By Ryan Lee Price

We measure tire pressure in pounds per square inch, engine displacement on modern cars in liters, while gas comes in gallons. Speedometers show both miles per hour and kilometers per hour, and the majority of U.S.-made vehicles contain a hodge-podge of bolts with varying specs, both metric and standard. Why has the U.S. held out so long against the metric system when it is embraced by 99 percent of the world? Expense? Tradition? Unimportant?

Only three countries -- Burma (Myanmar), Liberia, and the United States -- have yet to adopt the International System of Units (aka metric) as their official system of measurement. Though the U.S. has been increasing its use of metric units for many years, the pace has accelerated in the past three decades. In the early 1800s, the U.S. Coast and Geodetic Survey (the government's surveying and map-making agency) used meter and kilogram standards brought from France. In 1866, Congress authorized the use of the metric system in this country and supplied each state with a set of standard metric weights and measures.

In 1875, the United States solidified its commitment to the development of the internationally recognized metric system by becoming one of the original seventeen signatory nations to the Treaty of the Meter. The signing of this international agreement concluded five years of meetings in which the metric system was reformulated, refining the accuracy of its standards. The Treaty of the Meter, also know as the "Metric Convention," established the International Bureau of Weights and Measures (BIPM) in Sèvres, France, to provide standards of measurement for worldwide use.

Confusion about weights and measures in 19th Century United States is well displayed in this advertisement showing the different ways states measured a bushel.

In 1893, the United States adopted metric standards, developed through international cooperation under the auspices of BIPM, as the fundamental standards for length and mass. The meter and the kilogram have defined our customary measurements -- the foot, pound, quart, etc. -- ever since. The General Conference of Weights and Measures, the governing body that has overall responsibility for the metric system, and which is made up of the signatory nations to the Treaty of the Meter, approved an updated version of the metric system in 1960. This modern system is called Le Système International d'Unités or the International System of Units, abbreviated SI.

In 1968, Congress authorized a three-year study of systems of measurement in the U.S., with particular emphasis on the feasibility of adopting SI. The Department of Commerce conducted a detailed U.S. Metric Study, and the final report of the study: "A Metric America: A Decision Whose Time Has Come," concluded that the U.S. would eventually join the rest of the world in the use of the metric system of measurement. The study recommended that the United States implement a carefully planned transition to predominant use of the metric system over a 10-year period. Congress passed the Metric Conversion Act of 1975 "to coordinate and plan the increasing use of the metric system in the United States." The Act, however, did not require a 10-year conversion period. The act initiated a process of voluntary conversion and charged the newly-established U.S. Metric Board  with "devising and carrying out a broad program of planning, coordination, and public education, consistent with other national policy and interests, with the aim of implementing the policy set forth in this Act." The American public largely ignored the efforts of the Metric Board and in 1981, the Board reported to Congress that it lacked the clear Congressional mandate necessary to bring about national conversion. Due to this apparent ineffectiveness, and in an effort to reduce Federal spending, the US disestablished the Metric Board in the fall of 1982.

Congress, recognizing the necessity of the United States' conforming with international standards for trade, included new encouragement for U.S. industrial metrication in the Omnibus Trade and Competitiveness Act of 1988. This legislation amended the Metric Conversion Act of 1975 and designates the metric system as the "preferred system of weights and measures for United States trade and commerce." The legislation states that the Federal Government has a responsibility to assist industry, especially small business, as it voluntarily converts to the metric system of measurement.

A world map showing the metrication of its countries, color-coded by year of conversion. Data source: U.S. Metric Association.

This legislation required Federal agencies, with certain exceptions, to use the metric system in their procurement, grants and other business-related activities by the end of 1992. While not mandating metric use in the private sector, the Federal Government seeks to serve as a catalyst in the metric conversion of the country's trade, industry, and commerce. Regardless of these steps, a resource like this one is needed to compare the differences and similarities between U.S. and metric fasteners and will be for some time.

Metric vs. Standard
Inches Metric Inch Size
.1875 NE 3/16"
.1969 5mm NE
.2187 NE 7/32"
.2362 6mm NE
.2500 NE 1/4"
.2756 7mm NE
.2812 NE 9/32"
.3125 NE 5/16"
.3150 8mm NE
.3543 9mm NE
.3750 NE 3/8"
.3937 10mm NE
.4330 11mm NE
.4375 NE 7/16"
.4724 12mm NE
.5000 NE 1/2"
.5118 13mm NE
.5512 14mm NE
.5625 NE 9/16"
.5906 15mm NE
.6250 NE 5/8"
.6299 16mm NE
.6693 17mm NE
.6875 NE 11/16"
.7078 18mm NE
.7480 19mm NE
.7500 NE 3/4"
.7874 20mm NE
.8125 NE 13/16"
.8268 21mm NE
.8661 22mm NE
.8750 NE 7/8"
.9055 23mm NE
.9375 NE 15/16"
.9449 24mm NE
.9843 25mm NE
1.000 NE 1
NE = No full integer equivalent.

Does Your Fastener Measure Up?

The Society of Automotive Engineers, or SAE creates technical standards. Now an international organization, SAE standards advance mobility engineering throughout the world. More than 9,000 volunteer engineers, and other qualified professionals from around the world develop the standards by consensus. The standards apply to fasteners, such as nuts and bolts, as well as washers, torque-tension procedures, steels, hardenability, and many others.

 For more information, click here to learn more about converting U.S. and metric measurements in automotive repair. Confused? The U.S. government periodically issues General Tables of Units of Measurement in Appendix C of its Handbook 44: Specifications, Tolerances, and Other Technical Requirements for Weighing & Measuring Devices.


What is the future of the metric system in the United States? More and more, US manufacturers are using components from international sources, forcing its consumers to have both standard and metric tools readily available. How long until a ½-inch wrench becomes obsolete, if ever? Have an automotive repair project? Chilton is a valuable source for service procedures, specifications, graphics, wiring diagrams, and more. Subscribe at www.ChiltonDIY..com.

Not only is Ryan Lee Price a freelance writer specializing in automotive journalism and a former long-time magazine editor, he is part of the technical editorial team that provides content for most all of the ChiltonPRO and ChiltonDIY products. He currently resides in Corona, California, with his wife Kara and their two children.