Is 23 a prime number?
23 is a prime number. Here is how to check it, one division at a time.
The answer
Yes. 23 is a prime number: its only factors are 1 and 23.
How to check that 23 is prime
Try dividing 23 by the prime numbers, starting with 2. You only need primes: if 4 went into 23, then 2 would too.
- 23 ÷ 2 = 11 remainder 1, so 2 is not a factor.
- 23 ÷ 3 = 7 remainder 2, so 3 is not a factor.
Stop at 5: 5 × 5 = 25, which is more than 23. Any factor bigger than that would need a partner smaller than 5, and you have already tried those. So 23 is prime.
Where 23 comes among the primes
23 is the 9th prime number. The prime just before it is 19.
There are 25 prime numbers up to 100: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89 and 97.
Around 23
- 23 is not in any times table from 2 to 12. The answers either side of it are 22 (2 × 11 and 11 × 2) and 24 (2 × 12, 3 × 8, 4 × 6, 6 × 4, 8 × 3 and 12 × 2).
- The nearest square number is 25 (5 × 5), 2 above it.
- Primes 4 apart are called cousin primes, and 23 has one cousin: 19.
23 at a glance
- In words
- twenty-three
- In Roman numerals
- XXIII
- Odd or even
- Odd: it ends in 3
- Its factors add up to
- 24, or 1 without 23 itself, which makes 23 a deficient number
Related pages
More numbers
Questions parents ask
Is 23 one of a pair of twin primes?
No. Twin primes are two primes 2 apart, such as 11 and 13. Neither 21 nor 25 is prime, so 23 has no twin.
What is the next prime number after 23?
29, which is 6 more than 23. Every number in between has a factor other than 1 and itself.
What is 23 × 23?
23 × 23 = 529. The square of a prime has exactly three factors: 1, 23 and 529.
Number facts are the base of 11+ maths
Factors, multiples, primes and square numbers come up all over 11+ maths, from simplifying fractions to ratio. The free diagnostic checks maths, English and both kinds of reasoning, and shows what to practise next. No card is required.