Difference between the Pound and the Kilogram

The pound and the kilogram are two units that are used to measure the mass of an object. The kilogram is the SI unit of mass, while the pound is part of the imperial system. The imperial system is still used in many parts of the world, most notably the US.

The word ‘kilogram’ is derived from the French word ‘kilogramme.’ This French word was derived from words belonging to Greek and Latin. The Greek word ‘khiloi’ forms the first part of the word. It means ‘a thousand’. The Latin word ‘gramma’ forms the second part of the word. It means ‘a small weight’.

The pound, on the other hand, was derived from a word from the Proto-Germanic language. This word was in turn borrowed from a Latin word, ‘libra pondo.’ This means ‘the weight measured in libra.’

Definitions

• The kilogram was first called ‘the grave’ in earlier times. The grave was defined as the mass of 1 liter of water. This definition was used around the year 1793. In 1795, the word ‘kilogram’ came to be in general use. Around this time, it was defined as a thousand times the mass of 1 cm3 of water at the melting point. In 1799, an early prototype of the kilogram was manufactured called ‘Kilogramme des Archives’. However, this prototype failed to get widespread acceptance. Then at the Metre Convention of 1875, the kilogram’s other prototype was manufactured called the ‘International Prototype of Kilogram’ (IPK). This prototype had a mass equal to the mass of 1 dm3 of water at water’s maximum density. Water’s maximum density occurs at 4oC. In 2019, the General Conference on Weights and Measures (CGPM) defined the kilogram in terms of physics’ fundamental constants. The kilogram is now defined in terms of the speed of light ‘c’, the specific atomic transition frequency, and Planck’s constant ‘h’. This is done with the help of the unit Js (Joules-second). Js can be expressed as Kg⋅m2⋅s-1. Here, the meter is defined in terms of the speed of light ‘c’, and the second is defined in terms of the specific atomic transition frequency.
• The pound has been used to refer to many units in history. Not all of these units were identical. All these were only broadly similar to each other. The first known use of the unit of the pound is considered to be a very old unit called the Roman libra. This was used by the ancient Romans. It was approximately equal to 328.9 grams. This term is also responsible for the abbreviation of pound (lb) that we use today. In addition to these units, the pound has gone through several other variations. Some of them include the tower pound, the merchant pound, and the London pound. All these units are not in use today. A certain variation of pound called the troy pound is used to measure the mass of precious metals. This unit is commonly used in the trade, but it is only referred to in terms of ounces. If during a transaction the type of ounce is not specified, then the troy system is assumed.

Today the standard unit of the pound used all across the world is called the avoirdupois pound. On 1 July 1959, the US and the Commonwealth of Nations adopted the internationally accepted definition of the avoirdupois pound. It was defined as exactly 0.45359237 kg. In the UK, the resolution was adopted in 1963 by the Weight and Measures act. Thus, 1 avoirdupois pound is equal to 16 avoirdupois ounces.

Earlier, the UK tried to discontinue the practice of using pounds and other non-metric units by implementing the European Units of Measurement Directives. This required all the packaged goods to be labeled only in metric units like the kilogram. However, the UK abandoned this effort in 2007 and allowed the packaged goods to have labeling in both units.

Mass vs. Weight

The kilogram and the pound are both units of mass. Mass is often used interchangeably with weight. For example, the weight of a certain amount of apples can be 1 kg or 1 pound. This is not problematic in daily life. However, if we are required to measure the weight of an object with more precision than just a few percent, then the distinction becomes important. Weight is proportional to mass in a constant gravitational field, but the gravitational field around the Earth’s surface changes slightly with the location.

Summary: Difference between Pound and Kilogram