Greatest common factor (GCF) of two or more numbers is that number ; which can divide both or more Numbers, thereby leaving no remainder and is the greatest factor of the two number or more numbers. To make the understanding simple, anexample is explained below. Let I wantto find out the Gcf of 18 and 12 . By our commonidea we can say that 2, 3 and 6 can divide both the numbers but 6 is the greatest among 2, 3 and 6. Therefore 6 if the GCF of 12 and 18. GCF Or HCF (Highest common factor) can be calculated by prime factorisation . For lower classes it is the suitable method. 12=2x2x 3 18=2x3x3 The common factors are 23 ie 6. Watch this video.