A Regular Old PC Found the Largest Known Prime Number—Here’s How by Valentina Zarya @FortuneMagazine January 22, 2016, 3:58 PM EST E-mail Tweet Facebook Linkedin Share icons Earlier this month, the record for the largest prime number ever found was broken—by a number with 22 million digits. The number was found through a project called the Great Internet Mersenne Prime Search (GIMPS), in which volunteers use specially designed software to sort through potential candidates. What makes a number a prime is that it can only be divided by 1 and itself. For example, the number 5 can only be divided by the numbers 1 and 5. Same goes for 17, 23, 59, etc. The software finds prime numbers by multiplying different numbers of twos and then subtracting one. To find this record-breaking digit, the software multiplied together 74,207,281 twos, according to a GIMPS press release. The result is a number that has 5 million more digits than the previous record-holding prime, which only multiplied 57,885,161 twos. The number was found by professor Curtis Cooper on a PC in a lab at the University of Central Missouri. This is Dr. Cooper’s fourth record GIMPS project prime: His first record prime was discovered in 2005, eclipsed by the second in 2006, and third in 2013.