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spelling of metre and litre

Most of the English-speaking world spells the basic unit of length in metric systems “metre.” In the United States, it is spelled “meter.” Similarly, the unit Americans spell “liter” is elsewhere spelled “litre.”

The American spelling is due to Noah Webster (1758–1843), who condemned the “-re” ending in his influential speller and dictionary.

Not all American authorities approve of the “-er” ending; the American Society for Testing and Materials and the U.S. Metric Association prefer “metre” and “litre.”

American Society for Testing and Materials.
Standard for Metric Practice. (ASTM E 380-82)
American Society for Testing and Materials, Philadelphia, 1982.