American units, officially called US customary units, are the inch, foot, yard and mile for length, the gallon, quart, pint and fluid ounce for volume, and the pound and ounce for weight. Since 1959 every one of them has been defined exactly in metric terms: 1 inch = 2.54 cm and 1 pound = 0.45359237 kg.
If you grew up in the United States, you think in feet, pounds and degrees Fahrenheit. Most of the world, however, thinks in meters, kilograms and degrees Celsius. As a result, recipes, shipping labels, car specs and science homework constantly ask you to switch between the two.
The good news is that American units are not vague or old-fashioned at their core. In fact, each one has an exact metric definition, so a conversion is simple arithmetic rather than an estimate. However, the tricky part is knowing which version of a unit you have, because the same name (gallon, pint, ton, ounce, foot) can mean different things in different contexts.
In short: US customary units are a legal system of measurement tied to the metric system by exact factors, and most confusion comes from look-alike units such as the dry pint, the troy ounce and the British gallon.
The National Institute of Standards and Technology (NIST), the federal agency that keeps US measurement standards, publishes the official conversion tables in NIST Handbook 44, Appendix C and the SP 811 guide. Therefore, every factor on this page matches those tables.
Convert American Units of Length to Metric
Enter a length in feet, or switch the unit to inches, yards or miles. The converter then shows the result in meters, centimeters and kilometers using the exact 1959 definitions. For cooking volumes, use the converter on our US vs UK cooking measurements guide instead.
- American units are formally called US customary units; however, they are not the same as British imperial units.
- Since July 1, 1959, 1 inch = 2.54 cm and 1 pound = 0.45359237 kg exactly.
- The US gallon is 231 cubic inches (about 3.785 L); the British gallon is about 4.546 L.
- Because the US has separate liquid and dry volume units, a dry pint is bigger than a liquid pint.
- Everyday weights use the avoirdupois pound; precious metals use troy ounces, which are about 10% heavier.
- The US survey foot was retired for new work at the end of 2022.
- In addition, temperature is measured in degrees Fahrenheit; 32 F is freezing and 212 F is boiling at sea level.
Recommended Tools for Working in Two Unit Systems
The easiest way to avoid conversion errors is to measure in the unit you need in the first place. For example, a tape measure with inches on one edge and centimeters on the other lets you read both at once. Similarly, a kitchen scale with a unit button switches between ounces and grams, so you can follow a metric recipe without doing any math.
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Where American Units Came From
American units grew out of the English units that colonists brought in the 1600s and 1700s. After independence, the Constitution gave Congress the power to “fix the Standard of Weights and Measures”, but Congress never passed a full system. Instead, the Treasury adopted British-style standards for customs work in the 1830s. As a result, the US kept units such as the 231-cubic-inch wine gallon and the Winchester bushel.
Britain, meanwhile, reformed its own units in 1824 and created the imperial gallon. That reform is the main reason why US and British volumes still differ today.
After that came the metric connection. The Metric Act of 1866 made metric units legal for trade in the US. Then, in 1893, the Mendenhall Order declared that the yard and the pound would be defined from the international meter and kilogram. So, for more than a century, American units have rested on metric standards, even though everyday life still uses feet and pounds.
Finally, in 1959 the US, the UK, Canada, Australia, New Zealand and South Africa agreed on one shared yard and pound. According to NIST’s history of the inch, the US yard was shortened slightly and the imperial yard was lengthened, which made the inch exactly 25.4 mm. Later laws, including the Metric Conversion Act of 1975 and the 1988 trade act, encouraged voluntary metric use, but they did not replace the customary units.
American Units of Length and Area
First, length is the part of the system most people know. Each unit is a whole multiple of the one below it, although the multiples (12, 3 and 1,760) are not round numbers like 10 or 100.
| Unit | In smaller US units | Exact metric value |
|---|---|---|
| Inch (in) | – | 2.54 cm |
| Foot (ft) | 12 in | 30.48 cm |
| Yard (yd) | 3 ft | 0.9144 m |
| Mile (mi) | 5,280 ft = 1,760 yd | 1.609344 km |
| Fathom | 6 ft | 1.8288 m |
| Square foot | 144 sq in | 0.09290304 m² |
| Acre | 43,560 sq ft | about 4,046.86 m² (0.4047 ha) |
| Square mile | 640 acres | about 2.59 km² |
In addition, older land and travel units show up in deeds and books. For instance, a rod is 16.5 feet and a furlong is 660 feet. Likewise, the league was usually 3 miles, as we explain in what a league is in measurement. At sea, however, distances use the nautical mile, which is exactly 1,852 m and is not a customary unit at all.
American Units of Volume: Liquid vs Dry
Volume is where American units get confusing, because the US kept two separate sets. For example, liquid measures come from the old wine gallon of 231 cubic inches. Dry measures, by contrast, come from the Winchester bushel of 2,150.42 cubic inches. Because of this, a dry pint holds about 16% more than a liquid pint.
| Liquid unit | Equals | Metric | Dry unit | Metric |
|---|---|---|---|---|
| Fluid ounce | 2 tbsp | 29.57 mL | – | – |
| Cup | 8 fl oz | 236.59 mL | – | – |
| Pint | 16 fl oz | 473.18 mL | Dry pint | 550.61 mL |
| Quart | 2 pints | 946.35 mL | Dry quart | 1.101 L |
| Gallon | 4 quarts = 231 cu in | 3.785 L | Peck (8 dry qt) | 8.810 L |
| – | – | – | Bushel (4 pecks) | 35.24 L |
In daily life, you meet liquid units on milk jugs, gas pumps and recipes. Dry units, on the other hand, mostly appear at farm stands (a pint of berries, a peck of apples) and in grain trading. In contrast, kitchen cups and spoons are always liquid measures, even when you fill them with flour. Also, 3 teaspoons make a tablespoon, and 16 tablespoons make a cup.
American Units of Weight: Avoirdupois vs Troy
First of all, everyday weights in the US use the avoirdupois system: 16 ounces to the pound and 2,000 pounds to the short ton. Precious metals, however, are still weighed in troy ounces. Even so, both systems share the same grain (exactly 64.79891 mg), yet they group it differently.
| Unit | Grains | Metric | Used for |
|---|---|---|---|
| Grain (gr) | 1 | 64.79891 mg | Bullets, arrows, some medicines |
| Ounce, avoirdupois (oz) | 437.5 | 28.349523125 g | Food, mail, packages |
| Pound, avoirdupois (lb) | 7,000 | 0.45359237 kg exactly | Body weight, groceries |
| Troy ounce (oz t) | 480 | 31.1034768 g | Gold, silver, platinum |
| Short ton | 14,000,000 | 907.18474 kg | Trucks, freight, coal |
So a troy ounce is about 9.7% heavier than a regular ounce. Oddly, though, a troy pound has only 12 troy ounces, which makes it lighter (about 373.24 g) than an avoirdupois pound. Similarly, the British long ton of 2,240 pounds (about 1,016 kg) is another trap in older documents. For more on the everyday ounce, read how much is an ounce.
Temperature in American Units
Next, the US uses the Fahrenheit scale for weather, cooking and body temperature. On this scale, water freezes at 32 F and boils at 212 F at sea level, so there are 180 Fahrenheit degrees between the two points. Celsius, by comparison, puts 100 degrees in the same gap.
To convert, use F = C x 9/5 + 32, or C = (F – 32) x 5/9. For example, a normal body temperature of 37 C equals 98.6 F, and a room at 68 F is 20 C. Also, the two scales meet at -40, where -40 F equals -40 C. Scientists in the US, meanwhile, use Celsius and kelvin, just like the rest of the world.
American Units vs British Imperial Units
People often call American units “imperial”, but the two systems split in 1824. Today, length and weight are identical, thanks to the 1959 agreement. Volume, however, is not the same. If you compare the systems in detail in our metric vs imperial review, the gallon is the biggest difference.
| Unit | US customary | British imperial | Difference |
|---|---|---|---|
| Gallon | 3.785 L | 4.546 L | UK about 20% larger |
| Pint | 16 US fl oz (473 mL) | 20 UK fl oz (568 mL) | UK about 20% larger |
| Fluid ounce | 29.57 mL | 28.41 mL | US about 4% larger |
| Ton | Short ton, 2,000 lb | Long ton, 2,240 lb | UK 12% larger |
| Stone | Not used | 14 lb (6.35 kg) | Body weight in the UK |
| Inch, foot, mile, pound | Same | Same | None since 1959 |
American Units Update: The Survey Foot Retirement
Even after 1959, the US kept two feet. On the one hand, the international foot is exactly 0.3048 m. The older US survey foot, however, was 1200/3937 m, about 0.3048006 m, and surveyors kept it for land and mapping work. In fact, the gap is only 2 parts per million, or about 3.2 mm per mile. Over state-wide coordinate systems, though, it added up to errors of several feet.
So NIST and NOAA’s National Geodetic Survey retired it. As NIST’s survey foot page explains, from January 1, 2023 the US survey foot should be avoided except for historic and legacy data. As a result, one foot now means 0.3048 m everywhere in the US.
How to Convert American Units to Metric
- Identify the exact unit. Check whether you have a liquid or dry pint, a US or UK gallon, and an avoirdupois or troy ounce.
- Find the exact factor. Then use a NIST table or the converter above, for example 1 ft = 0.3048 m.
- Multiply, do not divide by guesswork. Multiply the US value by the factor to get metric; divide to go back.
- Keep full precision until the end. In other words, round only the final answer, not each step.
- Round to a sensible number of digits. A 6 ft board becomes 1.83 m, not 1.8288 m, unless you need machining accuracy.
- Check with a quick estimate. A meter is a little over a yard, and a kilogram is a little over 2 pounds, so your answer should be close to that.
Do and Don’t With US Customary Units
Do
- Write the system next to the unit, such as “US gal” or “UK pt”.
- Use exact factors from NIST for anything that gets built or sold.
- Weigh baking ingredients in grams when a recipe allows it, because weight is more consistent.
- Confirm whether old survey data uses the survey foot.
- Keep one unit system per drawing or spreadsheet.
Don’t
- Assume “ounce” means the same thing on a scale and a cup.
- Mix US and UK gallons in fuel or recipe math.
- Round 2.54 to 2.5 for anything precise.
- Price gold by the regular ounce; instead, use troy ounces.
- Convert dry produce pints with liquid factors.
Honest Limits of These Conversions
Of course, the factors on this page are exact definitions, but real measurements are not. A tape measure, for instance, has a tolerance class, and a kitchen cup can be off by a few percent. Also, rounding a conversion can make a number look more precise than the measurement behind it. For example, if a part was measured to the nearest 1/16 inch, quoting it to 0.01 mm adds no real accuracy. Finally, older documents may use units whose size changed over time, such as pre-1959 feet or regional bushels, so check the date before you convert.
When to Call a Professional
For most home tasks, a converter and a dual-unit tape are enough. Some jobs, however, need an expert because the stakes are higher. For example, land boundaries and legal descriptions should go to a licensed surveyor, especially when old plats use survey feet or chains. Similarly, medicine doses belong with a pharmacist or doctor, and you should use the dosing device that comes with the medicine. In addition, commercial scales and fuel pumps must be checked by your state weights and measures office, and precision parts may need a calibration lab.
American Units FAQs
What are American units called officially?
Officially, they are called United States customary units, or US customary units. They include inches, feet, miles, gallons, pounds and degrees Fahrenheit.
Are American units the same as imperial units?
Not entirely. Length and weight units have been identical since 1959, but US gallons, pints and fluid ounces differ from British imperial ones.
How are American units defined today?
Today, they are defined from metric units. One inch is exactly 2.54 cm, one pound is exactly 0.45359237 kg, and one US gallon is exactly 3.785411784 L.
Why does the US still use customary units?
Mainly because of cost and habit. In fact, metric is far from banned. Metric units have been legal since 1866 and are used in science, medicine and much of industry, but everyday life never switched.
How many feet are in a mile?
There are 5,280 feet, or 1,760 yards, in a mile. That equals exactly 1.609344 km.
What is the difference between a liquid pint and a dry pint?
A US liquid pint is about 473 mL, while a US dry pint is about 551 mL. For this reason, dry pints are used for berries and other produce.
Is a troy ounce heavier than a regular ounce?
Yes. A troy ounce is about 31.10 g, while a regular avoirdupois ounce is about 28.35 g, so the troy ounce is about 9.7% heavier.
What happened to the US survey foot?
NIST and NOAA retired it at the end of 2022. Since January 1, 2023, the international foot of exactly 0.3048 m is the only foot for new work.
How do I convert Fahrenheit to Celsius?
First, subtract 32, and then multiply by 5/9. For example, 68 F minus 32 is 36, and 36 times 5/9 is 20 C.
Which American units are closest to metric units?
For example, a liter is about 1.06 US quarts, a meter is about 1.09 yards, and a kilogram is about 2.2 pounds.
The Bottom Line on American Units
In short, American units look old, but they are precise. Since 1959, the inch, the foot, the pound and the gallon have each had an exact metric value, and since 2023 there is only one US foot. Therefore, most conversion mistakes come from picking the wrong version of a unit, not from the math.
So label your units, check for liquid vs dry and US vs UK, and use the converter at the top of this page when you need a quick length. For cooking and weight questions, our guides on ounces and spoon sizes cover the details.