What is the data source for onlineconversion.com values?

[chemebabak]

That is, what is the source or reference (NIST, etc.) that dictates the values. Who is the arbiter that 1609 m = 1 mile?

[JohnS]

That is, what is the source or reference (NIST, etc.) that dictates the values. Who is the arbiter that 1609 m = 1 mile?

NIST has a number of publications. NIST SP 811 has an appendix full of conversions. The main body of the publication is proper usage rules for the SI. NIST Handbook 44, Appendix C, and other Handbooks on their series of checking accuracy of net contents labeling, dispensing devices etc has definitions of Customary, SI and conversions between them.

As to your specific question the exact conversion is 1609.344 m = 1 mile. It stems from an agreement in 1958 with 5 other English-speaking nations to commonize the definition of the foot and pound. The US formally adopted those values 1959-07-01, published in the Federal Register. The foot is exactly 0.3048 m, and the mile being 5280 ft leads directly to the above. (The six nations were Australia, Canada, New Zealand, South Africa, UK, and US.)

As for authority, Congress delegates its Constitutional power to fix the system of weights and measures to the Secretary of Commerce, who in turn assigns it to NIST, which works for him.

Note that Customary units have no primary physical standards (they were retired to a museum in 1893, Mendenhall Order). Customary units are defined as declared values of their SI counterparts. Note that Imperial has some units different from Customary (Imperial gallon) and NPL in the UK is the authority for those, although NIST publishes those conversions for convenience.