Metric Act of 1866
The Metric Act of 1866, enacted July 28, 1866, legally
recognized the metric system of measurement in the US. It's
sometimes referred to as the Kasson Act, after Congressman John
A. Kasson of Iowa, who chaired the House Comittee on Coinage,
Weights, and Measures. The history
section below has more details on the reasons behind the law from
John Kasson's report to Congress.
Text of the law
The Act was codified as 15 USC 204 et seq., shown below. The following version shows the law as amended in 2007; refer to the history section for details.
Commerce and Trade
Weights and Measures and Standard Time
Weights, Measures, and Standards Generally
Sec. 204. Metric system authorized
It shall be lawful throughout the United States of America to employ
the weights and measures of the metric system; and no contract or
dealing, or pleading in any court, shall be deemed invalid or liable to
objection because the weights or measures expressed or referred to
therein are weights or measures of the metric system.
Sec. 205. Metric system defined
The metric system of measurement shall be defined as the
International System of Units as established in 1960, and subsequently
maintained, by the General Conference of Weights and
Measures, and as interpreted or modified for the United States
by the Secretary of Commerce.
Original bill and amendments
The Metric Act of 1866 was originally introduced as H.R. 596
in the 39th Congress. The House passed it on 17 May 1866;
the Senate passed it on 27 July 1866; and it was presented to
the President and signed the next day.
The 1866 Act included a now-obsolete definition of the metric system and tables of units, which you can view in the original 1866 bill
as passed by the House and sent to the Senate.
On 9 August 2007, the 1866 law was amended by Pub. L. 110–69, the America COMPETES Act. It replaced the old definition of the metric system by the modern-day definition of SI. The relevant pages are available here (PDF, 5 pp, 77K), extracted from the 148-page law. The extract includes the cover page and Sec. 1, containing the short title of the act, then jumps to the page containing Sec. 3013(c)(1), which amends the Metric Act of 1866.
The extract also includes the rest of Sec. 3013, which repeals an obsolete law that defined electrical and photometric units and amends the law defining time zones, primarily to officially establish UTC (Coordinated Universal Time) as the basis for clock times.
Following is some background on the reasons for the law, and the
values of the conversion factors, from John Kasson's report of
the Committee on Coinage, Weights, and Measures (39th Congress,
1st Session, H.R. Report No. 62, May 17, 1866):
|Congressman John A. Kasson|
House Committee on Coinage, Weights, and Measures
metric system is already used in some arts and trades in this
country, and is especially adapted to the wants of others. Some
of its measures are already manufactured at Bangor, in Maine, to
meet an existing demand at home and abroad. The manufacturers of
the well-known Fairbanks scales state: “For many years we
have had a large export demand for our scales with French
weights, and the demand and sale is constantly increasing.”
Its minute and exact divisions specially adapt it to the use of
chemists, aphothecaries, the finer operations of the artisan, and
to all scientific objects. It has always been and is now used in
the United States coast survey. Yet in some of the States, owing
to the phraseology of their laws, it would be a direct violation
of them to use it in the business transactions of the
community. It is therefore very important to legalize its use,
and give to the people, or that portion of them desiring it, the
opportunity for its legal employment, while the knowledge of its
characteristics will be thus diffused among men. Chambers of
commerce, boards of trade, manufacturing associations, and other
voluntary societies, and individuals, will be induced to consider
and in their discretion to adopt its use. The interests of trade
among a people so quick as ours to receive and adopt a useful
novelty, will soon acquaint practical men with its
convenience. When this is attained—a period, it is hoped,
not distant—a further act of Congress can fix a date for
its exclusive adoption as a legal system. At an earlier period it
may be safely introduced into all public offices, and for
In the schedule of equivalents provided in the
bill, extreme scientific accuracy is not expressed. The reasons
follow. The exact length of the meter in inches and the weight of
the kilogram in grains can of necessity be determined only
approximately. The most careful determinations of these
quantities now possible are liable to minute corrections
hereafter, as more numerous observations are made and better
instruments are used. Instead, therefore, of aiming at an
accuracy greater, perhaps, than is attainable, it is more
expedient to consult the convenience of the people by using the
simplest numbers possible in the schedule, and yet such as shall
be in fact more nearly exact than can ever be demanded in the
ordinary business of life. These numbers are to be used in
schools, and in practical life millions of times as multipliers
and divisors, and every unnecessary additional figure is justly
In a popular sense of the word, however, the
numbers in the schedule may be said to be exact. The length of
the meter, for example, is given as 39.37 inches. The mean of the
best English and the best American determinations differs from
this only by about the amount by which the standard bar changes
its length by a change of one degree of temperature. Such
accuracy is certainly sufficient for legal purposes and for
You can also
view images of some relevant pages
from the 1866 Journal of the House of Representatives and Journal
of the Senate on its introduction and passage.
You can view a handwritten original of the bill at
the National Archives and Records Administration (NARA): page
1 and page
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