The International System of Units

The International System of Units (SI), a globally recognized system of measurement, is the standard for measurements worldwide, used in almost every country except for the United States, Liberia, and Burma. Established in 1960, it has since become the standard for measurements worldwide.

Fundamental Physical Quantities

The International System recognizes seven fundamental physical quantities:

  1. Length
  2. Mass
  3. Time
  4. Temperature
  5. Electric Current Intensity
  6. Luminous Intensity
  7. Quantity of Substance

Measuring of System Units and Symbols

We assign symbols to represent each of the seven fundamental physical quantities and their corresponding units of measurement. When expressing the numerical value of a measured quantity, we write its symbol after the numerical value.

Why Only Seven Quantities?

One may wonder why the International System only recognizes seven quantities when many others can be measured in nature. Arithmetic operations allow us to derive other quantities from the seven fundamental physical quantities. For example, we can calculate the speed as a derived quantity by dividing the distance traveled by the time taken, resulting in a unit of (m/s).

Reference Values in the System of Units of Measurement

International bodies establish the reference values for each unit of measurement. These values are the basis for determining the multiples and submultiples of the units.

Multiples and Submultiples of System of Units

Multiples of the reference units of measurement are obtained by multiplying by 10, 100, 1000, etc. Submultiples are obtained by dividing by 10, 100, 1000, etc. These multiples and submultiples are convenient when measuring quantities that are very large or very small in relation to the unit of measurement.

Prefixes in the International System of Units

A prefix, a group of letters preceding the name of the unit of measurement, indicates each multiple or submultiple in the International System. For instance, the International System recognizes a centigram as the hundredth of a gram. The prefix is accompanied by a symbol that matches it.

Non-Decimal Units of Measurement

The International System also recognizes non-decimal units of measurement. For example, the second, the SI unit of measurement for time, has non-decimal multiples that are allowed to be used in the system. Similarly, angles are measured in degrees based on a non-decimal system. One degree is equal to 60 minutes and 60 seconds.


The International System of Units provides a standardized system of measurement used globally. It recognizes seven fundamental physical quantities and assigns symbols to the quantities and their corresponding units of measurement. Internationally established reference values define the units and prefixes indicate their multiples and submultiples. The system also recognizes non-decimal units of measurement, such as the second and degrees.


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