SI Overview

In order to understand SI, one needs to:

• Know the basic units. These are the meter, kilogram, Celsius and second. One should also be familiar with the litre.
• Be able to use prefixes to describe bigger and smaller amounts.
• Be able to do calculations in SI.
• If one is living the the USA (the only major country not to switch to SI), then one will also need to be able to convert between the USA measurement system and SI.

This page covers the first of these three topics. The fourth is covered on a separate page.

Basic Units

The meter is a measure of length. It is slightly over 39 inches, which is to say that it is slightly over a yard. One can write 'm' instead of 'meters'; for example one could say '8 meters' or '8 m'.

The kilogram is used as a measure of weight. You may hear that it is actually a measure of 'mass' rather than 'weight'; this is a topic for older students (it is discussed in the teacher notes page). On these pages, we will use it as a measure of 'weight'. A kilogram is equal to approximately 2.2 pounds. One can write 'kg' instead of 'kilogram'; for example one could say '8 kilograms' or '8 kg'.

Temperature is measured in degrees Celsius. The freezing point of water is equal to zero degrees in the Celsius system (written as 0°C), which is equal to 32°F. The boiling point of water is equal to 100 degrees in the Celsius system (written as 100°C), which is equal to 212°F. Room temperature is approximately 21°C or 70°F.

The unit of time is the second, which you are already familiar with.

In the USA system, volume can be measured in cups, quarts, gallons, and so on. In the SI, volume is measured in liters. One can write 'l' instead of 'liter'; for example one could say '8 liters' or '8 l'.

In some countries the unit of length is spelled "meter", while in others it is spelled "metre". Likewise the unit of volume may be spelled "liter" or "litre".

Metric Prefixes

In the metric system, the units of measurement (see above) are multiplied by 10 or divided by 10 in order to produce larger or smaller units; this is indicated by using a prefix. For example, a decimeter is a tenth of a meter and a dekameter is 10 meters. For even larger or smaller units, one continues to divide or multiple by 10. For example, one can either say '1000 meters' or one can say 'a kilometer', as kilo means '1000' (which is 10 * 10 *10). The following table lists all of the prefixes.

Many of these prefixs are not in common usage and consequently may not be understood by everyone. For example, most people will say '10 meters' rather than '1 dekameter'. Both are correct, but the former is much more frequently used and understood. Consequently, if you memorise the most common prefixes (micro, milli, centi, kilo, mega) you will probably find that you seldom need to use or remember the other ones.

The fact that all units are multiples or divisions of 10 greatly simplifies calculations and is a major reason that the metric system has been so widely adopted. A simple illustration of this is to first measure a room in yards and inches and calculate the amount of required carpet in square yards, then repeat the measurement in meters and centimeters and calculate the amount of carpet in square meters. The latter calculation is far quicker and simpler.

One can either use a prefix combined with the measurement (e.g. kilometer) or the multiplication symbol combined with the measurement symbol (e.g. km). Combining a symbol with a non-symbol (e.g. kmeter or kilom) is not done.

For a list of some of the common measures and prefixes, click on Table of Metric and Imperial units.

Notable Measures

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