What Is Smaller Than a Millisecond?

Below a millisecond the units keep going down in steps of 1,000: microsecond, nanosecond, picosecond, femtosecond, attosecond.

Each step down is a factor of a thousand, following the SI prefix ladder. There is no unit between them with a common name.

Where Each One Shows Up

• Microsecond — SSD reads, syscalls, network hops inside a datacentre
• Nanosecond — CPU instructions, cache and memory access, light travelling 30 cm
• Picosecond — signal propagation across a chip, laser pulse timing
• Femtosecond — molecular vibration, ultrafast laser chemistry
• Attosecond — electron motion; the 2023 Nobel Prize in Physics went to work at this scale

Where the Practical Floor Is

For anything you are likely to build, nanoseconds are the floor. Below that:

• Reading a clock takes 20-30 ns, so the measurement costs more than what you are measuring.
• Operating system clock resolution rarely goes finer than a few hundred nanoseconds.
• Network time synchronisation is accurate to milliseconds; hardware PTP reaches sub-microsecond, GPS a little further. Nothing routine reaches picoseconds.

Reading Subsecond Timestamps

A fractional timestamp's digit count tells you the unit:

• 12:00:00.123 — milliseconds, 3 digits
• 12:00:00.123456 — microseconds, 6 digits
• 12:00:00.123456789 — nanoseconds, 9 digits

Unix timestamps follow the same logic: 10 digits is seconds, 13 milliseconds, 16 microseconds. The [Unix timestamp converter](/tools/unix-timestamp) will tell you which reading produces a plausible date.