femtometre vs femtometer

femtometre and femtometer both are nouns.

Word	Noun	Adjective	Verb	Adverb
femtometre	Yes	No	No	No
femtometer	Yes	No	No	No

As nouns, femtometre and femtometer are synonyms defined as:

femtometre and femtometer: a metric unit of length equal to one quadrillionth of a meter

Other synonyms of femtometre include fermi.

femtometre (noun)	femtometer (noun)
a metric unit of length equal to one quadrillionth of a meter	a metric unit of length equal to one quadrillionth of a meter