Strictly speaking, the US doesn’t use the imperial system. It uses US customary units, a close cousin that split from British measures before Britain created the imperial system in 1824. The US never adopted metric as a mandatory system because Congress chose, in 1975, to make metric conversion voluntary, and most businesses and consumers never saw enough reason to switch. Yet every US customary unit is defined in metric today, so in a legal sense the country already runs on the metric system.
Is the US imperial or customary? #
The two systems share names but not always sizes. The US inherited English units from colonial times. In 1824, Britain reformed its own measures into the imperial system, and the US, independent by then, didn’t follow. That’s why:
| Unit | US customary | British imperial |
|---|---|---|
| Gallon | 3.785 L | 4.546 L |
| Pint | 16 fl oz (473 mL) | 20 fl oz (568 mL) |
| Fluid ounce | 29.57 mL | 28.41 mL |
| Ton | 2,000 lb | 2,240 lb |
| Stone | not used | 14 lb |
| Inch, foot, yard, mile, pound, ounce | same since 1959 | same since 1959 |
The length and weight units were brought into line in 1959, when the US and several Commonwealth countries agreed on an international yard of exactly 0.9144 m and an international pound of exactly 0.45359237 kg. Volume units never were. We cover the gallon split in US gallon vs imperial gallon.
Hasn’t metric been legal in the US for a long time? #
Yes, since 1866. A short timeline:
| Year | What happened |
|---|---|
| 1866 | The Metric Act makes the metric system legal to use in the US. |
| 1875 | The US signs the Treaty of the Metre, one of its original signatories. |
| 1893 | The Mendenhall Order defines the US yard and pound in terms of the meter and kilogram. |
| 1959 | The international yard and pound fix the inch at exactly 2.54 cm. |
| 1975 | The Metric Conversion Act, signed by President Ford, sets up voluntary conversion. |
| 1988 | The Omnibus Trade and Competitiveness Act amends it to make metric the “preferred” system for trade and commerce. |
| 1991 | Executive Order 12770 directs federal agencies to use metric. |
| 2007 | The America COMPETES Act updates the 1866 law to refer to the modern SI. |
| 2023 | The old US survey foot is retired in favor of the international foot. |
NIST’s metric policy page summarizes the current position: metric is preferred for trade and commerce, required of federal agencies, and voluntary for everyone else. NIST also retired the US survey foot on January 1, 2023, ending the last case where the US had two different feet.
Why didn’t the US go metric in the 1970s? #
The 1975 act made conversion voluntary, and voluntary conversion stalls when there’s no deadline. Changing road signs, building codes, machine tools, textbooks and packaging costs money up front, while the benefits arrive slowly and spread thinly. Many industries that trade internationally switched quietly on their own. Others, especially construction and anything aimed at consumers, stayed with feet, pounds and gallons because their customers did.
NIST’s metrication FAQ puts the current policy plainly: manufacturers “are in the best position to determine when/if metric adoption is good decision for their business.” Nothing forces the change.
Is the US one of only three countries that don’t use metric? #
That’s a popular claim, usually listing the US, Liberia and Myanmar, and NIST calls it a myth. Its FAQ says that “all countries have recognized and adopted the SI, including the United States,” and that the claim came from misreading 1960s literature that distinguished mandatory from voluntary adoption. What differs between countries is how much everyday life uses metric, not whether it’s recognized.
Where does the US already use metric? #
More places than people notice:
- Food labels list grams, and packaged goods carry dual labeling, such as “NET WT 16 OZ (454 g)”, under the Fair Packaging and Labeling Act.
- Medicine doses are in milligrams and milliliters.
- Soda comes in 2-liter bottles, and liquor and wine in 750 mL ones.
- Car engines are sized in liters.
- Running races are 5K and 10K.
- Science, medicine and much of manufacturing work in SI units.
- Electricity is billed in kilowatt-hours, built on the SI watt.
What does this mean when you convert? #
Because customary units are defined from metric ones, the conversions are exact, not estimates. An inch is exactly 2.54 cm, a pound exactly 0.45359237 kg and a US gallon exactly 3.785411784 liters. Any rounding in a conversion comes from how many decimals you keep, not from the definitions.
It also means Americans convert constantly: recipes from abroad, travel, science homework, car specs, medication. Convert Units Calculator is built for that daily back-and-forth. You type a number once and read it in every unit of the category, metric and US customary side by side, and version 1.0.5 adds British imperial gallons, pints and fluid ounces as separate units so the two systems don’t get mixed up. It uses the exact definitions above and works offline.
For the metric side of the story, see how the metric system works. For everyday conversions, the metric to imperial conversion chart has the common ones on a single page.
Frequently asked questions #
Is the US the only country that doesn’t use the metric system? #
No. According to NIST, every country, including the US, has recognized and adopted the SI. The US is unusual in how much everyday life still runs on customary units, but metric is legal, preferred for trade, and used widely in science, medicine and industry.
Is the imperial system the same as US customary units? #
No. They share many names, and length and weight units have been identical since 1959, but volumes differ. The imperial gallon is 4.546 L and the US gallon 3.785 L, and an imperial pint has 20 fluid ounces where a US pint has 16.
When did the US try to go metric? #
The Metric Conversion Act of 1975 set up a national program for voluntary conversion. Because it had no mandate or deadline, adoption was patchy. In 1988 Congress declared metric the preferred system for trade and commerce, and in 1991 federal agencies were directed to use it.
Does the UK still use the imperial system? #
Partly. Road signs use miles and mph, and beer is served in pints. Most trade and packaging is metric, and UK law restricts imperial units in trade to supplementary indications, with a few exceptions such as the pint for draught beer.
Will the US ever switch to metric? #
There’s no law requiring it. US policy remains voluntary for private business and consumers, so any change will keep happening industry by industry rather than all at once.