A unit of area, equal to 10^{−43} square metres. See **barn** and **inverse femtobarn. **

"Femto" means a factor of 10^{-15}, a thousandth of a millionth of a millionth. So a femtobarn is a barn (10^{-28} square metres) multiplied by 10^{-15}, which equals 10^{-43} square metres.

When writing pages for the general public, always give an approximate number of collisions as well as the figure in inverse femtobarns.

**One inverse femtobarn is equal to approximately 100 trillion proton-proton collisions**.

Be sure to include the word "approximately" or "about" before the collision number you cite - the figure is a general conversion rather than a precise estimate.

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