Pick a category, enter a value, and choose the units to convert from and to.
Length, weight/mass, volume, area, speed, time, digital storage, pressure, and energy each have a full set of common units — for example length covers meters, kilometers, centimeters, millimeters, miles, yards, feet, inches, and nautical miles. Temperature converts between Celsius, Fahrenheit, and Kelvin using their proper formulas rather than a simple ratio.
Every unit in a category is converted to a common base unit first (meters for length, kilograms for weight, liters for volume, and so on), then converted from that base into your target unit — so any combination within a category works, not just fixed pairs. Temperature is the one exception, since Celsius, Fahrenheit, and Kelvin scales don't share a zero point, so it uses direct formulas instead. The swap button flips the From and To units instantly, useful for checking a conversion in reverse.
Pressure covers pascals, kilopascals, bar, PSI (pounds per square inch, standard in the US for tire pressure and plumbing), atmospheres, and mmHg (used in medical blood-pressure and barometric contexts). Energy covers joules, kilojoules, calories, kilocalories (the "Calories" on nutrition labels), watt-hours, kilowatt-hours (what electricity bills are metered in), and BTU (British Thermal Units, still used for HVAC ratings in the US). These come up constantly in cross-border technical contexts — a European car's tire pressure spec in bar vs a US spec in PSI, or a US appliance's energy rating in BTU vs an EU rating in kWh.
Some conversion factors are internationally defined exact values (1 inch = 2.54cm exactly, part of the 1959 international yard and pound agreement), while others involve rounding depending on the standard used — the US survey foot and the international foot differ by about 2 parts per million, immaterial for daily use but occasionally relevant in land surveying and GIS work. For everyday use these differences are negligible, but they can matter in scientific, engineering, or legal contexts.
No — unit conversion only works within the same physical quantity (length to length, weight to weight), since converting across categories (like meters to kilograms) isn't mathematically meaningful without additional context like density or a specific material's properties.
All three, mixed within each category as needed. Metric (SI) units are the global default for science and most of the world's daily use, including India and virtually all of Europe. Imperial units (as historically used in the UK) and US customary units (the US's own variant, which differs from imperial in a few units like fluid ounces and gallons) are both represented for length, weight, and volume, since American users, and to a lesser extent British users for some everyday units like miles and pints, still use them daily.
The US customary gallon (3.785 liters) and the imperial gallon used historically in the UK (4.546 liters) are genuinely different volumes — about 20% apart — a holdover from different historical standardizations that never fully merged. This converter uses US customary volume units throughout, since that's the more commonly needed reference online; if you specifically need imperial (UK) gallons, note the roughly 1.2× difference when converting.
Worked example: 1 mile = 1,609.34 meters exactly as used here, so 5 miles converts to 8,046.7 meters, or 8.047 km. On the newer categories: 30 PSI (a common car tire pressure) converts to about 2.07 bar, and 1 kWh converts to exactly 3,600,000 joules or about 3,412 BTU — try entering these in the corresponding categories above.
Recipe units (teaspoons, tablespoons, cups) are volume measurements, same as the Volume category — this dedicated category just uses the exact US customary kitchen ratios (1 tbsp = 3 tsp, 1 cup = 16 tbsp) so you don't have to convert through liters mentally. Note this only converts volume to volume — converting cups of a specific ingredient to grams requires that ingredient's density, which varies (a cup of flour and a cup of honey weigh very different amounts), so that's intentionally not something a pure unit converter can do generically.
Converting a trip's fuel efficiency specifically? Use the dedicated Fuel Cost Calculator — it handles MPG, L/100km, and km/L together with trip cost. For the three most-searched single-pair conversions, the dedicated CM to Inches, KG to Pounds, and Celsius to Fahrenheit converters are quicker than picking units from the dropdowns above. Need to do arithmetic on a converted figure? The Basic Calculator is one click away. Converting an area or volume? Remember the factor squares or cubes — the Area Calculator and Volume Calculator explain why. Converting a cable run or an appliance rating? The Voltage Drop Calculator works in feet or metres, the Electricity Cost Calculator turns watts into a running cost in your own currency, and the Bandwidth Calculator handles the bits-versus-bytes conversion that catches everyone out. For derived quantities rather than single units, the Density Calculator and Speed Calculator handle mixed metric and imperial input directly. Building or landscaping? The Square Footage Calculator, Concrete Calculator and Gravel Calculator all take metric or imperial input and report both. Sizing heating or cooling? The BTU Calculator takes square feet or square metres. Curious what you would weigh elsewhere in the solar system? Weight on Other Planets uses NASA gravity figures. Cooking specifically? The Cooking Measurement Converter handles cups to grams per ingredient, which a general unit converter cannot — a cup of flour and a cup of honey differ by nearly threefold. Number bases are a conversion of a different kind — the hex, binary and decimal converter handles those.