The United States uses SI units far more than most Americans realize. While everyday life runs on inches, pounds, and gallons, the metric system is legally the preferred measurement system for trade and commerce, and it forms the hidden backbone of American science, medicine, manufacturing, and even the customary units themselves. Every U.S. customary unit is officially defined in terms of SI standards.
U.S. Customary Units Are Built on SI
Here’s the detail that surprises most people: the inch, the pound, and every other U.S. customary unit is formally defined by its relationship to a metric equivalent. One inch equals exactly 2.54 centimeters. One avoirdupois pound equals exactly 453.59237 grams. One U.S. gallon equals exactly 231 cubic inches, which works out to 3.785 liters. These aren’t approximations. They are the legal definitions maintained by the National Institute of Standards and Technology (NIST). When you measure something in feet and pounds, you’re ultimately measuring in meters and kilograms with a conversion factor applied.
This means the U.S. doesn’t maintain an independent measurement infrastructure. It piggybacks entirely on the international SI system, then translates those values into the customary units Americans use day to day.
The Legal Status of Metric in the U.S.
The metric system has been legal to use in America since 1866. Congress went further in 1988, amending the Metric Conversion Act of 1975 through the Omnibus Trade and Competitiveness Act to declare the metric system the “preferred system of weights and measures for trade and commerce.” That language matters, but it comes with a catch: adoption is voluntary. U.S. law permits either system, and the pace of metric use is left to individual companies, organizations, and people.
In 1991, Executive Order 12770 directed all executive branch departments and agencies to use the metric system in federal procurements, grants, and other business activities “to the extent economically feasible.” The order included an exception for situations where metric use would be impractical or cause U.S. firms to lose markets. This carve-out gave many industries a permanent reason to stick with customary units domestically, even as they used metric for exports.
Consumer Product Labels
If you pick up nearly any packaged product in your kitchen or bathroom, you’ll find metric units on the label. The Fair Packaging and Labeling Act requires that household consumer commodities display net contents in both metric and U.S. customary units. A bag of chips might read “Net wt 1 lb 8 oz (680g).” A bottle of soda might say “500 ml (1 pt 0.9 fl oz).” The metric figure can appear before or after the customary one.
The FDA applies the same dual-labeling rule to food and beverages specifically, requiring grams, kilograms, milliliters, or liters alongside ounces, pounds, and fluid ounces. Nutrition Facts panels go fully metric: protein is listed in grams, sodium in milligrams, dietary fiber in grams. Americans read these metric values constantly without thinking of them as “metric.” When you check that a snack has 20 grams of protein or 300 milligrams of sodium, you’re reading SI units.
Medicine and Healthcare
American medicine operates almost entirely in SI units. Pill bottles list active ingredients in milligrams. Liquid medications are dosed in milliliters. Patient weights in hospitals are recorded in kilograms, because drug dosing calculations depend on metric precision. Blood test results report cholesterol in milligrams per deciliter, blood glucose in the same units, and electrolytes in millimoles per liter.
The shift toward exclusive metric use in healthcare has been deliberate. The American Society of Health-System Pharmacists recommended in 2018 that the U.S. healthcare system adopt SI units exclusively for all patient and medication measurements. For oral liquid medications specifically, the National Council for Prescription Drug Programs has pushed to make milliliters the only unit on prescription container labels, replacing the patchwork of teaspoons, tablespoons, droppersful, and ounces that has historically caused dosing errors. Solid medications measured in milligrams have used metric for decades, but the liquid side has been slower to standardize.
This isn’t just a preference. Mixing measurement systems in medical settings creates real safety risks, and the steady push toward metric-only dosing reflects that.
Science and Education
Every scientific discipline in the United States uses SI units as the default. Research papers, lab equipment, and data reporting all follow metric conventions. This isn’t optional or cultural; it’s a requirement for participating in international science. A chemistry lab in Ohio and one in Munich use the same units.
American students encounter SI units early. The National Education Association has advocated since 1969 for teaching the SI system at all educational levels. The National Science Teaching Association adopted a position supporting metric instruction in 1999 and reaffirmed it in 2016. NIST develops educational resources targeting middle school students specifically, including materials on the seven SI base units (meter, kilogram, second, ampere, kelvin, mole, and candela). Metric fluency is treated as a core skill for anyone entering a STEM career, and science classes from middle school onward typically conduct experiments and report results exclusively in metric.
Federal Agencies and the Military
Federal procurement has been officially metric-preferring since the early 1990s. Agencies that deal with international partners, like NASA and the Department of Defense, use metric extensively. Military maps use kilometers. Ammunition is often designated in millimeters (5.56 mm, 9 mm, 7.62 mm). NASA famously lost the Mars Climate Orbiter in 1999 when one contractor used customary units while the rest of the mission used metric, a $125 million lesson in why mixing systems is dangerous.
In practice, the degree of metric adoption varies widely across federal agencies. Those with international obligations tend to be fully metric. Others, particularly those focused on domestic infrastructure, still work primarily in customary units because that’s what American contractors, builders, and suppliers use.
Manufacturing and International Trade
Any American manufacturer selling products internationally works in metric. Automotive parts, electronics, pharmaceuticals, and aerospace components are designed and specified in millimeters, grams, and liters to meet global supply chain requirements. The U.S. auto industry largely shifted to metric decades ago, which is why your car’s engine displacement is listed in liters and many of its bolts require metric wrenches.
Domestically focused industries tell a different story. Construction, real estate, and road signage remain firmly in feet, miles, and acres. Lumber is sold in nominal inches. Plumbing fittings use fractional inches. This split creates a two-track system where the same country operates in metric for global-facing work and customary units for local work.
Why Full Adoption Hasn’t Happened
The voluntary nature of U.S. metrication explains why progress has been uneven. Congress declared metric “preferred” but never mandated it for private industry or daily life. Retooling factories, retraining workers, replacing road signs, and changing consumer habits all carry real costs, and without a legal deadline, sectors that don’t face international pressure have little incentive to switch. The result is a country that technically prefers metric by law, relies on metric definitions for its own customary units, and uses metric daily in medicine, science, nutrition labels, and global trade, while still buying lumber in feet and milk in gallons.