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21 in words

21

twenty-one

21 is written twenty-one.

Ways to write it

As an ordinal: twenty-first
Roman numeral: XXI
Expanded form: 20 + 1

Place value

Place	Digit	Value
tens	2	20
ones	1	1

Number facts

Even or odd: Odd
Prime or composite: Composite
Factors: 1, 3, 7 and 21 (4 factors)
First 10 multiples: 21, 42, 63, 84, 105, 126, 147, 168, 189, 210
Square: 21 × 21 = 441
Square root: ≈ 4.58
Sum of the digits: 2 + 1 = 3
Rounded to the nearest 10: 20
Number before: 20
Number after: 22
In binary: 10101

Quick check

What number comes just after 21?
Show the answer
22
Is 21 even or odd?
Show the answer
Odd
What is 21 + 10?
Show the answer
31

Practice with a game

Count with Me

Count pictures up to 5, 10 or 20 and compare groups.

Place Value Quest

Ones, tens, hundreds and thousands.

Times Tables Challenge

Practice any times table from 2× to 12×.

Frequently asked questions

How do you write 21 in words?

21 is written "twenty-one".

Is 21 a prime number?

No. 21 is a composite number, because it has more than two factors.

What are the factors of 21?

The factors of 21 are 1, 3, 7 and 21.

How do you write 21 in Roman numerals?

21 in Roman numerals is XXI.