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Conversion of Units in Physics - Formula, Examples, History and Techniques

Since the beginning of human history, humans have encountered the necessity of measuring natural phenomena in numerically quantifiable values. For example, since prehistory, humans have tried to develop ways to measure the mass of objects. This need arose due to several practical processes that humans were developing, like trade. Now the question is, what reference should be used to determine the quantity of something? For example, how will we make sure that the mass of two similar-looking rocks is actually the same?

This problem was solved by defining units of quantities. For example, if we define one pound of mass as a fixed mass, then we can easily measure other masses with respect to one pound. Then we can find that how many pounds a certain rock weighs or how many onions should be packed in a sack to make it twenty pounds.

In the modern world, a unit is defined as a standard quantity that acts as a reference in a certain geographical region to measure physical quantities of the same kind. Smaller or bigger quantities of the same kind are expressed in the multiples of the base unit. Thus, the numerical value of a unit is always taken to be one.


The invention of units to measure physical quantities goes back to the stone age. People required units to measure quantities for a variety of tasks like house building, making cloth, or bartering goods with each other. The earliest uniform system of units is believed to be enforced in ancient Mesopotamia, Egypt, Indus Valley and Persia. Beginning in the middle ages, England adopted a uniform system of units. This law was passed in the Magna Carta, the famous document that is said to have birthed democracy. King John enforced this charter around 1215.

In the 21st century, the most widely used system of units is the Metric system. This system was first developed in France during the reign of Napoleon, who advocated the use of a uniform system of units to advance trade between different regions of the country. However, this system did not have universal acceptance until 1875, when 17 nations signed The Metric Conversion Treaty.

General Conference of Weights and Measures (CGPM) was established in the early part of the twentieth century. The CGPM came up with the SI system of units that became the standard all around the world.

Currently, the US is the only country that has not adopted the SI system fully. It instead operates on a dual system of both SI units and the US customary system.

Conversion of Units

Since several different units still exist all around the world, as evident in the US, it becomes a practical necessity to convert them from one unit to the other. Conversion of units may include adding or subtracting a fixed number of multiplying with a conversion factor to obtain the desired results. The specific situation decides the accuracy which is needed in such conversions. For example, a scientific laboratory requires a much higher accuracy as compared to a trading company that only needs to specify the conversion is rounded off numbers. Conversion of units can be done in two ways, soft conversion and hard conversion.

In soft conversion, the accuracy of the original measurement is preserved. This means that the accuracy of the converted unit must be the same as the original. This is done in situations requiring a high degree of accuracies like an experiment or a medical lab.

In hard conversion, the accuracy of the original may not be preserved. This is done when we only need a rough idea of the converted quantity.


A conversion factor is a number that is usually derived by obtaining the ratio of two units when converted into any one of those units. This factor can then be multiplied by any quantity to change its unit. An example is converting celsius to Fahrenheit. To convert, a conversion factor of 9/5 is multiplied by the celsius quantity.

Softwares are one of the premier conversion tools that are available to us. The basic code of conversion can be found in the functions library of any major application that we use, like spreadsheets. The calculator application that comes pre-loaded in our mobile handsets is another such example. As a result, the conversion tool has become one of the most ubiquitous applications that people use today. In fact, the easy availability of such tools sometimes hides the fact that we live in a society that still uses several different systems of units.





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