Greatest Common Factor

The Greatest Common Factor (GCF), also called the Highest Common Factor (HCF), is the largest number that will divide into two given numbers.

To find the GCF, list all the prime factors of each given number from smallest to largest. Start by dividing by 2, then 3, then 5, then 7, then 11 and so on.

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Examples

NUMBERS	ALL FACTORS	GREATEST COMMON FACTOR
18	2 × 3 × 3	2 and 3 are common of both 18 and 24. The GCF of 18 and 24 is 6, that is, 2 × 3.
24	2 × 2 × 2 × 3

NUMBERS	ALL FACTORS	GREATEST COMMON FACTOR
42	2 × 2 × 3 × 7	2 and 7 are common factors of both 42 and 70. The GCF of 42 and 70 is 14, that is, 2 × 7.
70	2 × 5 × 7