The Metric System (2024)

In the United States, measurements are generally provided in English units. For example, when driving to a nearby town to see a friend or family member, we may say “it’s 20 miles from here.” In many parts of the world, a person referring to the same distance may say “it’s 32 kilometers from here.” People who use the term “kilometer” to describe distance are employing the metric system.

The metric system, or more accurately the International System of Units (SI), is an international system of measurement and the unit system most often used in STEM (science, technology, engineering, and math). Scientists around the world use and understand the SI. (In writing, “S” appears before “I” because in French the “International System of Units” is written “Systѐme International d’Unités” [Bryant 1976]).

Unlike the English system of units that uses different names and unit sizes for volumes (e.g., gallons, pints, quarts), weights (e.g., pounds, stones, tons, short tons, long tons), and distances (e.g., feet, yards, miles), the SI uses the same base units and the same prefixes for all types of measurements.

Prefixes are provided in scales that function around 7 units known as metric system (or SI) base units. As shown in Table 1, base units include the meter (m), the kilogram (kg), the kelvin (K), the second (s), the ampere (A), the candela (cd), and the mole (mol).

When you report data in metric (SI) or non-metric (non-SI) units, it is often necessary to use a prefix so numbers are not extremely large or extremely small when written. As examples of how scientists use units in their work, consider the following:

1. Most bacteria are 0.0000002 to 0.00001 meters in length (Carter n.d.). Instead of expressing the length as 0.0000002 meters, we could write 2 x 10^–7meters (in scientific notation) or 0.2 µm (pronounced “micrometers”).Note:Although “micron,” which denoted the micrometer, was part of the scientific vernacular for many years, the term is no longer accepted in academic science circles.The mathematical transition is, as follows, based on the SI prefixes provided in Table 3:

2. NASA (National Aeronautics and Space Administration) and NOAA (National Oceanic and Atmospheric Administration) often report the units of carbon flowing into and out of the atmosphere, the terrestrial biosphere, and the oceans, and carbon stocks in units of gigatons (a non-SI unit with a metric prefix) (Riebeek 2011). In NASA’s and NOAA’s carbon maps, the largest carbon reservoir on Earth is located in the deep ocean (below the level of photosynthesis and respiration). NASA and NOAA assign a value of 37,000 gigatons to this carbon reservoir. In tons this is:

Note that this is an example of a non-metric (or non-SI) unit used with the metric prefixes shown in Table 3. Many of the non-SI units shown in Table 2 are often used in this way—especially in earth science, physical science, and engineering.