What a Unit Converter Is
A unit converter translates a quantity from one unit of measurement into another. The International System of Units (SI) is the modern form of the metric system and is the global standard for science, engineering and commerce. It defines seven base units from which all others derive: the metre, the kilogram, the second, the ampere, the kelvin, the mole and the candela. Since the 2019 redefinition of the SI, every base unit is fixed by universal physical constants such as the speed of light and the Planck constant rather than by physical artefacts.
Despite SI being nearly universal in scientific publication, imperial and US customary units remain in daily use in the United States and in specialised fields such as aviation (feet for altitude, nautical miles for distance) and food recipes. A good unit converter smooths this friction by letting users work in whichever system they prefer without manual arithmetic.
How It Works
For dimensional quantities like length and weight, every unit in a category has a defined ratio to a chosen base unit. To convert between two units, multiply by the first ratio and divide by the second. For example, converting 5 feet to metres uses 5 times 0.3048, giving 1.524 metres. Then to express that in centimetres, divide by 0.01, giving 152.4 centimetres.
Temperature is a special case because the scales do not share a common zero. Celsius and Kelvin differ by a constant offset of 273.15. Fahrenheit and Celsius are linearly related by F equals C times 1.8 plus 32. Rankine, used in some engineering fields, equals Fahrenheit plus 459.67. All four form a family of linear transformations, so you can always convert through a common intermediate such as Celsius.
How to Use This Tool
- Pick a category from the dropdown: length, weight, temperature, volume, area, speed or time.
- Enter the number you want to convert into the From field.
- Choose the source unit.
- Choose the target unit.
- The result appears instantly in the To field.
- Use Swap to flip the two units, or Reset to clear the inputs.
Common Use Cases
- Cooking: Convert between US cups, UK cups, millilitres and grams for recipes from different regions.
- Travel: Convert miles to kilometres for driving and knots to km/h for flight apps.
- Fitness: Switch between pounds and kilograms, inches and centimetres for gym logs and health forms.
- DIY and construction: Convert feet and inches to metres for lumber and tile calculations.
- Science homework: Convert pressure, speed or area between SI and imperial for physics problems.
- Shipping: Compute volumetric weight by converting cubic centimetres to kilograms using the carrier factor.
Common Mistakes
- Mixing US and UK gallons. A US gallon is 3.78541 litres, a UK gallon is 4.54609 litres.
- Confusing mass (kilogram) with weight (newton). Pounds-force and pounds-mass are also different.
- Forgetting that temperatures have offsets, not just scale factors.
- Using decimal commas in locales that expect decimal points, or vice versa.
- Rounding too aggressively on precision conversions such as chemistry concentrations.
Weight vs Mass and Imperial Quirks
In physics, weight is force and mass is matter. A kilogram is a unit of mass. A newton is the SI unit of force, and on Earth one kilogram of mass weighs about 9.81 newtons. In everyday speech people say they weigh 70 kilograms, which is loose but harmless. In engineering problems, mixing up the pound-mass (lbm) and the pound-force (lbf) is a classic source of unit errors, and the slug is an older unit of mass that still appears in American engineering textbooks.
Volume units are similarly tricky. The litre is the practical SI unit and equals one cubic decimetre. The US fluid gallon and the UK imperial gallon diverged in 1824 when Britain redefined its gallon based on 10 pounds of water. Recipes crossing regions silently fail when a cook assumes their local gallon is the same as the one in the book.
Privacy
All calculations run in your browser. The numbers you enter are never uploaded, logged or stored.
Frequently Asked Questions
Which units are part of the SI base?
The seven SI base units are the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance and the candela for luminous intensity. All other units derive from these.
What are some common metric to imperial conversions?
1 metre equals 3.2808 feet, 1 kilometre equals 0.6214 miles, 1 kilogram equals 2.2046 pounds, and 1 litre equals 0.2642 US gallons. These are definitional values from the 1959 international yard and pound agreement.
Is temperature conversion linear?
Yes, but with offsets. Celsius to Fahrenheit is F equals C times 1.8 plus 32. Celsius to Kelvin is K equals C plus 273.15. Rankine equals Fahrenheit plus 459.67. All four scales are linear transformations of each other.
Is a US gallon the same as a UK gallon?
No. A US liquid gallon is 3.78541 litres while a UK or imperial gallon is 4.54609 litres, about 20 percent larger. The two have been different since 1824 when Britain redefined its gallon.
What is the difference between mass and weight?
Mass is the amount of matter in an object, measured in kilograms. Weight is the force gravity exerts on that mass, measured in newtons. In everyday speech people use weight when they mean mass, which works fine on Earth but confuses physics problems.
Are historical units like furlong and stone still supported?
This tool focuses on the units you meet in daily life and engineering. Historical units like furlong, league, chain, stone and hundredweight exist in dedicated conversion references but are not included here.
Is my input stored?
No. All conversions run in your browser with JavaScript. Values are not uploaded, saved or shared.