# Microsecond

A **microsecond** (symbol µs) is one millionth of a second, that is, \( 10^{-6} \) s or 0.000001 s, formed with the [SI prefix](https://www.edgepedia.org/si-prefix) micro.<sup>[1](https://www.bipm.org/en/committees/cg/cgpm/11-1960/resolution-12)</sup><sup> • </sup><sup>[2](https://www.collinsdictionary.com/us/dictionary/english/microsecond)</sup> It sits between the millisecond (one thousandth of a second) and the nanosecond (one billionth of a second), and it is the natural time scale of fast electronics, network timing, and atomic-clock distribution systems.

| Key fact | Detail |
|---|---|
| Value and symbol | \( 10^{-6} \) s; symbol µs, from the Greek letter mu used for the micro prefix<sup>[1](https://www.bipm.org/en/committees/cg/cgpm/11-1960/resolution-12)</sup><sup> • </sup><sup>[3](https://www.oxfordlearnersdictionaries.com/us/definition/english/microsecond)</sup> |
| Prefix origin | The micro prefix, factor \( 10^{-6} \), was adopted by the 11th CGPM in 1960 (Resolution 12) as part of the original SI prefix set<sup>[1](https://www.bipm.org/en/committees/cg/cgpm/11-1960/resolution-12)</sup> |
| Frequency relation | A 1 MHz sine wave completes one cycle every microsecond; period is the reciprocal of frequency, \( T = 1/f \)<sup>[4](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication960-12e2.pdf)</sup> |
| Light travel | In vacuum, light travels about 300 meters in one microsecond<sup>[5](https://scixa.com/article?slug=the-speed-of-light-and-computing)</sup> |
| Timing distribution | GPS receivers determine time to within 100 nanoseconds; NTP with a GPS pulse-per-second signal reaches accuracy on the order of a few microseconds<sup>[6](https://www.gps.gov/gps-and-telling-time)</sup><sup> • </sup><sup>[7](https://www.ntp.org/reflib/exec/)</sup> |
| Telecom clocks | Stratum 1 clocks, the highest quality in the telecommunications timing hierarchy, keep time to within about one microsecond per day<sup>[8](https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-st-sy)</sup> |
| Storage latency | NAND flash memory access takes tens of microseconds, against tens of nanoseconds for DRAM and several milliseconds for spinning disk<sup>[9](https://technav.ieee.org/topic/latency/)</sup> |

## Definition and notation

The microsecond is not an independent unit but the second, the [SI base unit](https://www.edgepedia.org/si-base-unit) of time, combined with the prefix micro. The second itself is defined by fixing the caesium frequency \( \Delta\nu_{\mathrm{Cs}} \), the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, at exactly 9,192,631,770 Hz, so one second is the duration of that many periods of the corresponding radiation.<sup>[10](https://www.bipm.org/en/si-base-units/second)</sup> A microsecond is therefore \( 10^{-6} \) of 9,192,631,770 caesium periods.

The prefix micro was adopted at the creation of the SI by the 11th [General Conference on Weights and Measures](https://www.edgepedia.org/general-conference-on-weights-and-measures) in 1960, in the same resolution that established the prefix series from tera down to pico.<sup>[1](https://www.bipm.org/en/committees/cg/cgpm/11-1960/resolution-12)</sup> Its symbol µ is the only SI prefix symbol taken from the [Greek alphabet](https://www.edgepedia.org/greek-alphabet) rather than the Latin one, and prefix symbols attach to unit symbols without a space, giving µs.<sup>[11](https://www.bipm.org/utils/common/pdf/si_brochure_8.pdf)</sup> Older engineering texts sometimes write µsec, but µs is the standard form.<sup>[12](https://science.osti.gov/-/media/wdts/suli/pdf/SULI_Deliverables_Guidelines.pdf)</sup>

## Frequency and period

The microsecond appears wherever frequencies reach the megahertz range. Since the period of a repetitive event is the reciprocal of its frequency, a 1 MHz signal has a period of exactly one microsecond, and a 32,768 Hz quartz crystal, the most common time base in watches and timers, ticks once every 30.518 µs.<sup>[4](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication960-12e2.pdf)</sup> Digital stopwatches commonly resolve 10 milliseconds, so microsecond-level interval measurement requires electronic counters rather than handheld devices.<sup>[4](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication960-12e2.pdf)</sup>

## Time distribution and synchronization

Distributing time to microsecond accuracy is a practical engineering target across several systems. In telecommunications, stratum 1 clocks, the top of the network timing hierarchy, hold frequency offset to \( 1 \times 10^{-11} \) or less, which keeps them within about one microsecond per day of reference time.<sup>[8](https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-st-sy)</sup> GPS provides a common source: each satellite carries atomic clocks, and a receiver decoding the signals synchronizes itself to within 100 billionths of a second without owning an atomic clock.<sup>[6](https://www.gps.gov/gps-and-telling-time)</sup> Combining a GPS receiver and pulse-per-second signal with operating system support, [Network Time Protocol](https://www.edgepedia.org/network-time-protocol) achieves accuracy on the order of a few microseconds.<sup>[7](https://www.ntp.org/reflib/exec/)</sup>

For finer work, the IEEE 1588 [Precision Time Protocol](https://www.edgepedia.org/precision-time-protocol) with hardware timestamping in network interface cards enables sub-microsecond round-trip time measurement by bypassing software stack delays.<sup>[9](https://technav.ieee.org/topic/latency/)</sup> Industrial automation protocols such as PROFINET, EtherCAT, and the IEEE/IEC 60802 TSN profile enforce cycle times in the 250-microsecond to 1-millisecond range while synchronizing clocks to sub-microsecond accuracy.<sup>[13](https://technav.ieee.org/topic/low-latency-communication/)</sup> [Electric power](https://www.edgepedia.org/electric-power) grids also work at this scale: synchrophasor measurements usually target 1 µs accuracy, which corresponds to a phase angle error of only 0.022° at 60 Hz.<sup>[14](https://tf.nist.gov/general/pdf/2941.pdf)</sup>

Operating systems expose microsecond-level timing directly. Windows offers GetSystemTimePreciseAsFileTime and KeQuerySystemTimePrecise for UTC-synchronized timestamps with a resolution of 1 microsecond or better, and the SunOS kernel historically included a system clock with 1 µs resolution.<sup>[15](https://learn.microsoft.com/th-th/windows/win32/sysinfo/acquiring-high-resolution-time-stamps)</sup><sup> • </sup><sup>[16](https://www.ietf.org/ietf-ftp/rfc/rfc1589.txt.pdf)</sup> Path asymmetry between client and server remains a fundamental barrier to microsecond-level accuracy in internet timekeeping, which is why hardware timestamping and calibrated paths matter.<sup>[17](https://arxiv.org/abs/2011.06713)</sup>

## Physical scale

One microsecond bounds how far signals can travel. Light in vacuum covers about 300 meters in a microsecond, roughly a large data center hall or a campus of buildings, so a microsecond of latency corresponds to a fast network hop at that scale.<sup>[5](https://scixa.com/article?slug=the-speed-of-light-and-computing)</sup> This relationship links physical distance to timing budgets in system design: propagation over hundreds of meters already consumes a microsecond, which matters when synchronization targets are themselves sub-microsecond.

## Context among time units

The microsecond occupies the middle of the electronic timing range. DRAM access takes tens of nanoseconds, [NAND flash](https://www.edgepedia.org/nand-flash) tens of microseconds, and spinning disk several milliseconds, so the microsecond marks the boundary between memory-class and storage-class latency.<sup>[9](https://technav.ieee.org/topic/latency/)</sup> At the accurate end of timekeeping, the best cesium oscillators such as NIST-F1 hold frequency to about \( 3 \times 10^{-16} \), a time error of about 0.03 nanoseconds per day, so a microsecond of accumulated error corresponds to tens of millions of days of ideal cesium performance.<sup>[18](https://www.nist.gov/pml/time-and-frequency-division/timekeeping-and-clocks-faqs)</sup> Optical clocks under development, such as strontium lattice clocks that would not gain or lose a second in 15 billion years, are candidates for redefining the second itself.<sup>[19](https://www.nist.gov/how-do-you-measure-it/how-do-you-measure-second)</sup>

## References

1. [Resolution 12 of the 11th CGPM (1960): SI prefixes, BIPM](https://www.bipm.org/en/committees/cg/cgpm/11-1960/resolution-12)
2. [Microsecond definition, Collins English Dictionary](https://www.collinsdictionary.com/us/dictionary/english/microsecond)
3. [Microsecond noun, Oxford Advanced Learner's Dictionary](https://www.oxfordlearnersdictionaries.com/us/definition/english/microsecond)
4. [NIST Recommended Practice Guide: Stopwatch and Timer Calibrations (2009 edition), NIST SP 960-12](https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication960-12e2.pdf)
5. [The Speed of Light and Computing: The Ceilings Every Computer Must Hit, Scixa](https://scixa.com/article?slug=the-speed-of-light-and-computing)
6. [GPS and Telling Time, GPS.gov](https://www.gps.gov/gps-and-telling-time)
7. [Executive Summary: Computer Network Time Synchronization, NTP.org](https://www.ntp.org/reflib/exec/)
8. [Time and Frequency from A to Z, St to Sy, NIST](https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-st-sy)
9. [Latency, IEEE Technology Navigator](https://technav.ieee.org/topic/latency/)
10. [SI base unit: second, BIPM](https://www.bipm.org/en/si-base-units/second)
11. [The International System of Units (SI brochure), 8th edition, BIPM](https://www.bipm.org/utils/common/pdf/si_brochure_8.pdf)
12. [Units and abbreviations guidelines, OSTI SULI Deliverables Guidelines](https://science.osti.gov/-/media/wdts/suli/pdf/SULI_Deliverables_Guidelines.pdf)
13. [Low latency communication, IEEE Technology Navigator](https://technav.ieee.org/topic/low-latency-communication/)
14. [Metrological and Legal Traceability of Time Signals, NIST](https://tf.nist.gov/general/pdf/2941.pdf)
15. [Acquiring high-resolution time stamps, Microsoft Learn](https://learn.microsoft.com/th-th/windows/win32/sysinfo/acquiring-high-resolution-time-stamps)
16. [RFC 1589: Kernel Model for Precision Timekeeping, IETF](https://www.ietf.org/ietf-ftp/rfc/rfc1589.txt.pdf)
17. [iHorology: Lowering the Barrier to Microsecond-level Internet Time, arXiv](https://arxiv.org/abs/2011.06713)
18. [Timekeeping and clocks FAQs, NIST](https://www.nist.gov/pml/time-and-frequency-division/timekeeping-and-clocks-faqs)
19. [How Do You Measure a Second?, NIST](https://www.nist.gov/how-do-you-measure-it/how-do-you-measure-second)

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*Topic: Encyclopedia › Physical world and mathematics › Measurement and time › Timekeeping and time standards › Units of time › Second and subsecond units*

*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Sep 30, 2026; Oct 15, 2026 · Last review: Oct 15, 2026*

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