Different countries and fields use different measurement systems. The metric system is the international standard used in science and most countries. The imperial system is still common in the United States. Accurate conversion is essential in engineering, cooking, trade, and travel.
1 meter = 3.28084 feet | 1 kilometer = 0.621371 miles | 1 inch = 2.54 centimeters. These base conversions underpin all other length unit calculations.
Celsius to Fahrenheit: multiply by 9/5 and add 32. Fahrenheit to Celsius: subtract 32 and multiply by 5/9. Kelvin = Celsius + 273.15. Kelvin is used in scientific applications and has no negative values.
Mass measures the amount of matter in an object (kg or lbs). Weight is the force of gravity on that mass (Newtons). For everyday conversion purposes the distinction does not affect calculations.