
Enter any whole number up to 13 digits long (one less than 10 million million or 10 trillion in USA and 10 billion in Britain, France and German), press the factorise button and all the factors are listed in the list box bellow the number entered.
The factors of a number are all the prime numbers that when multiplied together equal the number, for example the factors of 21 are 1, 3 and 7 (1 x 3 x 7 = 21). If the same number is listed more than once in the factor list then it is required the number of times it is listed in the calculation to recreate the number, for example the factors of 24 are 1, 2, 2, 2 and 3 (1 x 2 x 2 x 2 x 3 = 24).
Prime numbers are numbers that can only be divided by the number 1 and the number itself and so the only factors of a prime number are 1 and the number itself. If you enter a prime number in the 'number to factorise' box and press the 'factorise' button then the list will show just two entries, 1 and the number you requested to be factorised. Conversely if you enter a number to factorise and the only factors shown are 1 and the number you requested to be factorised then it is a prime number. That is how prime numbers are proved to be prime numbers, you try to find all its factors and if it only has the two (1 and itself) it is a prime number.

