Metric System of Measurement
(Correctly called "SI ")

The metric system is a system of measuring based on:

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!

How Big are They?

The length of this guitar is about 1 meter

A dictionary weighs about 1 kilogram .

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

Larger or Smaller
Those measurements are useful for things in everyday life ...

... but how do we talk about really big and really small things?

We use Metric Number Prefixes like "kilo" (a thousand) and "milli" (one thousandth) and add them in front of the measurement.

So something that is 1,000 meters is a "kilo meter"

Something that is one thousandth of a meter is a "milli meter"

In fact the kilogram already uses this method, because it is a thousand grams. So one thousandth 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
10^{1}
10^{2}
10^{3}
10^{6}
10^{9}
10^{12}
10^{15}
10^{18}
10^{21}
10^{24}

Example A million liters would be 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 second would be called a millisecond and abbreviated ms

Making Other Units
The meter, kilogram and second can be used in combination with each other .

This will make different Units of Measurement to mean other amounts, such as area, volume, energy, pressure, and velocity.

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

Area
Square Meter
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 m^{2} (square meters).

Volume
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 .

The Unit is written m^{3} (cubic meters).

Liter
So, a cube that is 1 meter on each side is a cubic meter (m^{3} ) ...

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

1 m^{3} = 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} m^{3}

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:

Day
A day is 24 hours so:

1 day = 24 × 60 × 60 = 86,400 second
Speed
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 traveling at 1 m/s it moves 1 meter every second .

Speed in kilometers per hour (km/h)
A bit more complicated, but a kilometer has 1,000 meters, and an hour has 3,600 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
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/s^{2} :

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

And a Newton is actually 1 kg · m / s^{2} (one kilogram-meter per second-squared). One way of looking at this is how much force it takes to make 1 kg accelerate at 1 m/s^{2} .

But even if you don't fully understand this, it shows you that force is a combination of the three basic units .

Language: Metre vs Meter
Metre is the English spelling, Meter is the American spelling.

Also litre is an English spelling, and liter is the American spelling.

SI
The original Metric System was first developed in France back in 1670.

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

So you 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
metre
m
Mass
kilogram
kg
Time
second
s
Electrical Current
ampere
A
Temperature
kelvin
K
Amount of substance
mole
mol
Luminous intensity
candela
cd