Metric System of Measurement

(Correctly called "SI")

The metric system is a system of measuring. It has three main units:

m   the meter for length
kg   the kilogram for mass
s   the second for time
With those three simple measurements
we can measure nearly everything in the world!

Examples:

Meter

The length of this guitar
is about 1 meter:
1 meter
  When unfolded this ruler
measures 2 meters:
ruler
     

Kilogram

1 kilogram

This gold bar has a
mass of 1 kilogram.

 

 

Dictionary
A dictionary also has a
mass of about 1 kilogram.

Second

1 second is about as long as it takes to say "one thousand and one"

Larger or Smaller

But what if we want to talk about really big or really small things?

Answer: we can use Metric Number Prefixes

  • like "kilo" (a thousand)
  • and "milli" (one thousandth)
  • and so on

Example: something that is 1,000 meters is a "kilometer"

Something that is one thousandth of a second is a "millisecond"

In fact the kilogram already uses this method, as it's a thousand grams, a kilogram.

So one thousandth (1/1000) of a kilogram is simply a "gram"

Here is a quick summary of the special prefixes:

Large Numbers

Name deca hecto kilo mega giga tera peta exa zetta yotta
Symbol da h k M G T P E Z Y
Factor 101 102 103 106 109 1012 1015 1018 1021 1024

Example: A million liters is called a megaliter and abbreviated ML

Small Numbers

Name deci centi milli micro nano pico femto atto zepto yocto
Symbol d c m µ n p f a z y
Factor 10-1 10-2 10-3 10-6 10-9 10-12 10-15 10-18 10-21 10-24

Example: A thousandth of a meter is called a millimeter and abbreviated mm

Making Other Units

We can also combine the meter, kilogram and second to make new Units of Measurement!

Example: Speed

Speed is how far something moves over a period of time

So it can be measured in meters per second

It means: How many meters does something travel in one second

We can write it as meters/second, or simply m/s

Here are a few common units that are based on the meter, kilogram and second:

Area

Square Meter

square

Area is length by length, so the basic unit of area is a square that is 1 meter on each side.

The Unit is meters × meters, which is written m2 (square meters).

Volume:

Cubic Meter

cube

Volume is length by length by length, so the basic unit of volume is a cube that is 1 meter on each side.

The Unit is meters × meters × meters, which is written
m3 (cubic meters).

Liter

So, a cube that is 1 meter on each side is a cubic meter (m3) ...

... and that is also equal to 1,000 liters.

1 m3 = 1,000 Liters

Liter is abbreviated L (some people use lowercase l, but that looks too much like 1).

So a liter is actually one-thousandth of a cubic meter.

1 Liter = 1/1000 m3

Another way of thinking about a liter is:

  • A box that is 0.1 meters (10 cm) on each side,
  • One square meter that is millimeter thick.

Time

Hour

An hour is 60 minutes, and a minute is 60 seconds, so an hour is:

  • 60 × 60 = 3,600 seconds

Day

A day is 24 hours so:

  • 1 day = 24 × 60 × 60 = 86,400 second

Speed

Speed in meters per second (m/s)

square

This is a combination of two units (meters and seconds) to make a new one (m/s).

If something is traveling at 1 m/s it moves 1 meter every second.

Speed in kilometers per hour (km/h)

A kilometer has 1000 meters, and an hour has 3600 seconds, so a kilometer per hour is:

  • 1000 / 3600 = 1/3.6 = 0.277... m/s

How did I know to make it 1000/3600, and not 3600/1000 (the other way around)? Read how to Safely Convert From One Unit to Another.

Acceleration

Acceleration is how fast Speed changes.

If something accelerated from a Speed of 5 m/s (5 meter per second) to 6 m/s (6 meters per second) in just one second, it has accelerated by 1 meter per second per second!

That is two lots of "per second" and is written m/s2:

Force

Force is usually measured in the Unit of Newtons, an important measurement in Physics and Engineering.

A Newton is how much force it takes to make 1 kg accelerate at 1 m/s2.

Which is written as 1 kg · m / s2 (one kilogram-meter per second-squared).

So force is actually based on the meter, kilogram and second.

So force is a combination of the three basic units.

SI

The Metric System had its beginnings back in 1670 by a mathematician called Gabriel Mouton.

The modern version, (since 1960) is correctly called "International System of Units" or "SI" (from the French "Système International").

So we should really call it "SI", but mostly people just call it "Metric".

A few special units are also needed to complete the SI System:

  • ampere for electric current,
  • kelvin for temperature,
  • mole for the amount of substance, and
  • candela for luminous intensity

So the complete list is:

Quantity Name Symbol
Length meter m
Mass kilogram kg
Time second s
     
Electrical Current ampere A
Temperature kelvin K
Amount of substance mole mol
Luminous intensity candela cd