Americans measure height in feet and inches, road trips in miles, and gasoline in gallons, while nearly everyone else uses meters, kilometers and liters. The two systems can be converted exactly, because since 1959 every US customary unit has been defined in terms of a metric one. For example, an inch is exactly 2.54 centimeters, not approximately.
This page lists the customary units of length, mass and volume, gives their exact metric definitions, and shows how to convert in both directions with everyday examples.
Contents
- US customary, English and imperial: what is the difference?
- Exact definitions
- Quick conversions you can do in your head
- Worked examples
- Convert step by step with conversion factors
- Familiar sizes in both systems
- Common mistakes
- Practice problems
- Frequently asked questions
- Related calculators
- See also
US customary, English and imperial: what is the difference?
People often say "English system" or "imperial system" for what is used in the United States, but the names are not interchangeable. The US customary system descends from the units used in England before 1824. The British imperial system was standardized in 1824 and changed some of them, mainly the volume units. Length and weight units are the same in both. Volume units are not: a US gallon is about 3.785 L, while an imperial gallon is about 4.546 L, roughly 20% larger.
In 1959 the United States, the United Kingdom and other countries agreed on one international yard and one international pound, defined as exact metric values. From that moment, every customary unit became a fixed multiple of a metric unit.
Exact definitions
Length
| Unit | Symbol | Equivalent in customary units | Exact metric value |
|---|---|---|---|
| inch | in | 1/12 ft | 2.54 cm = 25.4 mm |
| foot | ft | 12 in | 0.3048 m |
| yard | yd | 3 ft = 36 in | 0.9144 m |
| mile | mi | 5,280 ft = 1,760 yd | 1,609.344 m |
Everything starts with the inch: $latex 1\ \text{in} = 2.54\ \text{cm}$ exactly. The rest follows by multiplication, for example $latex 1\ \text{ft} = 12\times 2.54 = 30.48\ \text{cm} = 0.3048\ \text{m}$.
Inches and centimeters on one ruler
Drag the orange handle along the ruler, or use the slider or the boxes. 1 inch is exactly 2.54 cm.
Mass
| Unit | Symbol | Equivalent | Exact metric value |
|---|---|---|---|
| ounce | oz | 1/16 lb | 28.349523125 g |
| pound | lb | 16 oz | 0.45359237 kg |
| US ton (short ton) | 2,000 lb | 907.18474 kg |
Everyday language uses ounces and pounds for weight, but here they are units of mass, defined through the pound: $latex 1\ \text{lb} = 0.45359237\ \text{kg}$ exactly.
Volume
| Unit | Symbol | Equivalent | Exact metric value |
|---|---|---|---|
| fluid ounce | fl oz | 1/128 gal | 29.5735295625 mL |
| cup | c | 8 fl oz | 236.5882365 mL |
| pint | pt | 2 cups = 16 fl oz | 0.473176473 L |
| quart | qt | 2 pints = 32 fl oz | 0.946352946 L |
| gallon | gal | 4 quarts = 128 fl oz | 3.785411784 L |
The US gallon is defined as 231 cubic inches, so its metric value follows from the inch: $latex 231\times 2.54^3 = 3785.411784\ \text{cm}^3 = 3.785411784\ \text{L}$.
Quick conversions you can do in your head
| To convert | Multiply by | Memory aid |
|---|---|---|
| inches to centimeters | 2.54 | An inch is a bit more than 2.5 cm |
| feet to meters | 0.3048 | A foot is about 30 cm |
| miles to kilometers | 1.609344 | A mile is about 1.6 km |
| pounds to kilograms | 0.45359237 | A little less than half |
| gallons to liters | 3.785411784 | A gallon is almost 3.8 L |
To go the other way, divide by the same factor. For example, 5 L is $latex 5\div 3.785411784$ gallons.
Worked examples
Example 1: height in centimeters
A person is 5 ft 10 in tall. Convert everything to inches first: $latex 5\times 12 + 10 = 70$ in. Then multiply by 2.54.
$$70\times 2.54 = 177.8\ \text{cm}$$
Example 2: a marathon in kilometers
A runner covers 26.2 mi. Multiply by 1.609344.
$$26.2\times 1.609344 = 42.1648\ \text{km} \approx 42.16\ \text{km}$$
The official distance is 26.219 mi, which is the familiar 42.195 km.
Example 3: pounds to kilograms
A bag weighs 12 lb.
$$12\times 0.45359237 = 5.44310844\ \text{kg} \approx 5.443\ \text{kg}$$
Example 4: gallons to liters
A tank holds 2.5 gal.
$$2.5\times 3.785411784 = 9.46352946\ \text{L} \approx 9.46\ \text{L}$$
Example 5: fuel economy
A car gets 30 miles per gallon. Convert miles to kilometers and gallons to liters:
$$30\ \frac{\text{mi}}{\text{gal}} = \frac{30\times 1.609344\ \text{km}}{3.785411784\ \text{L}}$$
$$= 12.75\ \text{km/L}$$
Many countries quote the inverse, liters per 100 km: $latex 100\div 12.7543 = 7.84$ L/100 km.
Example 6: a speed limit
A highway sign says 65 mph.
$$65\times 1.609344 = 104.60736\ \text{km/h} \approx 104.6\ \text{km/h}$$
Convert step by step with conversion factors
A reliable method is to multiply by a fraction that equals 1. Write the exact equivalence as a fraction, put the unit you want to get rid of in the denominator, and let the units cancel like numbers. To convert 60 mph to meters per second you need miles to meters and hours to seconds:
$$60\ \frac{\text{mi}}{\text{h}}\times\frac{1609.344\ \text{m}}{1\ \text{mi}}\times\frac{1\ \text{h}}{3600\ \text{s}}$$
$$= 26.8224\ \frac{\text{m}}{\text{s}}$$
Miles and hours cancel, and meters per second remain. If a unit does not cancel, you wrote one of the fractions upside down. The same idea works for any pair of units, however strange, and it is what a conversion calculator does behind the scenes.
Familiar sizes in both systems
| Object | Customary | Metric |
|---|---|---|
| Letter-size paper | 8.5 in × 11 in | 21.59 cm × 27.94 cm |
| Basketball hoop height | 10 ft | 3.048 m |
| Football field between the goal lines | 100 yd | 91.44 m |
| Bag of flour | 5 lb | 2.268 kg |
| Large soda bottle | 0.53 gal | 2 L |
The customary numbers in the first four rows are simple because those objects were designed in inches, feet, yards and pounds. The soda bottle is the opposite: the metric size is the round one. Products sold in many countries often print both.
Getting the units right is not a classroom detail. In 1999 NASA lost the Mars Climate Orbiter because one team's software reported thrust in pound-force seconds while another team's software expected newton-seconds.
Common mistakes
- Using 2.5 instead of 2.54. The error looks small, but it adds up: over 100 inches it is 4 cm.
- Mixing the US and imperial gallon. A UK fuel or recipe figure in gallons is about 20% bigger than a US one.
- Confusing fluid ounces and weight ounces. A fluid ounce measures volume (about 29.57 mL) and a weight ounce measures mass (about 28.35 g). They are only close for water-like liquids.
- Multiplying when you should divide. Check the direction: a mile is longer than a kilometer, so the number of miles must be smaller than the number of kilometers.
- Rounding too early. Keep the full factor during the calculation and round the final answer.
- Mixing up the three tons. A US ton is 2,000 lb, a long ton is 2,240 lb, and a metric ton (tonne) is 1,000 kg.
Practice problems
1. Convert 8 in to centimeters.
$latex 8\times 2.54 = $ 20.32 cm.
2. Convert 180 cm to feet and inches.
$latex 180\div 2.54 = 70.87$ in. That is 5 ft with $latex 70.87-60 = 10.87$ in left over, so about 5 ft 11 in.
3. A baby weighs 3 lb 8 oz. How many kilograms is that?
3 lb 8 oz is $latex 3.5$ lb, and $latex 3.5\times 0.45359237 = $ 1.588 kg (rounded).
4. Convert 5 L to gallons.
$latex 5\div 3.785411784 = $ 1.32 gal (rounded).
5. A speed limit is 100 km/h. What is it in miles per hour?
$latex 100\div 1.609344 = $ 62.14 mph (rounded).
6. A soda can holds 12 fl oz. How many milliliters is that?
$latex 12\times 29.5735295625 = 354.88$, so about 354.9 mL (cans are labeled 355 mL).
Frequently asked questions
Why does the United States still use customary units?
The customary system was already in daily use when the metric system spread, and changing road signs, tools, packaging and habits is costly. Metric is legal and preferred in US science, medicine and the military, but daily life remains mostly customary.
Is an American pint the same as a British pint?
No. A US pint is 16 fl oz (0.473 L). An imperial pint is 20 imperial fl oz, about 0.568 L.
What is the exact definition of an inch?
Exactly 2.54 centimeters, as fixed by the international agreement of 1959.
How many feet are in a mile?
5,280 feet, which is also 1,760 yards.
Is a US ton the same as a tonne?
No. A US ton (short ton) is 2,000 lb, about 907 kg. A tonne, or metric ton, is exactly 1,000 kg, about 2,205 lb. Check which one a source means before converting, because the difference is about 10%.
Should I memorize the exact factors?
Memorize 2.54 cm per inch, because everything in the length column follows from it. For the rest, keep the rough values (1.6 km per mile, 0.45 kg per pound, 3.8 L per gallon) for estimates and use the exact numbers for any answer that will be graded or sold.
Related calculators
- Inches to Centimeters Calculator
- Feet to Centimeters Calculator
- Pounds to Kilograms Calculator
- Kilograms to Pounds Calculator
- Gallons to Liters Calculator
- Ounces to Milliliters Calculator
- Kilometers per Hour to Meters per Second Calculator
- Speed, Distance and Time Calculator