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GCF (HCF) of 15 and 9

"Greatest Common Factor" Calculator

What is the Greatest Common Divisor of 15 and 9?

Answer

GCF (HCF) of 15 and 9 is

3

Common factors: 1, 3

3 divides both: 15 ÷ 3 = 5, 9 ÷ 3 = 3 — and 5 and 3 share no factor but 1, so no larger number divides both.

15 = 3 × 5

9 = 3²

Prime	15	9	Lowest power
3	3	3²	3
5	5	—	—

— = power 0: the prime is not a factor of this number, so it cannot be in the GCF.

GCF = 3

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See Also

Least Common Multiple - Find the Least Common Multiple (LCM) of two numbers
Greatest Common Factor of 3 Numbers - Find the Greatest Common Factor (GCF) of three numbers
Least Common Multiple of 3 Numbers - Find the Least Common Multiple (LCM) of three numbers

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What is the GCF (HCF) of 15 and 9. The GCF of 15 and 9 is 3 — the biggest number that can divide both integers. Step-by-step GCF (GCD) calculation by factoring.

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GCF Table

Number 1	Number 2	GCF
15	5	5
15	6	3
15	9	3
15	10	5
15	12	3
15	16	1
15	17	1
15	18	3
15	20	5
15	21	3
15	22	1
15	23	1
15	24	3
15	25	5
15	26	1
15	27	3
15	28	1
15	29	1
15	30	15
15	32	1
15	33	3
15	35	5
15	36	3
15	38	1
15	40	5
15	42	3
15	44	1
15	45	15
15	48	3
15	49	1

FAQ

What is the Greatest Common Divisor of 15 and 9?

GCF (HCF) of 15 and 9 is 3

How to find the GCF of 15 and 9?

You can find the GCF of 15 and 9 by listing all factors of both numbers and picking the greatest common one, or by prime factorization. The GCF of 15 and 9 is 3.

What prime factors do 15 and 9 share?

The prime factorizations of 15 and 9 have 3 in common, and multiplying those shared primes gives 3 — exactly the greatest common factor.

How does the GCF reduce the fraction 15/9?

Divide the numerator and the denominator by the GCF 3: 15 ÷ 3 = 5 and 9 ÷ 3 = 3. So 15/9 in lowest terms is 5/3.