Square Root of 21 Here we will define, analyze, simplify, and calculate the square root of 21. We start off with the definition and then answer some common questions about the square root of 21. Then, we will show you different ways of calculating the square root of 21 with and without a computer or calculator. We have a lot of information to share, so let's get started!

Square root of 21 definition
The square root of 21 in mathematical form is written with the radical sign like this √21. We call this the square root of 21 in radical form. The square root of 21 is a quantity (q) that when multiplied by itself will equal 21.

21 = q × q = q2

Is 21 a perfect square?
21 is a perfect square if the square root of 21 equals a whole number. As we have calculated further down on this page, the square root of 21 is not a whole number.

21 is not a perfect square.

Is the square root of 21 rational or irrational?
The square root of 21 is a rational number if 21 is a perfect square. It is an irrational number if it is not a perfect square. Since 21 is not a perfect square, it is an irrational number. This means that the answer to "the square root of 21?" will have an infinite number of decimals. The decimals will not terminate and you cannot make it into an exact fraction.

21 is an irrational number

Can the square root of 21 be simplified?
You can simplify 21 if you can make 21 inside the radical smaller. We call this process "to simplify a surd". The square root of 21 cannot be simplified.

How to calculate the square root of 21 with a calculator
The easiest and most boring way to calculate the square root of 21 is to use your calculator! Simply type in 21 followed by √x to get the answer. We did that with our calculator and got the following answer with 9 decimal numbers:

21 ≈ 4.582575695

How to calculate the square root of 21 with a computer
If you are using a computer that has Excel or Numbers, then you can enter SQRT(21) in a cell to get the square root of 21. Below is the result we got with 13 decimals. We call this the square root of 21 in decimal form.

SQRT(21) ≈ 4.5825756949558

What is the square root of 21 rounded?
The square root of 21 rounded to the nearest tenth, means that you want one digit after the decimal point. The square root of 21 rounded to the nearest hundredth, means that you want two digits after the decimal point. The square root of 21 rounded to the nearest thousandth, means that you want three digits after the decimal point.

10th: √21 ≈ 4.6

100th: √21 ≈ 4.58

1000th: √21 ≈ 4.583

What is the square root of 21 as a fraction?
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
≈ 4.58/1
≈ 458/100
≈ 4 29/50

What is the square root of 21 written with an exponent?
All square roots can be converted to a number (base) with a fractional exponent. The square root of 21 is no exception. Here is the rule and the answer to "the square root of 21 converted to a base with an exponent?":

b = b½

21 = 21½

How to find the square root of 21 by long division method
Here we will show you how to calculate the square root of 21 using the long division method with one decimal place accuracy. This is the lost art of how they calculated the square root of 21 by hand before modern technology was 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 is on top. The square root of 21 with one digit decimal accuracy is 4.5.

Square Root of a Number
Please enter another number in the box below to get the square root of the number and other detailed information like you got for 21 on this page.

Notes
Remember that negative times negative equals positive. Thus, the square root of 21 does not only have the positive answer that we have explained above, but also the negative counterpart.

We often refer to perfect square roots on this page. You may want to use the list of perfect squares for reference.

Square Root of 22
Here is the next number on our list that we have equally detailed square root information about.