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Fundamental Units of Measurement

by Ron Kurtus

You measure things by defining a standard unit and then stating the measurement in terms of multiples of that unit. A fundamental unit of measurement is a defined unit that cannot be described as a function of other units.

The International System of Units (SI) defines seven fundamental units of measurement. They can be applied to the various Physical Science areas of study. However, there are some questions about the definitions.

Distance, time and mass are the fundamental units.

This lesson will answer those questions. Useful tool: Units Conversion

Measuring in units

In order to measure something, you need to define a unit of measurement. "Unit" refers to 1. In this way, all measurements are multiples of that unit. For example, the unit of mass is the kilogram. Thus, the measurement of mass is in multiples—or fractions—of 1 kilogram.

Originally, the English unit foot was the length of the King's foot. Thus, a distance of 25 feet was 25 time the foot unit of measurement.

Unfortunately, each King had a different sized foot, so that brought about some confusion. Finally, they agreed on a standard length for a foot that would not vary.

The units of measurement are defined as standard and do not vary.

SI base units

The International System of Units (SI) defined seven basic units of measure from which all other SI units are derived.

These SI base units or commonly called metric units are:

Measure Unit Symbol Area of Science
Time Second s All
Length or distance Meter or Metre m All
Mass Kilogram kg Physics
Electric Current Ampere A Physics
Temperature Kelvin K Physics
Luminous Intensity Candela cd Optics
Amount of Substance Mole mol Chemistry

Although these SI base quantities are supposed to be a set of mutually independent dimensions, some may well be interdependent.

Problem areas

There are some variations or problem areas in these fundamental units.


Although multiples or fractions of a meter are useful in most sciences, the unit is impractical in Astronomy. Instead, the fundamental unit of length in Astronomy is the light-year, which is the distance light travels in kilometers in one year.


It would seem more intuitive to define the fundamental of mass as a gram. However, the SI decision was to say that 1000 grams or a kilogram was fundamental.

Electric current

Since electric current is man-made and depends on a number of factors, the ampere does not seem appropriate as a fundamental unit. The ampere is defined as:

"The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in a vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per meter of length."

That does not seem too fundamental.


Units of measurement that are considered fundamental, such that they cannot be described as a function of other units. They often related specifically to the various sciences. However, there are some problems with the definitions.

Look to the source of things



Physical Science Resources

Units of measurement - Wikipedia

General Tables of Units of Measurement - National Institute of Standards and Technology

Units of Measurement and SI Base Units - Includes conversions

Fundamental unit - Wikipedia

Base Units of the International System (SI) - University of North Carolina at Chapel Hill

SI base unit - Wikipedia


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