Microsecond vs Nanosecond: What's the Difference?
A microsecond is 1,000 nanoseconds. See what lives at each scale, why nanosecond precision is often fictional, and how to read the units correctly.
A microsecond is 1,000 nanoseconds. One millionth of a second against one billionth.
| Unit | Symbol | In a second | In scientific notation |
|---|---|---|---|
| Microsecond | µs | 1,000,000 | 10⁻⁶ s |
| Nanosecond | ns | 1,000,000,000 | 10⁻⁹ s |
A Nanosecond Is a Foot of Cable
The most useful intuition for a nanosecond comes from Grace Hopper, who handed out 30 cm lengths of wire to illustrate how far light travels in one. That is the hard floor on latency: no signal crosses a room in less than a few nanoseconds, regardless of how fast the hardware is.
A microsecond, at 1,000 times longer, covers about 300 metres of cable.
What Lives at Each Scale
Nanoseconds — a single CPU instruction (under 1 ns on modern hardware), an L1 cache hit (~1 ns), an L3 hit (~10-40 ns), a main memory read (~80-100 ns).
Microseconds — an SSD read (50-150 µs), a syscall (1-10 µs), a network round trip within a rack (20-100 µs), a context switch (1-5 µs).
The jump from memory to storage is where nanoseconds become microseconds, and it is the largest performance cliff in most systems.
Nanosecond Precision Is Often Fictional
A timestamp with nanosecond digits rarely has nanosecond accuracy. Several things get in the way:
- Clock resolution — the OS may only update its clock every few hundred nanoseconds, or worse.
- Call overhead — reading the clock itself takes 20-30 ns, which is inside the value you are trying to measure.
- NTP synchronisation — network time sync is accurate to milliseconds, not nanoseconds. Only PTP or GPS gets below that.
So a log line reading 12:00:00.123456789 is precise to nine digits and accurate to perhaps six. Precision is how many digits you print; accuracy is how many of them mean anything.
Measuring Durations at This Scale
Use a monotonic clock, not the wall clock. A wall clock can be adjusted backwards by NTP mid-measurement and produce a negative duration. See [what is a monotonic clock](/blog/what-is-a-monotonic-clock).