# Size measures for items related to cells

This page lists order of magnitude estimates for the sizes of various microscopic items that are related to cells. The goal is to give a rough sense of what individual sizes mean, since these are too small for us to compare with our day-to-day intuitions.

## Units for length

### Units

Unit for length (full word) Unit for length (shorthand) In terms of meters (m) What's it used to measure?
femtometer fm ${\displaystyle 10^{-15}}$ nucleus of an atom (not nucleus of a cell). Rarely used in biology
picometer pm ${\displaystyle 10^{-12}}$ radius and diameter of an atom or size of a molecule. Used in biology when talking about biological membranes and passage of ions through membranes
angstrom A ${\displaystyle 10^{-10}}$ sizes of molecules, wavelengths of light
nanometer nm ${\displaystyle 10^{-9}}$ sizes of very large molecules, substructures and organelles of cells, wavelengths of light
micron or micrometer ${\displaystyle \mu }$m ${\displaystyle 10^{-6}}$ sizes of cells and cellular organelles
millimeter mm ${\displaystyle 10^{-3}}$ Rarely used in cell biology. May be used for tissues and cell clusters

### Conversion between units

Unit for length (full word) Unit for length (shorthand) Number of fm Number of pm Number of angstroms Number of nm Number of ${\displaystyle \mu }$m Number of mm
femtometer fm 1 ${\displaystyle 10^{-3}}$ ${\displaystyle 10^{-5}}$ ${\displaystyle 10^{-6}}$ ${\displaystyle 10^{-9}}$ ${\displaystyle 10^{-12}}$
picometer pm ${\displaystyle 10^{3}}$ 1 ${\displaystyle 10^{-2}}$ ${\displaystyle 10^{-3}}$ ${\displaystyle 10^{-6}}$ ${\displaystyle 10^{-9}}$
angstrom A ${\displaystyle 10^{5}}$ ${\displaystyle 10^{2}}$ 1 ${\displaystyle 10^{-1}}$ ${\displaystyle 10^{-4}}$ ${\displaystyle 10^{-7}}$
nanometer nm ${\displaystyle 10^{6}}$ ${\displaystyle 10^{3}}$ 10 1 ${\displaystyle 10^{-3}}$ ${\displaystyle 10^{-6}}$
micron or micrometer ${\displaystyle \mu }$m ${\displaystyle 10^{9}}$ ${\displaystyle 10^{6}}$ ${\displaystyle 10^{4}}$ ${\displaystyle 10^{3}}$ 1 ${\displaystyle 10^{-3}}$
millimeter mm ${\displaystyle 10^{12}}$ ${\displaystyle 10^{9}}$ ${\displaystyle 10^{7}}$ ${\displaystyle 10^{6}}$ ${\displaystyle 10^{3}}$ 1