A prime number is a whole number greater than 1 with exactly two factors: 1 and itself. The prime numbers up to 100 are 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. There are 25 of them. 1 is not prime, and 2 is the only even prime.
Primes are the building blocks of every whole number: any number above 1 is either prime or can be written as a product of primes. Children meet primes in Years 5 and 6, and prime factors, HCF and LCM are core GCSE skills.
The highlighted numbers are prime.
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 |
| 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 |
| 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 |
| 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 |
| 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 59 | 60 |
| 61 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 |
| 71 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 |
| 81 | 82 | 83 | 84 | 85 | 86 | 87 | 88 | 89 | 90 |
| 91 | 92 | 93 | 94 | 95 | 96 | 97 | 98 | 99 | 100 |
Every other number from 4 to 100 is composite: it has more than two factors. 1 is neither prime nor composite, because it has only one factor.
Every whole number greater than 1 can be written as a product of prime numbers in exactly one way. Split the number into any factor pair, then keep splitting until every branch ends in a prime. This is a factor tree.
Write both numbers as products of primes. The highest common factor (HCF) uses the primes they share, each to the lower power. The lowest common multiple (LCM) uses every prime that appears, each to the higher power.
Calling 1 a prime number. It has only one factor, so it is not prime.
Forgetting that 2 is prime because it is even. It is the only even prime.
Assuming every odd number is prime. 9, 15, 21, 27, 51, 57 and 91 are all odd but composite.
Leaving a composite number in a factor tree, such as 60 = 2 × 5 × 6. Keep splitting until every factor is prime.
Use the free HCF and LCM calculator to see a factor tree, the product of primes, a Venn diagram and the full method for any two to four numbers.
Untimed practice with worked answers. Every answer is a whole number. This extra activity does not count towards your daily progress.
MathsDaily sets ten fresh questions a day across the whole curriculum, with new numbers every morning. Primes, factors and multiples come up regularly in the Year 5 to Year 8 and GCSE sets, with a tip and a worked solution if you get stuck.
Start Year 6 MathsDaily →No. A prime number has exactly two factors, 1 and itself. 1 has only one factor, so it is not prime. It is not composite either.
Yes. 2 has exactly two factors, 1 and 2. It is the smallest prime and the only even prime: every other even number has 2 as a factor.
There are 25: 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.
No. Prime numbers must be greater than 1, and 0 has infinitely many factors.
A prime factor is a factor of a number that is also prime. The prime factors of 60 are 2, 3 and 5, because 60 = 2² × 3 × 5.
Free A4 PDFs with worked answers. See every printable, Year 1 to A-level.