Squares and Square Roots
First learn about Squares, then Square Roots are easy.
How to Square A Number
To square a number: multiply it by itself.
Example: What is 3 squared?
3 Squared  =  = 3 × 3 = 9 
"Squared" is often written as a little 2 like this:
This says "4 Squared equals 16"
(the little 2 says
the number appears twice in multiplying)
Squares From 0^{2} to 6^{2}
0 Squared  =  0^{2}  =  0 × 0  =  0 
1 Squared  =  1^{2}  =  1 × 1  =  1 
2 Squared  =  2^{2}  =  2 × 2  =  4 
3 Squared  =  3^{2}  =  3 × 3  =  9 
4 Squared  =  4^{2}  =  4 × 4  =  16 
5 Squared  =  5^{2}  =  5 × 5  =  25 
6 Squared  =  6^{2}  =  6 × 6  =  36 
The squares are also on the Multiplication Table: 
Negative Numbers
We can also square negative numbers.
Example: What happens when we square (−5) ?
Answer:
(−5) × (−5) = 25
(because a negative times a negative gives a positive)
That was interesting!
When we square a negative number we get a positive result.
Just the same as squaring a positive number:
(For more detail read Squares and Square Roots in Algebra)
Square Roots
A square root goes the other way:
3 squared is 9, so a square root of 9 is 3
A square root of a number is ...
A square root of 9 is ...
It is like asking:
What can we multiply by itself to get this?
To help you remember think of the root of a tree: "I know the tree, but what root made it?" In this case the tree is "9", and the root is "3". 
Here are some more squares and square roots:
4 
16 

5 
25 

6 
36 
Decimal Numbers
It also works for decimal numbers.
Try the sliders below. Note: the numbers here are only shown to 2 decimal places.
Using the sliders (remembering it is only accurate to 2 decimal places):
 What is the square root of 8?
 What is the square root of 9?
 What is the square root of 10?
 What is 1 squared?
 What is 1.1 squared?
 What is 2.6 squared?
Negatives
We discovered earlier that we can square negative numbers:
Example: (−3) squared
(−3) × (−3) = 9
And of course 3 × 3 = 9 also.
So the square root of 9 could be −3 or +3
Example: What are the square roots of 25?
(−5) × (−5) = 25
5 × 5 = 25
So the square roots of 25 are −5 and +5
The Square Root Symbol
This is the special symbol that means "square root",
it is sort of like a tick, and actually started hundreds of years ago as a dot with a flick upwards. It is called the radical, and always makes mathematics look important! 
We use it like this:
and we say "square root of 9 equals 3"
Example: What is √25?
25 = 5 × 5, in other words when we multiply 5 by itself (5 × 5) we get 25
So the answer is:
√25 = 5
But wait a minute! Can't the square root also be −5? Because (−5) × (−5) = 25 too.
 Well the square root of 25 could be −5 or +5.
 But when we use the radical symbol √ we only give the positive (or zero) result.
Example: What is √36 ?
Answer: 6 × 6 = 36, so √36 = 6
Perfect Squares
The Perfect Squares (also called "Square Numbers") are the squares of the integers:
Perfect Squares 

0  0 
1  1 
2  4 
3  9 
4  16 
5  25 
6  36 
7  49 
8  64 
9  81 
10  100 
11  121 
12  144 
13  169 
14  196 
15  225 
etc... 
Try to remember them up to 12.
Calculating Square Roots
It is easy to work out the square root of a perfect square, but it is really hard to work out other square roots.
Example: what is √10?
Well, 3 × 3 = 9 and 4 × 4 = 16, so we can guess the answer is between 3 and 4.
 Let's try 3.5: 3.5 × 3.5 = 12.25
 Let's try 3.2: 3.2 × 3.2 = 10.24
 Let's try 3.1: 3.1 × 3.1 = 9.61
 ...
Getting closer to 10, but it will take a long time to get a good answer!
At this point, I get out my calculator and it says: 3.1622776601683793319988935444327 But the digits just go on and on, without any pattern. So even the calculator's answer is only an approximation ! 
Note: numbers like that are called Irrational Numbers, if you want to know more.
The Easiest Way to Calculate a Square Root
Use your calculator's square root button! 
And also use your common sense to make sure you have the right answer.
A Fun Way to Calculate a Square Root
There is a fun method for calculating a square root that gets more and more accurate each time around:
a) start with a guess (let's guess 4 is the square root of 10)  
b) divide by the guess (10/4 = 2.5) c) add that to the guess (4 + 2.5 = 6.5) d) then divide that result by 2, in other words halve it. (6.5/2 = 3.25) e) now, set that as the new guess, and start at b) again 
 Our first attempt got us from 4 to 3.25
 Going again (b to e) gets us: 3.163
 Going again (b to e) gets us: 3.1623
And so, after 3 times around the answer is 3.1623, which is pretty good, because:
3.1623 x 3.1623 = 10.00014
Now ... why don't you try calculating the square root of 2 this way?
How to Guess
What if we have to guess the square root for a difficult number such as "82,163" ... ?
In that case we could think "82,163" has 5 digits, so the square root might have 3 digits (100x100=10,000), and the square root of 8 (the first digit) is about 3 (3x3=9), so 300 is a good start.
Square Root Day
The 4th of April 2016 is a Square Root Day, because the date looks like 4/4/16
The next after that is the 5th of May 2025 (5/5/25)