The US uses inches, feet, and miles because Congress never made the metric system mandatory. The country has flirted with metric conversion since 1866 and signed conversion acts in 1866 and 1975, but each act made metric legal rather than required. By the time the rest of the world had switched, US industry, infrastructure, and everyday life were so deeply built around imperial units that changing became economically and politically impossible.
Where the inch came from
The inch is medieval. King Edward II of England in 1324 defined an inch as "three barleycorns, round and dry, laid end to end", about 25 mm at the time, surprisingly close to the modern 25.4 mm. Before that, English measurements traced back to Roman units (the Roman uncia, meaning "twelfth", gave us both "inch" and "ounce") and even earlier to body-part measurements: the foot from a literal foot, the cubit from elbow to fingertip.
By the 1500s, England had standardized on the foot, yard, and inch. Henry VIII's and Elizabeth I's standards (kept as physical metal rods in Westminster) defined the legal units that traveled to the American colonies. When the United States declared independence in 1776, it inherited the British system intact.
The metric system arrives, but late
The metric system was invented in revolutionary France in the 1790s. It uses base-10 units (10 mm in a cm, 100 cm in a meter, 1000 m in a km) which makes mental arithmetic far easier than imperial (5,280 feet per mile, 16 ounces per pound, 12 inches per foot). Most of Europe adopted metric in the 1800s. Latin America followed. Asia and Africa adopted it through colonial influence or post-independence reform.
The US considered it. Thomas Jefferson sent a French naturalist to bring back a copy of the new French metric standards in the 1790s, but the ship was captured by pirates. By the time another delegation made the trip, the political moment had passed. Congress made metric "legal" through the Metric Act of 1866 but never required it.
The 1975 conversion attempt
President Gerald Ford signed the Metric Conversion Act of 1975, which created the US Metric Board and aimed for voluntary conversion. It produced miles of road-sign experiments (kilometers on highways), nutrition labels in grams, and a generation of confused schoolchildren learning both systems. Then President Ronald Reagan abolished the Metric Board in 1982 because no measurable conversion was happening. The board's job was advisory; without legal force, US industry had no reason to spend money switching.
Why we still use inches today
Three reasons keep the US on imperial. Cost of switching: billions of dollars of road signs, blueprints, machinery, textbooks, and consumer goods would need replacement. Even a partial conversion of road distances would require new signage on every highway. Industry inertia: the auto industry, construction, aerospace, and household goods all evolved imperial standards. A 2x4 stud, a half-inch drill bit, a quarter-inch socket, these are physical objects with millions of compatible tools and materials. Cultural attachment: Americans think in feet and inches for height, pounds for weight, miles for distance, and Fahrenheit for weather. Metric feels foreign to most US adults even after decades of partial exposure.
Where metric quietly won
US science is fully metric. So is medicine: prescriptions are in milliliters and milligrams, lab results in micrograms per deciliter. The US military uses metric for almost all operations (kilometers for ranges, kilograms for weight). The automotive industry shifted to metric in the 1970s and 80s; modern cars use metric bolts, metric pressures, and metric fuel economy targets (although consumer-facing MPG stayed). Even the National Football League uses metric in some training contexts.
What stayed imperial: road distances, speed limits, body height and weight, oven temperatures, recipe measurements, consumer product packaging, real estate (square feet), aviation altitude (worldwide), and weather temperatures.
The 1959 inch is global
One thing changed in 1959: the international yard and pound agreement made the inch exactly 25.4 mm worldwide. Before 1959, US inches and UK inches differed by tiny amounts (parts per million), enough that precision engineering had to specify which standard it followed. After 1959, an inch is an inch globally. This is why the conversion factor of 25.4 mm per inch is presented as exact in every reference.
Will the US ever go fully metric?
Probably not as a sudden switch. The cost is enormous, public demand is zero, and political will is absent. What is happening gradually: younger Americans are more comfortable with metric than their grandparents were, scientific and medical contexts have already converted, and global commerce pushes US industries to dual-label products. The endgame is likely permanent bilingualism: Americans will continue to think in inches and pounds at home while moving seamlessly to metric for science, medicine, and international trade. Other countries have done the same thing in reverse (the UK uses metric officially but pints, miles, and feet survive in everyday life).
For converting between the two systems day to day, the constants are: 1 inch = 2.54 cm exactly, 1 foot = 30.48 cm, 1 mile = 1.609 km, 1 pound = 453.6 grams, 1 gallon (US) = 3.785 liters. Use these factors or our converters and the inch-versus-metric question is just a math problem, not a culture war.