GCF and LCM of 8 and 15

The GCF of 8 and 15 is 1. The LCM of 8 and 15 is 120.

• GCF (greatest common factor, also called GCD or HCF) of 8 and 15 = 1
• LCM (least common multiple) of 8 and 15 = 120

8 and 15 share no common factor except 1, so they are coprime (relatively prime). When two numbers are coprime their LCM is simply their product: 8 × 15 = 120.

Method 1: list the factors (GCF)

Factors of 8: 1, 2, 4, 8.
Factors of 15: 1, 3, 5, 15.
Common factors: 1. The greatest of these is 1.

Method 2: prime factorization

8 = 2 × 2 × 2
15 = 3 × 5

GCF: multiply the prime factors both numbers share: there are none, so the GCF is 1.

LCM: multiply every prime the maximum number of times it appears in either number: 2 × 2 × 2 × 3 × 5 = 120.

Method 3: Euclidean algorithm (GCF)

Divide the larger number by the smaller and repeat with the remainder until it is 0. The last non-zero remainder is the GCF.

1. 15 = 1 × 8 + 7
2. 8 = 1 × 7 + 1
3. 7 = 7 × 1 + 0

The last non-zero remainder is 1, so GCF(8, 15) = 1.

LCM by listing multiples

Multiples of 8: 8, 16, 24, 32, 40, 48, 56, 64, 72, ...
Multiples of 15: 15, 30, 45, 60, 75, 90, 105, 120, 135, ...
The first number in both lists is 120.

Check: GCF × LCM = product

1 × 120 = 120 = 8 × 15. This always holds for two numbers, so you can find the LCM as (8 × 15) ÷ 1 = 120.

Simplifying the fraction 8/15

8/15 is already in lowest terms because the GCF of 8 and 15 is 1.

GCF and LCM of similar pairs

See also: factors of 8 · factors of 15 · 8/15 as a decimal.

Different numbers? Use the GCF and LCM calculator (any amount of numbers) or browse all GCF and LCM pairs.