A simplified representation of a helium atom, having an estimated (calculated) radius of 31 picometres^[1]

A picometre (American spelling: picometer; symbol pm) is a unit of length in the metric system, equal to one trillionth, i.e. (¹/_{1,000,000,000,000}) of a metre, which is the current SI base unit of length. It can be written in scientific notation as 1×10⁻¹² m, or as 1 E−12 m in engineering notation — both meaning 1 m / 1,000,000,000,000.

It equals a millionth of a micrometre (formerly called a micron), and was formerly called micromicron, stigma, or bicron.^[2] The symbol µµ was once used for it.^[3] It is also a hundredth of an angstrom, an internationally recognised but non-SI unit of length.

The picometre's length is of an order such that its application is almost entirely confined to particle physics and quantum physics. Atoms are between 62 and 520 pm in diameter. Smaller units still may be used to describe smaller particles (some of which are the components of atoms themselves), such as hadrons and the upper limits of possible size for fermion point particles.

The Laser Interferometer Space Antenna (LISA) probe had been planned to be launched in 2025 to directly detect gravitational waves and would have been able to measure relative displacements with a resolution of 20 picometres over a distance of 5 million kilometres, yielding a strain sensitivity of better than 1 part in 10²⁰. 2011 mission changes have put the fate of the LISA project in jeopardy.

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