Unit conversion is the process of changing a measurement from one unit to another while preserving the same quantity. For example, 1 kilometer equals 1,000 meters — both describe the same distance, just expressed differently.
The metric system uses meters as the base unit for length. The imperial/US customary system uses inches, feet, yards, and miles. Common conversions: 1 inch = 2.54 cm, 1 mile = 1.60934 km, 1 foot = 30.48 cm.
The metric system uses grams and kilograms. Imperial uses ounces and pounds. 1 pound = 453.592 grams, 1 kilogram = 2.20462 pounds.
Volume measures how much space a substance occupies. The metric unit is liters (L) and milliliters (mL). US units include fluid ounces, cups, pints, quarts, and gallons. 1 US gallon = 3.78541 liters.
Speed is distance per unit of time. Common units: km/h (kilometers per hour), mph (miles per hour), m/s (meters per second), and knots (nautical miles per hour). 1 mph = 1.60934 km/h.
Unit conversion comes up constantly — following a recipe written in cups when you only have a kitchen scale, reading a weather forecast in Celsius when you're used to Fahrenheit, buying hardwood flooring measured in square feet for a room you measured in meters, or checking a car's fuel efficiency in km/L versus miles per gallon. Fluency with common conversions saves time and prevents costly mistakes.
Length: 1 inch = 2.54 cm; 1 foot = 30.48 cm; 1 mile = 1.609 km; 1 km = 0.621 miles; 1 meter = 3.281 feet.
Weight: 1 kg = 2.205 lbs; 1 lb = 453.6 g; 1 stone = 14 lbs = 6.35 kg. The stone is still commonly used for body weight in the UK and Ireland.
Temperature: °F = (°C × 9/5) + 32; °C = (°F − 32) × 5/9. Key reference points: 0°C = 32°F (water freezes), 100°C = 212°F (water boils at sea level), 37°C = 98.6°F (normal human body temperature), 20°C = 68°F (comfortable room temperature).
Volume: 1 liter = 33.8 fl oz; 1 US gallon = 3.785 liters; 1 US cup = 236.6 mL.
Speed: 1 km/h = 0.621 mph; 100 km/h = 62.1 mph (a handy reference for highway driving).
The metric system is the official standard in most of the world and is universally used in science and medicine. The imperial system remains dominant for everyday use in the United States. Canada officially adopted metric in the 1970s, but imperial persists strongly in everyday life — people still give their height in feet and inches, weigh themselves in pounds, and measure rooms in feet. Construction and renovation in Canada frequently uses both systems simultaneously, which is why a good unit converter is essential.
The SI system is the modern form of the metric system, formally defined by an international treaty. It establishes seven base units: the meter (length), kilogram (mass), second (time), ampere (electric current), kelvin (temperature), mole (amount of substance), and candela (luminous intensity). All other units — such as newtons, watts, and pascals — are derived from these seven. SI prefixes like kilo- (×1,000), centi- (÷100), and milli- (÷1,000) make it easy to scale measurements up or down.
American recipes use volume measurements (cups, tablespoons, teaspoons) while most of the world uses weight (grams). Key conversions for the kitchen: 1 US cup = 240 mL; 1 tablespoon = 15 mL; 1 teaspoon = 5 mL; 1 US fluid ounce = 29.6 mL. Note that converting cups to grams also depends on the ingredient — 1 cup of water weighs 240 g, but 1 cup of all-purpose flour weighs only about 120–130 g because flour is less dense.
This free unit converter lets you instantly convert between hundreds of measurement units across all major categories: length (meters, feet, inches, miles, kilometers), weight (kilograms, pounds, ounces, grams), temperature (Celsius, Fahrenheit, Kelvin), volume (liters, gallons, fluid ounces), speed (km/h, mph, knots), and area (square meters, square feet, acres). It is the most complete free unit conversion calculator online.
Whether you are following a recipe in a different measurement system, calculating flooring needs, converting speeds for travel, or doing science homework, this unit converter handles it all with precision. Simply select a category, choose the "from" and "to" units, and type your value for an instant conversion — no math required.