Factors, primes, square roots and multiples
Pick a number to see its factors, check whether it is prime, find a square root or list its multiples.
Factors of 2 to 100
Pick a number to see its factors, factor pairs and prime factors. Numbers in blue and underlined are prime: their only factors are 1 and the number itself.
What factors are
A factor of a number divides it exactly, with nothing left over. The factors of 12 are 1, 2, 3, 4, 6 and 12, because each of them goes into 12 a whole number of times.
Factors come in pairs that multiply to make the number: 1 × 12, 2 × 6 and 3 × 4. Finding one factor gives you its partner for free, which is why the quickest way to find them all is to start at 1 and work up until the pairs repeat.
Is it prime? 1 to 100
A prime number has exactly two factors, 1 and itself. There are 25 primes up to 100, shown in blue and underlined. Pick any number to see the check that proves whether it is prime, including why 1 is not.
Square roots
The square root of a number, times itself, makes the number. The square numbers up to 144 have whole number roots; 2, 3, 5 and 8 do not, and their pages show why.
Multiples of 2 to 20
The multiples of a number are its times table: the multiples of 4 are 4, 8, 12, 16 and so on. Each page lists the first 12, shades every one up to 100 on a hundred square and gives a quick way to spot one.
Questions parents ask
What is a factor?
A factor of a number divides it exactly, with nothing left over. 4 is a factor of 12 because 12 ÷ 4 = 3.
What is a factor pair?
Two numbers that multiply to make the number. 3 × 4 = 12, so 3 and 4 are a factor pair of 12.
What is a prime number?
A number with exactly two factors: 1 and itself. 7 is prime because only 1 × 7 makes 7. 1 is not prime, because it has only one factor.
What is a multiple?
A number in a times table. The multiples of 4 are 4, 8, 12, 16 and so on. 12 is a multiple of 4, and 4 is a factor of 12.
What is a square root?
The number you multiply by itself to make another number. 5 × 5 = 25, so the square root of 25 is 5. It is written √25.
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.