Unit Converter
Pick the quantity, the unit you have and the unit you want. The conversion uses exact factors, shows the math and lists the same value in the other units.
Educational use: results are estimates made for study and reference. Checking the answer and deciding what to do with it is the sole responsibility of whoever uses the tool. Read the legal notice.
On this page
- How to convert a measurement
- How the conversion works under the hood
- The path of a conversion
- Conversions everyone looks up
- Solved examples
- Mistakes that trip up conversions
- Questions about conversions
- Where the units we use came from
- When you will need it
- Rounding and units that vary
- The conversion that cost a spacecraft
- Glossary of measurements
- Other useful tools
How to convert a measurement
- Choose the quantity (length, mass, temperature and so on). The unit lists update to match.
- Type the value you have.
- Select the starting unit ("From") and the target unit ("To").
- Click Convert. Besides the result, the tool shows the factor it used and the same value in every other unit of that quantity, which is handy for comparing.
If you just want to know "how many kilometers is 1 mile," type 1, pick mile under "From" and kilometer under "To." The step-by-step work includes the line "1 mi = 1.609344 km" for you to keep.
How the conversion works under the hood
For proportional quantities (almost all of them), the conversion goes through a base unit. Each unit has a factor that says how many base units it is worth. The math is always the same:
For example, for length the base unit is the meter: 1 km is worth 1,000 m and 1 mile is worth 1,609.344 m. So 10 km in miles is 10 × 1,000 ÷ 1,609.344 = 6.2137 mi.
Temperature is different
Temperature scales do not all start at the same zero, so multiplying is not enough. Each pair has its own formula:
°C = (°F − 32) × 5/9
K = °C + 273.15the Kelvin scale starts at absolute zero, and the tool does not accept values below it
Digital data: KB or KiB?
There are two systems. The decimal one uses powers of 1,000 (1 KB = 1,000 bytes, 1 MB = 1,000,000 bytes), and the binary one uses powers of 1,024 (1 KiB = 1,024 bytes, 1 MiB = 1,048,576 bytes). Disk makers use the decimal system, and many operating systems show the binary one, which is why a "1 TB" drive shows up as about 931 GB (actually GiB) on your computer.
The path of a conversion
Every proportional conversion is a two-step trip: it leaves your unit, passes through the base unit and arrives at the target unit.
Conversions everyone looks up
Length
| Unit | Equals |
|---|---|
| 1 inch | 2.54 cm (exact) |
| 1 foot | 30.48 cm = 12 inches |
| 1 yard | 91.44 cm = 3 feet |
| 1 mile | 1,609.344 m = 1.609 km |
| 1 nautical mile | 1,852 m |
Mass
| Unit | Equals |
|---|---|
| 1 pound (lb) | 453.59237 g |
| 1 ounce (oz) | 28.3495 g = 1/16 of a pound |
| 1 metric ton | 1,000 kg |
Volume (kitchen included)
| Measure | In mL |
|---|---|
| Teaspoon | 5 mL (cooking standard) |
| Tablespoon | 15 mL |
| Brazilian cup | 240 mL |
| US cup | 236.588 mL |
| Pint (US) | 473.176 mL |
| Gallon (US) | 3,785.41 mL |
Reference temperatures
| Point | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Water freezes | 0 | 32 | 273.15 |
| Room temperature | 20 | 68 | 293.15 |
| Water boils (sea level) | 100 | 212 | 373.15 |
Solved examples
1. Height in feet and inches
Someone is 5 feet 11 inches tall. How many centimeters is that?
2. The American recipe
A recipe calls for 2 US cups of milk. How many mL is that?
If the recipe is Brazilian, 2 cups of 240 mL come to 480 mL. The difference is small, but it can matter in delicate recipes such as cakes and bread.
3. Oven in degrees Fahrenheit
A recipe says to set the oven to 350 °F. What is that in Celsius?
Rounded, about 175 to 180 °C.
4. The disk that "lost space"
A 500 GB (decimal) drive shows up as how many GiB on your computer?
No space went missing: these are two different ways of counting.
5. Speed in another country
A speed limit of 100 km/h is how many miles per hour?
Mistakes that trip up conversions
Forgetting that areas and volumes use the squared or cubed factor
1 m = 100 cm, but 1 m² = 10,000 cm² and 1 m³ = 1,000,000 cm³. If you convert an area with the length factor, you will be way off.
Confusing the ounce of mass with the fluid ounce
The ounce (oz) can measure mass or volume. 1 oz of mass is about 28.35 g, and 1 fl oz (US fluid ounce) is about 29.57 mL. They are not the same thing.
US gallon versus imperial gallon
The US gallon is 3.785 liters. The imperial (UK) gallon is 4.546 liters. Check which system your value comes from.
Converting temperature by multiplying only
Because the scales do not start from the same zero, you have to add or subtract 32 (or 273.15) as well as multiply. Doubling the degrees Celsius does not double the degrees Fahrenheit.
Rounding too much midway
In chained conversions, keep all the digits and round only at the end.
Questions about conversions
How do I convert kilometers to miles?
Divide by 1.609344. 10 km ÷ 1.609344 = 6.2137 miles. To go the other way, multiply by 1.609344.
How many centimeters are in an inch?
Exactly 2.54 cm. This value is set by international agreement.
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 (1.8) and add 32. 100 °C × 1.8 + 32 = 212 °F.
How do I convert Fahrenheit to Celsius?
Subtract 32 and multiply by 5/9. (350 − 32) × 5/9 ≈ 176.7 °C.
How many mL is a cup?
It depends on the standard. The Brazilian recipe cup is usually 240 mL. The US cup is 236.588 mL.
What is the difference between MB and MiB?
MB uses powers of 1,000 (1 MB = 1,000,000 bytes). MiB uses powers of 1,024 (1 MiB = 1,048,576 bytes). That is why 1 GB is about 0.93 GiB.
How do I convert m² to cm²?
Multiply by 10,000, since 1 m² = 100 cm × 100 cm. To go the other way, divide.
How many liters are in a cubic meter?
1,000 liters.
How do I convert km/h to m/s?
Divide by 3.6. 90 km/h ÷ 3.6 = 25 m/s.
Why does the converter not accept very low temperatures?
Because nothing can be colder than absolute zero (−273.15 °C or 0 K). If the value you type is below that, the tool warns you.
Where the units we use came from
For centuries, every region had its own measures: spans, cubits, leagues, arrobas. Trade suffered for it, and the French Revolution decided to create a system "for all people, for all time." The metric system was born, with the meter initially defined as one ten-millionth of the distance from the North Pole to the Equator.
Today the International System of Units (SI) defines all of its units through constants of nature. Countries that still use miles, pounds and gallons, such as the United States, also have those units officially tied to the SI: since 1959, the yard has been exactly 0.9144 meter and the pound exactly 0.45359237 kilogram. That is where the exact factors this tool uses come from.
When you will need it
- Cooking: recipes from other countries, with cups, ounces and degrees Fahrenheit.
- Travel: road signs in miles, temperature in °F, fuel in gallons.
- Online shopping: clothing, screen and electronics measurements in inches.
- Computing: file sizes, connection speeds and disk space.
- Study and technical work: converting data from problem statements, standards and catalogs.
- Sports: distances in miles and paces in minutes per mile.
Rounding and units that vary
The factors used are the officially defined ones where they exist (such as 1 inch = 2.54 cm and 1 pound = 0.45359237 kg), and the rest follow widely adopted values. Even so, some units are not universal: cups, spoons, gallons and ounces vary by country and by use, and the tool indicates which standard is in use in the name of the unit.
The display is rounded (up to ten decimal places for unit conversions, and up to six for the other calculations), but the internal math uses the full precision of the browser. For uses where accuracy is critical, such as dosages, engineering or construction projects, confirm the result against the standard or the original source of the measurement.
Bottom line: this tool is educational. Results do not replace your own checking, and how you use them is entirely your responsibility. More details in the legal notice.
The conversion that cost a spacecraft
In 1999, NASA's Mars Climate Orbiter was lost as it approached Mars. The investigation found that one navigation system supplied data in pound-force seconds while another expected newton seconds. A unit conversion was missing, the trajectory went wrong and the spacecraft entered the Martian atmosphere at a much lower altitude than planned.
The moral of the story applies to everyday life too: always check which unit a number is in before you use it.
- Only three countries (the United States, Liberia and Myanmar) have not officially adopted the metric system, according to the most cited classification, although many others keep traditional units for specific uses.
- The foot did come, of course, from the human foot, but the "standard foot" was defined much later, and it varied from city to city.
Glossary of measurements
- Base unit
- The reference unit of a quantity, used as a bridge in conversions (meter for length, kilogram for mass).
- Conversion factor
- A number that relates two units, such as 1 inch = 2.54 cm.
- SI
- The International System of Units, adopted by most countries.
- Prefix
- The part of the name that indicates a power of 10: kilo (1,000), milli (0.001), mega (1,000,000).
- Absolute zero
- The lowest possible temperature: 0 K or −273.15 °C.
- Byte
- A group of 8 bits, the basic unit of digital storage.
- Nautical mile
- A unit used in sea and air navigation: 1,852 meters.