# 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: When unfolded this ruler measures 2 meters:

### Kilogram

 This gold bar has a mass of 1 kilogram. 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"

Example: 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.

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

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

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

These prefixes are also used for computers! A megabyte is a million bytes, a gigabyte is a billion bytes, etc.

How to remember?

Remember this for large values (each one a thousand times bigger):

"kilo mega giga tera"

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

"milli micro nano pico"

## 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

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

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)

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