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!



The length of this guitar is
about 1 meter:
1 meter

When unfolded this ruler
measures 2 meters:


This gold bar has a mass
of 1 kilogram:
1 kilogram

A dictionary also has a mass of
about 1 kilogram:


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


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

a very short time of one thousandth of a second is a "millisecond"

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

Here is a quick summary of the special prefixes:

Common Big and Small Numbers

Name The Number Prefix Symbol
trillion 1,000,000,000,000 tera T
billion 1,000,000,000 giga G
million 1,000,000 mega M
thousand 1,000 kilo k
hundred 100 hecto h
ten 10 deka da
unit 1    
tenth 0.1 deci d
hundredth 0.01 centi c
thousandth 0.001 milli m
millionth 0.000 001 micro µ
billionth 0.000 000 001 nano n
trillionth 0.000 000 000 001 pico p


A thousand meters is a kilometer and abbreviated km (k for kilo, m for meter)

A million liters is a megaliter and abbreviated ML

A millionth of a second is a microsecond and abbreviated µs (µ is the greek symbol "mu")

These prefixes are used for computers too! A megabyte (MB) is a million bytes, a gigabyte (GB) is a billion bytes, etc.

How to remember?

For large values (each one a thousand times bigger) say:

"kilo mega giga tera"

And for small values (each one a thousand times smaller) say:

"milli micro nano pico"

Try saying them both a few times, they sort of rhyme and become easy to remember.

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 more simply m/s

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


Square Meter


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

The Unit is meters × meters, which is written m2


Cubic Meter


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

The Unit is meters × meters × meters, which is written m3


A cube that is 1 meter on each side 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 = 11000 m3

Another way of thinking about a liter is:



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


A day is 24 hours so:


Speed in meters per second (m/s)


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

If something is moving 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:

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 is how fast velocity changes.

When a runner accelerates from 5 m/s (5 meter per second) to 6 m/s (6 meters per second) in just one second, they accelerate by 1 meter per second per second!

And yes, "per second" is used twice!
It can be thought of as (m/s)/s but is usually written m/s2



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.

1 Newton = 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.


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:

So the complete list is:

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

All other units (such as speed, force, power and many more) are "derived units" that are defined using these seven base units.