fem·​to·​me·​ter ˈfem(p)-tə-ˌmē-tər How to pronounce femtometer (audio)
: a unit of length equal to 10−15 meters : one quadrillionth of a meter

Examples of femtometer in a Sentence

Recent Examples on the Web This explains why the proton and neutron have particular sizes to them, where each one has a radius that’s a little smaller than a femtometer. Ethan Siegel, Forbes, 20 Apr. 2021 Finally, the results are in: Hessels and company have pegged the proton’s radius at 0.833 femtometers, give or take 0.01, a measurement exactly consistent with Pohl’s value. Quanta Magazine, 11 Sep. 2019 Pohl and his team were surprised to find a lower value for the radius, pegging it at 0.84 femtometer—well outside the range of potential sizes established by earlier measurements. Dan Falk, Scientific American, 28 Nov. 2019 The result: their measurement of 0.833 femtometers (just under one-trillionth of a meter) agrees with the smaller value from the 2010 study. Jennifer Ouellette, Ars Technica, 5 Sep. 2019

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'femtometer.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


femto- + meter entry 3

First Known Use

1971, in the meaning defined above

Time Traveler
The first known use of femtometer was in 1971

Dictionary Entries Near femtometer

Cite this Entry

“Femtometer.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/femtometer. Accessed 7 Dec. 2023.

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