many physicists think that at the Planck scale, our current theories of space and time stop working, so a shorter interval may have no meaning. However, nobody has tested this, and the Planck time is not an official SI unit.
Smallest Unit of Time: Three Answers Compared
Because people ask this question in different ways, here is how the three answers line up side by side.
| Question | Answer | Size in seconds | Status |
|---|---|---|---|
| Smallest named SI unit | Quectosecond (qs) | 10^-30 s | Official since 2022 (BIPM) |
| Shortest interval measured | About 247 zeptoseconds | 2.47 x 10^-19 s | Measured, Science, 2020 |
| Smallest meaningful time in physics | Planck time | about 5.39 x 10^-44 s | Theoretical, not an SI unit |
| Smallest unit on a typical stopwatch | 1/100 second | 10^-2 s | Everyday use |
| Base unit of time | Second (s) | 1 s | Defined by caesium-133 |
Meanwhile, the everyday picture is very different. For example, a blink takes a few hundred milliseconds, and many sports timing systems report to the hundredth or thousandth of a second. On the other hand, old and informal units go the other way: a medieval “moment” was about 90 seconds, as our guide to how long a moment is shows.
How to Read and Convert Tiny Time Units
Once you know the pattern, then, converting between these units is quick. Follow these steps:
- Find the exponent. Look up the unit in the prefix table, for example zepto = 10^-21.
- Subtract exponents to compare. To compare zepto (-21) with atto (-18), subtract: -18 minus -21 = 3, so 1 attosecond = 1,000 zeptoseconds.
- Move the decimal point. Each step of 3 in the exponent moves the decimal three places, for example 247 zs = 0.247 as.
- Use scientific notation for large gaps. Write 5.39 x 10^-44 rather than a long string of zeros, because it is easy to miscount zeros.
- Check with the converter. For seconds and milliseconds, enter your value in the converter above to confirm the result.
Stopwatches for Everyday Timing
None of the units below a microsecond show up in daily life, of course. For sports, lab classes, cooking and reaction-time games, however, a good digital stopwatch with 1/100-second resolution is the practical tool. So look for a clear display, a lap or split function, and a memory if you time several runs.
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Do and Don’t When Talking About Tiny Times
Do
- Say which question you mean: named unit, measured record or theory.
- Use scientific notation for anything below a nanosecond.
- Check the prefix order: atto, zepto, yocto, ronto, quecto.
- Cite the BIPM for unit names and definitions.
Don’t
- Call the zeptosecond the smallest unit; it is just the smallest measured.
- Treat the Planck time as an SI unit.
- Confuse stopwatch display resolution with real accuracy.
- Mix up caesium (the clock atom) with calcium.
Honest Limits
A few caveats apply to the numbers on this page. First, the 247-zeptosecond figure is a reported result from one experiment; newer work may set a shorter record, so treat it as the record as of this writing. Second, the Planck time value also depends on the measured gravitational constant, which is known less precisely than most constants, so we give it to four figures only. Third, a stopwatch reading to 0.01 second is not accurate to 0.01 second, because human reaction time adds roughly a fifth of a second or more. Therefore, for timing to the millisecond, use electronic start and stop triggers. Similarly, for a feel of what longer spans mean, see how long 900 seconds is.
Smallest Unit of Time: FAQs
What is the smallest unit of time?
The smallest named SI unit of time is the quectosecond, 10^-30 seconds, added in 2022. The shortest interval measured is about 247 zeptoseconds, and the Planck time, about 5.39 x 10^-44 s, is the smallest in theory.
Is the zeptosecond the smallest unit of time?
No. The zeptosecond (10^-21 s) is the scale of the shortest measurement so far, but the yoctosecond, rontosecond and quectosecond are all smaller named units.
What is the shortest time ever measured?
About 247 zeptoseconds, the time a photon took to cross a hydrogen molecule, measured by Goethe University physicists and published in Science in 2020.
What is a quectosecond?
A quectosecond is 10^-30 seconds, a millionth of a millionth of a millionth of a millionth of a millionth of a second. In fact, the prefix quecto was adopted by the CGPM in 2022.
What is Planck time?
Planck time is about 5.391 x 10^-44 seconds, built from the Planck constant, the gravitational constant and the speed of light. Still, it is a theoretical scale, not an SI unit.
How is a second defined?
One second is exactly 9,192,631,770 cycles of the radiation from the hyperfine transition of the caesium-133 atom, as defined by the BIPM.
How many zeptoseconds are in a second?
There are 10^21 zeptoseconds in one second, which is one sextillion. One zeptosecond is a trillionth of a billionth of a second.
What is the smallest unit on a stopwatch?
Most digital stopwatches display hundredths of a second. However, electronic sports timing often reports thousandths, which is a millisecond.
Which is smaller, an attosecond or a femtosecond?
An attosecond is smaller, because one femtosecond equals 1,000 attoseconds.
Can anything happen faster than Planck time?
Current physics cannot describe events shorter than the Planck time, so most physicists treat it as a practical limit. Even so, whether time is truly divisible below it is still an open question.
The Smallest Unit of Time: Bottom Line
To sum up, the smallest unit of time has three honest answers. The quectosecond, 10^-30 s, is the smallest official SI name. Next, about 247 zeptoseconds is the shortest interval anyone has measured. Finally, the Planck time, about 5.39 x 10^-44 s, is where known physics runs out.
Above all, these rest on the same base: the second, defined by 9,192,631,770 cycles of caesium-133. For everyday timing, however, milliseconds are as small as you will need, and the converter above handles them.