Convert between units of length, weight, temperature, volume, area, and speed with instant results.
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About the Unit Converter — Free Online Measurement Conversion Tool
This free online unit converter handles 8 measurement categories — length, weight, temperature, volume, area, speed, energy, and pressure — with dozens of units in each category. Results update instantly as you type, with no button press required. Whether you need to convert kilometres to miles, kilograms to pounds, Celsius to Fahrenheit, litres to gallons, or any other common unit combination, this unit converter online delivers precise results in seconds.
Unit conversion is one of the most universally needed calculations across science, engineering, cooking, travel, fitness, and everyday life. The coexistence of the metric system (used by most of the world) and imperial/US customary units (used primarily in the United States) means that unit conversion is a daily necessity for anyone working across international contexts — reading a foreign recipe, interpreting a medical result in different units, comparing product specifications from different countries, or understanding weather forecasts while travelling. A reliable unit converter online eliminates the mental arithmetic and fraction-of-a-second rounding errors that manual conversion introduces.
How to Use the Unit Converter
Select a Category from the dropdown: Length, Weight/Mass, Temperature, Volume, Area, Speed, Energy, or Pressure.
Enter the Value you want to convert in the input field. Results update instantly as you type each digit.
Select the From unit (what you have) and the To unit (what you want).
The converted result appears immediately. Click the ⇄ swap button to reverse the conversion direction instantly.
How the Unit Conversion Works
For most categories, each unit has a precise conversion factor relative to a base unit (for example, all length units convert relative to metres). The formula is: result = value × (fromFactor / toFactor). Temperature conversions are handled differently because temperature scales have different zero points — they are offset rather than purely proportional — so direct formulas are used: Celsius to Fahrenheit: F = C × 9/5 + 32, Fahrenheit to Celsius: C = (F − 32) × 5/9, Celsius to Kelvin: K = C + 273.15.
Example Conversions
100 km to miles: 100 × 0.621371 = 62.14 miles
72°F to Celsius: (72 − 32) × 5/9 = 22.22°C
5 kg to pounds: 5 × 2.20462 = 11.02 lbs
Tips for Using the Unit Converter
Use the swap button for reverse conversions: If you typed a conversion from miles to kilometres and now need to convert kilometres back to miles, press the ⇄ swap button to instantly reverse the From and To units without reselecting them from the dropdowns. This is the fastest way to check a bidirectional conversion.
Convert cooking measurements with volume and weight: Recipes from different countries use different volume units (US cups vs. metric millilitres vs. UK fluid ounces) and sometimes weight instead of volume. Use the volume category for liquid measurements and the weight category for dry ingredients. Note that volume-to-weight conversions for dry ingredients require knowing the ingredient's density, which this converter does not calculate.
Use pressure conversion for tyre inflation: Tyre pressure is expressed in PSI (pounds per square inch) in the US and bar or kPa in Europe and elsewhere. The pressure category converts between PSI, bar, kPa, and atmospheres — useful when checking your car's tyre pressure with a gauge calibrated in different units than your vehicle's specification.
Convert pace units in the speed category: Runners often work in minutes per kilometre or minutes per mile, while vehicle speeds use km/h or mph. The speed category converts between all common speed units, making it useful for comparing race pace targets across metric and imperial marathon training plans.
Bookmark for travel and international work: If you regularly travel between metric and imperial countries or work with international specifications, bookmark this unit converter as a quick-access reference. The instant-update design means you can type directly into the value field and get results in one action, faster than a search engine query.
Frequently Asked Questions about Unit Converter
Conversions use the full precision of JavaScript's 64-bit IEEE 754 floating-point numbers, which provides approximately 15–17 significant decimal digits of precision. Results are rounded to 8 decimal places for display — sufficient for virtually all practical applications including engineering calculations, scientific conversions, and everyday measurement needs. For cases where extreme precision is critical (analytical chemistry, precision manufacturing), use the displayed 8-decimal-place result rather than rounding further.
Most unit conversions are purely multiplicative — you multiply by a fixed factor. This works because the units share a common zero point (0 metres = 0 feet = 0 inches). Temperature scales do not share a zero point: 0°C is not the same as 0°F and not the same as 0 K. This means you cannot simply multiply to convert between them — you must apply an offset formula as well. The converter handles all temperature combinations (Celsius, Fahrenheit, Kelvin, Rankine) using the correct offset-aware formulas automatically.
The unit converter supports 8 categories: Length (metres, feet, inches, miles, kilometres, yards, nautical miles, light years, and more), Weight/Mass (kilograms, pounds, ounces, grams, stone, metric tons), Temperature (Celsius, Fahrenheit, Kelvin, Rankine), Volume (litres, gallons, fluid ounces, cups, millilitres, cubic metres), Area (square metres, square feet, acres, hectares, square miles), Speed (km/h, mph, m/s, knots), Energy (joules, calories, kilowatt-hours, BTUs), and Pressure (Pa, bar, PSI, atm, kPa). Each category includes the most commonly needed units for that measurement type.
Yes — this unit converter online is completely free. No account is required, no app is downloaded, and there are no usage limits. The tool works in any modern browser on desktop, tablet, and smartphone. All conversions happen instantly in your browser with no server round-trip — just type your value and see the result update in real time. The tool is available at all times without any registration or sign-up process.
Select the appropriate category, then choose a metric unit in the From dropdown (e.g. Kilometer, Kilogram, Celsius, Litre) and an imperial unit in the To dropdown (e.g. Mile, Pound, Fahrenheit, Gallon) or vice versa. The converter supports all standard metric-to-imperial combinations in each category. The swap button lets you instantly reverse the direction, so if you converted 5 kg to pounds and now need pounds to kg, press ⇄ and type the pound value directly.
Yes — the converter handles very large numbers (e.g. converting light years to kilometres: 1 light year = 9,461,000,000,000 km) and very small numbers (e.g. converting nanometres to metres: 1 nm = 0.000000001 m) using JavaScript's floating-point number format. For extremely large or small values, the result may be displayed in scientific notation (e.g. 9.461e12 km). The precision remains accurate within JavaScript's 64-bit floating-point range, which covers values from approximately 5×10⁻³²⁴ to 1.8×10³⁰⁸.
Mass is the amount of matter in an object (measured in kilograms, grams, pounds) and does not change regardless of location. Weight is the force exerted on that mass by gravity (measured in Newtons) and varies depending on gravitational field strength. On Earth, mass and weight are proportional — a 70 kg person experiences 686 N of gravitational force. The unit converter's Weight/Mass category uses mass units (kg, lb, oz, g) as they are universally used in everyday weighing applications, even though the technically correct term for these measurements is mass rather than weight.
Yes — the unit converter is fully mobile-optimised. The dropdowns, input field, and swap button are all touch-friendly and scale to fit smartphone and tablet screens. The number keyboard appears automatically on iOS and Android when you tap the value field. Results update in real time as you type, without needing to press a calculate button. The converter works on iOS Safari and Android Chrome without any app installation and is accessible at any time without an internet connection once the page has loaded.