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

The picometre (International spelling as used by the International Bureau of Weights and Measures; SI symbol: pm) or picometer (American spelling) 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 SI base unit of length. It can be written in scientific notation as 1×10−12 m, in engineering notation as 1 E−12 m, and is simply 1 m / 1,000,000,000,000.

It equals one millionth of a micrometre (also known as a micron), and was formerly called micromicron, stigma, or bicron.[2] The symbol µµ was once used for it.[3] It is also one 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, quantum physics, chemistry and acoustics. Atoms are between 62 and 520 pm in diameter, and the typical length of a carbon-carbon single bond is 154 pm. 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 is planned for launch in 2034 to directly detect gravitational waves and will measure relative displacements with a resolution of 20 picometers over a distance of 5 million kilometers, yielding a strain sensitivity of better than 1 part in 1020.

See also


  1. ^ "Atomic radius". WebElements: the periodic table on the web. 
  2. ^ Deza, Elena;  
  3. ^ How Many? A Dictionary of Units of Measurement; Russ Rowlett and the University of North Carolina at Chapel Hill; http://www.unc.edu/~rowlett/units/dictB.html