Mass is a physical quantity representing the amount of matter contained in an object. In everyday life, it's often called 'weight,' but strictly speaking, mass and weight are different. Mass is the quantity of the object itself and remains constant regardless of location, while weight is affected by gravity and differs between Earth's surface and the Moon. However, in everyday conversation and commerce, mass and weight are used interchangeably, and this tool supports both uses.
Difference between mass and weight
Mass represents the amount of matter contained in an object and doesn't change regardless of where it's measured. Units are kilograms (kg), grams (g), etc. Weight is the magnitude of gravitational force acting on an object and changes depending on location. Units are Newtons (N). For example, a person with 60kg mass remains 60kg on the Moon, but their weight becomes about one-sixth of Earth's. However, in everyday life, mass and weight aren't distinguished, as in 'body weight 60kg' or 'package weight 5kg.'
Metric system mass units
In the metric system based on the International System of Units (SI), mass units are defined with the gram as the basis.
Milligram (mg)
1 milligram = 0.001 grams = 0.000001 kilograms. Used when measuring very small masses. Used for pharmaceutical ingredient amounts ('500mg vitamin C'), nutrition labels ('1.2mg sodium equivalent'), chemical reagent quantities in experiments, etc. 1 gram equals 1,000 milligrams.
Gram (g)
One of the basic mass units. Most commonly used unit in everyday life. Used for food weight ('200g sugar,' '300g meat'), letter and package weight, precious metal weight ('gold 1g = 8,000 yen'), nutrition labels, etc. 1 gram equals about 0.035 ounces, about 0.002 pounds.
Kilogram (kg)
1 kilogram = 1,000 grams. One of seven basic SI units and the standard unit of mass. Used for human body weight ('body weight 65kg'), bulk food purchases ('10kg rice'), package weight, athlete weight classes, etc. 1 kilogram equals about 2.205 pounds, about 35.274 ounces. Currently, kilograms are defined based on Planck's constant (revised 2019).
Ton (t)
1 ton = 1,000 kilograms = 1,000,000 grams. Used when expressing very heavy object masses. Used for vehicle weight ('vehicle weight 1.5 tons'), ship cargo capacity, construction material weight, agricultural production ('annual production 5,000 tons'), etc. Also called 'metric ton' or 'tonne,' symbol 't.' Note that this differs from American 'short ton' (about 907kg) and British 'long ton' (about 1,016kg).
Imperial system mass units
Traditional unit system mainly used in America and Britain.
Ounce (oz)
1 ounce = about 28.35 grams. Unit used when measuring small masses. Used in American cooking recipes ('8 ounces flour'), precious metal weight, postal weight, baby products ('8 ounce bottle'), etc. 1 pound equals 16 ounces. Note that 'troy ounce' (about 31.1g) used in precious metal trading differs from regular ounces.
Pound (lb)
1 pound = 16 ounces = about 453.592 grams = about 0.454 kilograms. Most commonly used mass unit in America and Britain. Used for human body weight ('weight 150 pounds'), food weight ('1 pound steak'), dumbbell and barbell weight ('25 pound dumbbell'), package weight limits, etc. Symbol is 'lb' (from Latin libra pondo) or 'lbs' (plural).