(Correctly called "")

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:

When unfolded this ruler measures 2 meters:

This gold bar has a mass of 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 one"

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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

Examples:

something that's 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's a quick summary of the special prefixes:

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  

Examples:

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, and so on.

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"

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:

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

The Unit is meters × meters, which is written m2

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

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

A cube that's 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:

A box that's 0.1 meters (10 cm) on each side,

One square meter that's millimeter thick

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

60 × 60 = 3,600 seconds

A day is 24 hours so:

1 day = 24 × 60 × 60 = 86,400 second

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.

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 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.

And 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:

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  
           
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.

2204, 2205, 980, 981, 982, 983, 984, 985, 2206, 3609

Copyright © 2026 Rod Pierce