# Square Root of 21

In this article we're going to calculate the square root of 21 and explore what the square root is and answer some of the common questions you might. We'll also look at the different methods for calculating the square root of 21 (both with and without a computer/calculator).

## Square Root of 21 Definition

In mathematical form we can show the square root of 21 using the radical sign, like this: √21. This is usually referred to as the square root of 21 in radical form.

So what is the square root? In this case, the square root of 21 is the quantity (which we will call q) that when multiplied by itself, will equal 21.

√21 = q × q = q2

## Is 21 a Perfect Square?

In math, we refer to 21 being a perfect square if the square root of 21 is a whole number.

In this case, as we will see in the calculations below, we can see that 21 is not a perfect square.

To find out more about perfect squares, you can read about them and look at a list of 1000 of them in our What is a Perfect Square? article.

## Is The Square Root of 21 Rational or Irrational?

A common question is to ask whether the square root of 21 is rational or irrational. Rational numbers can be written as a fraction and irrational numbers cannot.

A quick way to check this is to see if 21 is a perfect square. If it is, then it is a rational number. If it's not a perfect square then it's an irrational number.

We already know if 21 is a perfect square so we also can see that √21 is an irrational number.

## Can the Square Root of 21 Be Simplified?

21 can be simplified only if you can make 21 inside the radical symbol smaller. This is a process that is called simplifying the surd. In this example square root of 21 cannot be simplified.

√21 is already in its simplest radical form.

## How to Calculate The Square Root of 21 with a Calculator

If you have a calculator then the simplest way to calculate the square root of 21 is to use that calculator. On most calculators you can do this by typing in 21 and then pressing the √x key. You should get the following result:

√21 ≈ 4.5826

## How to Calculate the Square Root of 21 with a Computer

On a computer you can also calculate the square root of 21 using Excel, Numbers, or Google Sheets and the SQRT function, like so:

SQRT(21) ≈ 4.582575694956

## What is the Square Root of 21 Rounded?

Sometimes you might need to round the square root of 21 down to a certain number of decimal places. Here are the solutions to that, if needed.

10th: √21 ≈ 4.6

100th: √21 ≈ 4.58

1000th: √21 ≈ 4.583

## What is the Square Root of 21 as a Fraction?

We covered earlier in this article that only a rational number can be written as a fraction, and irrational numbers cannot.

Like we said above, since the square root of 21 is an irrational number, we cannot make it into an exact fraction. However, we can make it into an approximate fraction using the square root of 21 rounded to the nearest hundredth.

√21

≈ 4.6/1

≈ 458/100

≈ 4 29/50

## What is the Square Root of 21 Written with an Exponent?

All square root calculations can be converted to a number (called the base) with a fractional exponent. Let's see how to do that with the square root of 21:

√b = b½

√21 = 21½

## How to Find the Square Root of 21 Using Long Division

Finally, we can use the long division method to calculate the square root of 21. This is very useful for long division test problems and was how mathematicians would calculate the square root of a number before calculators and computers were invented.

### Step 1

Set up 21 in pairs of two digits from right to left and attach one set of 00 because we want one decimal:

21
00

### Step 2

Starting with the first set: the largest perfect square less than or equal to 21 is 16, and the square root of 16 is 4 . Therefore, put 4 on top and 16 at the bottom like this:

 4 21 00 16

### Step 3

Calculate 21 minus 16 and put the difference below. Then move down the next set of numbers.

 4 21 00 16 5 00

### Step 4

Double the number in green on top: 4 × 2 = 8. Then, use 8 and the bottom number to make this problem:

8? × ? ≤ 500

The question marks are "blank" and the same "blank". With trial and error, we found the largest number "blank" can be is 5.

Now, enter 5 on top:

 4 5 21 00 16 5 00

That's it! The answer shown at the top in green. The square root of 21 with one digit decimal accuracy is 4.6. Notice that the last two steps actually repeat the previous two. To add decimal places to your answe you can simply add more sets of 00 and repeat the last two steps.