Metric vs. Imperial: Why the US Still Uses Miles, Pounds, and Fahrenheit
Why most of the world measures in meters, kilograms, and Celsius, why the United States still uses miles, pounds, and Fahrenheit, and what the base units of the metric system actually are.
Published
Nearly every country in the world uses the metric system (the International System of Units, or SI) as its official system of measurement. The United States is the most prominent exception, continuing to use miles, pounds, and Fahrenheit in everyday life despite having formally recognized the metric system as preferred since 1975.
Where the imperial system came from
The units now called "imperial" — inches, feet, miles, pounds, and ounces — evolved over centuries from Roman, Anglo-Saxon, and medieval English measures, many based on everyday physical references like the length of a foot or the weight a person could carry. Britain formalized these into the imperial system in 1824. The United States inherited earlier versions of these units from British colonial rule and has kept largely similar definitions ever since, even after the UK itself moved toward metric.
Why the US hasn't fully switched to metric
The Metric Conversion Act of 1975 declared the metric system the preferred system of measurement for US trade and commerce, and established a board to coordinate voluntary conversion. Unlike similar efforts in other countries, it set no mandatory deadlines or requirements, and public and industry resistance — along with the cost of replacing tools, signage, and manufacturing standards — led the effort to stall by the 1980s.
In practice, the US already relies on metric units extensively: science, medicine, the military, and manufacturing for export all use metric measurements. Metric units also appear on US product labels alongside imperial ones. It's specifically everyday consumer contexts — road signs, recipes, weather reports, body weight — where imperial units have persisted.
What the metric system is built on
The metric system's core strength is that its units scale by powers of ten, so converting between them is a matter of moving a decimal point rather than multiplying by an irregular ratio. The International System of Units defines seven base units:
- Metre (m) — length
- Kilogram (kg) — mass
- Second (s) — time
- Ampere (A) — electric current
- Kelvin (K) — temperature
- Mole (mol) — amount of substance
- Candela (cd) — luminous intensity
Every other metric unit, from square meters to kilowatt-hours, is derived from these seven.
Frequently asked questions
- Which countries still use the imperial system?
- The United States is the most prominent example, using miles, pounds, and Fahrenheit in everyday life. Liberia and Myanmar are also frequently cited as non-metric, though both have officially adopted metric units to varying degrees. The United Kingdom uses a mix, with miles on road signs but metric units in most other contexts.
- Has the US ever tried to switch to the metric system?
- Yes. The Metric Conversion Act of 1975 declared metric the preferred system and encouraged voluntary conversion, but it set no deadlines or mandates, and the effort largely stalled. Metric units are already standard in US science, medicine, the military, and manufacturing for export.
- What are the base units of the metric system?
- The International System of Units (SI) defines seven base units: the metre (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). Every other metric unit is derived from these.
- Is the imperial system less accurate than the metric system?
- No — both systems can express the same physical quantity to any level of precision. The practical difference is that metric units scale by powers of ten (making conversions and calculations simpler), while imperial units use irregular ratios such as 12 inches per foot and 16 ounces per pound.