A Helium atom,
having a radius of 31 picometres.

A picometre (American spelling: picometer, symbol pm) is a unit of length in the metric system, equal to one trillionth (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 (engineering notation) or (exponential notation) ? both meaning .

It equals a millionth of a micrometre (formerly called a micron), and was formerly called micromicron, stigma, or bicron.^[1] The symbol µµ was once used for it.^[2]

It equals a hundredth of an Ångström, an internationally recognized non-SI unit of length.

See also

• Metric system
• Orders of magnitude (length)
• Conversion of units, for comparison with other units of length.

References

1. ↑
2. ↑ 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