Factors of 21
Every whole number that divides 21 exactly, the factor pairs, and how to find them yourself.
The answer
The factors of 21 are 1, 3, 7 and 21.
Factor pairs of 21
| Pair | Makes |
|---|---|
| 1 × 21 | 21 |
| 3 × 7 | 21 |
Prime factors of 21
- Prime factorisation
- 21 = 3 × 7
- Is 21 prime?
- No. It has 4 factors, not just 1 and 21.
- Is 21 a square number?
- No. 4 × 4 = 16 and 5 × 5 = 25, so 21 falls between two square numbers.
How to find the factors of 21
21 is odd, so every even number you try will fail. Its digits add up to 3, a multiple of 3, so 3 is a factor. Work up from 1 to 4, and write down both numbers in every pair you find.
- 21 ÷ 1 = 21, so 1 × 21 is a pair.
- 2 does not go into 21 exactly.
- 21 ÷ 3 = 7, so 3 × 7 is a pair.
- 4 does not go into 21 exactly.
Stop at 5: 5 × 5 = 25, which is more than 21. The next pair you would find is 7 × 3, which is 3 × 7 turned round, so you already have every pair.
21 in the times tables
3 × 7 = 21. Practise the 3 times table and 7 times table.
More numbers
Check whether 21 is prime.
Questions parents ask
What is the largest factor of 21, apart from 21 itself?
7. Divide 21 by its smallest factor above 1, which is 3: 21 ÷ 3 = 7.
What do the factors of 21 add up to?
32. Leaving out 21 itself, they add up to 11, which is less than 21, so 21 is called a deficient number.
Which number after 21 has the same number of factors?
22. Like 21, it has exactly 4 factors.
Continue your number practice
Explore the diagnostic for another set of practice questions.