Here is an online calculator — enter a number and press "Factor" to compute its prime factors:
This page, and the javascript that drives it, is derived fromJavaScript Prime Factor CalculatorCalculator Notes:
Notice about the calculator's output that two adjacent numbers imply multiplication — the list of factors for the default example is "25 3 52 41659," which if fully spelled out would be "2 × 2 × 2 × 2 × 2 × 3 × 5 × 5 × 41659". The use of exponential notation (22 = 2 × 2) is just a way to shorten the display of repetitive factors. To see the point of this, enter 4503599627370496 as a number to factor above. The result (252) means "two multiplied by itself 51 times."
Factoring takes the least amount of time for composite numbers, and primes require the most computation time (remember this for a later discussion). If you type in a difficult number (a number that is prime and/or has a lot of digits) and if your computer is not fast, after a few seconds most browsers will complain and ask whether you want to stop the calculation.
The worst-case prime for this calculator is 9007199254740881, the largest prime below 253-1 (the upper numerical limit for this calculator). On some browsers (but not all) this value will produce a long computation and may cause your browser to complain — but the calculator will eventually identify it as prime. Because JavaScript is a relatively slow interpreted language, on a modern desktop and depending on the selected browser, this particular computation might require as much as 45 seconds (a C version of this program requires only 1/3 second).