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Metric System of Measurement

(Correctly called "SI")

The metric system is a system of measuring based on:

the meter for length

the kilogram for mass

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 "kilometer"

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

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	10¹	10²	10³	10⁶	10⁹	10¹²	10¹⁵	10¹⁸	10²¹	10²⁴

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 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 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 bit more complicated, but a kilometer has 1,000 meters, and an hour has 3,600 seconds, so a kilometer per hour is:

1000 / 3600 = 3.6 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.

And a Newton is actually 1 kg · m / s² (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/s².

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 "Système International").

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