Convert between common units of length, weight, temperature, and volume.
Length: millimeters, centimeters, meters, kilometers, inches, feet, yards, miles.
Weight: milligrams, grams, kilograms, ounces, pounds, stone, metric tons.
Temperature: Celsius, Fahrenheit, Kelvin.
Volume: milliliters, liters, cups, pints, quarts, gallons (US), fluid ounces.
Pick a category first — Length, Weight, Temperature, or Volume — using the buttons above the input fields. Each category loads its own list of units so you never have to scroll through options that don't apply. Type a number into the "From" field, choose the unit you're converting from and the unit you want to convert to, and the result appears instantly below. There's no need to press a calculate button; the conversion runs live as you type. If you want to flip the direction of the conversion, click the circular arrow between the two fields and the units swap places automatically.
Length is the most frequently converted unit type, mostly because the US uses feet and inches while most of the world uses meters and centimeters. A few reference points worth knowing: 1 inch equals 2.54 centimeters, 1 foot equals 0.3048 meters, and 1 mile equals 1.60934 kilometers. So a 6-foot-tall person is about 1.83 meters, and a 5-kilometer run is roughly 3.1 miles. These figures are exact conversion factors, not rounded estimates, so the calculator above will always match manual math done with these numbers.
Weight conversions trip people up because "pounds" and "kilograms" don't share a clean round-number relationship. One kilogram equals 2.20462 pounds, and one pound equals 0.453592 kilograms. For quick mental math, multiplying kilograms by 2.2 gets you close enough for everyday use — a 70 kg adult weighs about 154 lb. For precise figures, such as shipping weights or medical dosages, use the exact calculator above rather than the rounded shortcut.
Temperature is different from length and weight because it doesn't convert by simple multiplication — it needs an offset. To convert Celsius to Fahrenheit, multiply by 9/5 and add 32: °F = (°C × 9/5) + 32. To go the other way, subtract 32 first, then multiply by 5/9: °C = (°F − 32) × 5/9. Kelvin is used mainly in science and equals Celsius plus 273.15, with no negative values possible since 0 K is absolute zero. Water freezes at 0°C (32°F, 273.15 K) and boils at 100°C (212°F, 373.15 K) at sea level.
Cooking and baking are where volume conversions come up most, especially when following a recipe written in a different measurement system. One US cup equals 236.588 milliliters, one US gallon equals 3.78541 liters, and one liter equals roughly 4.2 cups. Note that the US and UK define some volume units differently — this calculator uses US customary units (US cups, US gallons, US fluid ounces), which is the standard used in most American recipes and product labeling.
Most of the world uses the metric system (meters, kilograms, liters, Celsius) because it's based on powers of ten, making conversions within the system simple — 1 kilometer is always 1,000 meters, no matter what you're measuring. The United States, along with a small number of other countries, primarily uses the imperial/US customary system (feet, pounds, gallons, Fahrenheit), which has historical roots but doesn't scale as cleanly (1 mile is 5,280 feet, and 1 gallon is 128 fluid ounces). This is exactly why a converter tool is useful: the two systems don't share simple round-number relationships, so converting by hand means memorizing or looking up conversion factors, while a calculator does the exact math instantly.
Rounded conversion shortcuts are fine for a quick mental estimate, but they can introduce meaningful error over larger numbers or repeated calculations. For example, rounding 1 mile to "1.6 km" instead of the precise 1.60934 km creates a small error that grows if you're converting distances for a long road trip or a scientific measurement. This calculator always uses full-precision conversion factors internally, then rounds only the final displayed result — so chaining multiple conversions together (like feet to meters to kilometers) won't compound rounding errors the way manual shortcuts can.
Unit conversion isn't just a classroom topic — it shows up constantly in daily life. Travelers crossing between countries need to convert temperature forecasts, road distances, and luggage weight limits. Home cooks following a recipe from a different country often need to convert cups to milliliters or Fahrenheit oven temperatures to Celsius. Online shoppers comparing products from international retailers frequently need to convert dimensions or weights to know if an item will fit or how much shipping will cost. Students and professionals in science, engineering, and healthcare regularly convert between metric and imperial units because equipment, textbooks, and regulations don't always agree on which system to use.
For fast mental estimates (not exact — use the calculator above for precision): 1 inch ≈ 2.5 cm, 1 foot ≈ 30 cm, 1 mile ≈ 1.6 km, 1 kg ≈ 2.2 lb, 1 liter ≈ 1.06 quarts, 1 US gallon ≈ 3.8 liters, and 0°C ≈ 32°F while every additional 5°C ≈ 9°F. These rounded figures are handy for quick estimates in conversation or shopping, but for anything involving money, medicine, construction measurements, or science, the exact calculator above should be used instead of rounded approximations.
Is this converter accurate for scientific or engineering use? Yes — all conversion factors used are standard, internationally recognized values, not rounded approximations. Results are shown to six significant figures for most units.
Why do temperature conversions need a different formula than length or weight? Length and weight scales both start at zero and grow proportionally, so converting is just multiplication. Temperature scales start at different points (Celsius and Fahrenheit have different zero points), so an offset has to be added or subtracted as well.
Does this tool store or share the numbers I enter? No. All conversions run directly in your browser using JavaScript — nothing is sent to a server or saved.
Why do US and UK recipes sometimes give different results for the same cup measurement? A US cup (236.588 ml) is slightly smaller than a UK/imperial cup (284.131 ml). This calculator uses US customary volume units, so if you're converting a UK recipe, double-check which cup size the original source intended.
Last reviewed by Mehmed on July 11, 2026.